Project Small Bore Pt2

Last time we started building a small-bore engine for maximum power. But there is no point having a good engine and putting the wrong head on it.

I’m going to use the 12G202 head, as these are the best to modify for our purposes. They are a bit smaller than the 12G295 or 12G206 heads, which gives you a bit more metal to play with when grinding. As far as a standard head goes, you can’t go past the 12G295 or 12G206.

I do not like the use of the 1275-type heads (12G940, etc) as these are too big for the small-bore motors. Yes, they will make the horsepower if you rev them hard enough, but I can still make more horsepower, with a better spread of torque, using the smaller heads.

The smaller heads have much better air-speed and better filling, which in turn produces a better spread of torque throughout the rev range – and it is torque that accelerates the car. When using 1.218” valves the 295 head flows the same as the 940 head with 1.3”, but the 295 head is doing it with smaller volume right through.

To give an example, let’s look at the 970S and 998 Cooper motors. The 970S uses a Mk1 Cooper S head, with smaller combustion chambers – therefore higher compression – and larger inlet valves. Both engines make the same torque – 57 ft lbs – but the 998 Cooper engine makes this at 3,000 rpm, while the 970S engine makes it at 5,000 rpm – some 2,000rpm higher. My point is, bigger is not always better.

Formular jr1  Formular jr2

The Cooper Formula Junior head, showing (from left to right) the combustion chambers, and extra head stud holes.

As an aside, there was a one-off type of engine, of which only around 40 were made, built for John Cooper’s Formula Junior race cars. It is not well known, but worth having a look at. This was based on a 950cc A-series block, with a bore of 2.661” (67.6mm) and stroke of 3” (76.2mm) – giving a capacity of 1095cc – and produced 98 hp at 7,800 rpm with a 40mm Weber. The first version used a smaller cam and twin SU carbs, and produced 94hp.

It was dry-sumped and had special rods, but the best part was the cylinder head. This engine had a small-bore head (with two casting numbers – AEA626 and 12A185) which was almost the same as the 12G295 head. But, it had slightly bigger valves (both inlet and exhaust). More importantly, it had two extra head studs, one at each end, like the Cooper S heads, to prevent blowing the head gasket.

Getting back to our project motor, I have modified the combustion chambers on our 202 Head, ported it out and fitted bigger valves – 1.280” inlet and 1.062” exhaust. Be careful not to overdo the porting. You don’t need to go any bigger than on a 12G295 head. In fact, with a bit of cleaning up of the inlet ports, the 202 head is ideal for a road car.

Grinding Comb polishing Comb

Grinding out the combustion chamber (left) and polishing it off.

12g295 Std



Std 12G202

12g202 Mod

Modified 12G202

With a decent manifold and 1.75” SU carb fitted, this modified head has a good high gas flow of 115 cubic feet per minute (cfm) @ 25”, compared with the standard head at 70cfm.

This brings me to the importance of a good manifold. It is no good having a well-ported head and then putting a poor-flowing manifold on it, as the manifold is really just an extension of the inlet ports. If the manifold doesn’t flow efficiently then you have just wasted time and money on doing the head. I’ve seen some manifolds drop the airflow of a good Cooper S head by as much as 20%.

The length of the manifold is also a consideration, and here longer IS better. In this I am talking only about the length of the manifold. Don’t think that long ram tubes fitted with a short manifold will be as good, because they won’t. The critical length when dealing with carburettors is the distance from the valve to where the fuel enters the air-flow. This, of course, is not the case with fuel-injection.

So, the two things you need for a good manifold are length and volume. The Weber manifold and SU manifold that I make are both a tight squeeze under the bonnet, but they are designed for performance, not looks. My Weber manifold is about 5.25” (133mm) long, but has a very large volume for its size. My SU manifold has both volume and length – around 7” (178mm) along the centre-line of the manifold – and makes good torque on any size engine.


RE7 Weber

The old “Swan-neck” Weber manifold is disliked by many people, but it is actually not a bad manifold, if you are not after maximum torque and horsepower. So, if you are wanting a nice, mild engine, you don’t have to put the same type of manifold on, that you would use if looking for say 150hp. I’ve seen 90hp with these manifolds, which still makes for a lively road car.

Colin Dodds from Sprite Parts in Castle Hill recently tested five of the most common manifolds for Weber carburettors on a flow bench, then compared the best three with a Maniflow and my SU manifold while mounted to the head. Following is Colin’s report, reproduced with permission.

We regularly get asked what is the “best” Weber manifold to fit to your A-series engine. I’ll admit, I’m biased, and I do have a favourite, but largely it also depends on what you want to define as “best”. Certainly “best” has to include “fit for the purpose” and in that context you have to consider whether the manifold actually does fit. Some manifolds made for Sprites and Midgets don’t really fit a Mini, and some made nicely for a Mini don’t produce the best results for a Sprite/Midget.

Anyway, a little bored and wanting some concrete statistics, I gathered up all the manifolds I could find and wandered off for a scientific test on a flow bench. For the purpose of this exercise, I will define “best” as the one that produces the highest cfm (cubic feet per minute) of airflow at a partial vacuum of 25 inches of water (which I believe is the “standard” of measurement).

Now, let’s start with a little theory (sorry).  Engines need fuel and air mixed in the correct ratio to ignite efficiently, and the more of this mix you can deliver to the engine, the more power the engine is theoretically capable of producing. The actual power output of the engine depends of course on many more factors which will include cylinder head design, camshaft profile and timing, valve size and lift, exhaust system, and compression ratio. A fairly standard 948cc engine might produce maximum power using only a moderately effective carby and inlet manifold. However, as your engine increases in capacity and performance capability, so it needs more fuel/air to enable it to produce the best results. So it is fair to say that the manifold that flows the most air, is CAPABLE of producing the most power.

We had five manifolds on hand, and I know this doesn’t represent all the ones available world-wide.  However, they would be the ones most used in Australia. They can be described as follows, but sorry if I don’t include your favourite manifold (nope, no Cannon ones were available).

Man A: LYNX Mini manifold, steeply upswept “swan-neck”. Fits easily in a Mini, which is why it was designed that way.

Commonly accepted as a poor performer.

Man B: LYNX flat 5” A-series manifold. Fits a Sprite or Midget, will not fit a Mini without cutting a hole in the firewall.

Man C: Warneford or Redline 5” manifold. Upswept to clear the firewall of a Mini, also fits nicely in a Sprite/Midget.

Man D: Warneford or Redline 6” manifold, longer (duh!) and not so upswept. Needs firewall modification to fit a Mini.

Weber comes VERY close to the inner mudguard on Sprite/Midget.

Man E: Limited-production hand-finished 5” cast manifold from Russell Engineering.

Fits comfortably in either Sprite/Midget or Mini.

And so to the results of the shoot-out.

Man A: (LYNX Mini) – 104 cfm

Man B: (LYNX 5”) – 136 cfm

Man C: (Warneford 5”) – 139 cfm

Man D: (Warneford 6”) – 175 cfm

Man E: (RE 5”) – 207 cfm    (Russell Engineerings SU manifold was independently flow tested in the UK)

I had two people suggest that we re-run the tests, but this time with the manifolds attached to a cylinder head.  That way we could see the extent to which the manifolds hindered air flow into the head. Well, that sounded like a really good idea, so that’s just what we did.  We grabbed a nicely modified 12G940 head from a 1275cc engine, with good porting etc, and 1.4” inlet valves, set it up on the flow bench, and took readings with and without manifolds.  We didn’t bother testing the two poorest performing manifolds as we really weren’t interested in the result.

I have to admit, the results weren’t exactly what I was expecting. I knew the raw numbers would change, but I wasn’t expecting any change in the order of ranking of the manifolds – but that is what happened. We re-ran the test just to check, and nothing changed.  All tests were done with the same vacuum as previously, and with .500” of valve lift.  That’s about the lift you would get with a performance camshaft and 1.5:1 rockers.

146 cfm through the head alone

144 cfm (2 cfm or 1% drop) with the RE

131 cfm (15 cfm or 10% drop) with the 6” Warneford/Redline

130 cfm (16cfm or 11% drop) with the Maniflow

113 cfm (33 cfm or 23% drop) with the 5” Warneford/Redline

Just for the hell of it, we also attached the RE single SU manifold, and it flowed 141 cfm (5 cfm or 3% drop).

flow bench flow bench2

And that was WITH a 1.75” SU attached to the manifold!




RE7 Weber

Now, nothing sounds quite like a Weber, but there are more important things than how it sounds. You can still get the same horsepower from an SU, but for racing conditions a Weber is more drivable. For normal road use, even if you are a bit of a “Boy Racer”, the SU is great. It is also a lot quieter than the Weber, possibly more economical and a lot less smelly, which is all the better if you are driving your Mini a fair bit. Still, if you really have to have that Weber sound then there is no comparison.

It is very important to remember what you are building the motor for. If it s going to be a daily driver, or used for weekend cruises, then build it for that. Don’t go overboard with a big head, lumpy cam, manifold and exhaust which would be more suited to a race engine. Instead, aim to get a good spread of torque from low down. It will be much better to drive than a motor which doesn’t start to make power until around 4,500-5,000rpm.

When I build race engines I still chase torque as the most important factor. Getting out of corners quickly and accelerating away is more important in most racing than outright top speed. When it comes to top speed in a Mini, I’ve done a lot of testing with a GPS (no chance for tacho, tyres or other variables to throw out the results) and, without a doubt, a Mini on narrow tyres and no flares, with 145-150hp, will not go over about 123mph on a flat road. But, a similar Mini with 115-120hp will still do 118-120mph.

This is because of the drag that is inherent in the design of the Mini body, and it takes a massive amount of horsepower to overcome this. Yes, Minis have recorded faster speeds at Bathurst, but that is on Conrod Straight, which is down hill! I would happily sacrifice 10-15hp in the interest of getting an extra 5-7 ft lbs of torque.

So, with our head complete, and our choice of manifold sorted out, we need to look at fitting the head and setting the valve timing. As I mentioned in the last issue, when timing the camshaft I always have the head fitted, to put tension on the chain or belt.

Before fitting the rocker gear, I’ll make a quick point on what sort of gear to use. Unless you are looking for power above 5,500rpm then do not use rockers with a ratio greater than 1.3:1, as they will cost you power and torque. When fitting the rocker gear, make sure that the rocker pad is centred on the valve tip when the valve is on half lift.

Now, with the head on the engine and the rocker gear fitted and adjusted, it is important with a performance engine to get the valve timing the same for all valves. By this I mean, when the valves actually start to open, and it has nothing to do with ignition timing.

To do this we need to use a dial indicator on the valve cap and a degree wheel on the crankshaft. With number one cylinder at Top Dead Centre (TDC) on the firing stroke, set the pointer on the degree wheel to Zero.

Now, rotate the engine and check the opening point on number one valve. That is when the dial indicator just starts to move by about one-thou. Repeat this process for the other three inlet valves.

You may find that three of the valves open at say 35    Before Top Dead Centre (BTDC), and one opens at 29    BTDC. This often happens with Mini rockers, both standard and after-market. You may get a bit of a shock when you start checking them.

To get the 29   valve to open at 35   you may have to close the tappet up, so that it is set at say 0.012” rather than 0.016”. Don’t worry about the difference in settings as it is more important to get all the valve timings the same.

On some big-power engines I’ve had tappet clearances vary from 0.015” to 0.023” on the one engine. Remember, get the valve timing the same and you will make more power.

If you are a little surprised about this variance, remember there are four factors that can change the tappet clearance. These are the radius on the cam-followers, the length of the pushrod, the length of the tappet adjuster and the ratio of the rocker.

I can’t stress enough the importance of getting these valve timings the same. A difference of 0.002” can mean a loss of three to four ft lbs of torque across the rev range.

timing wheel measring

With degree wheel set to 0 (left), rotate engine until valve opens one thou (0.001”)

The ratio of rockers is also a bit of a concern. As I said, unless you after power above 5,500rpm, there is no need to use greater than 1.3:1.

Some rockers which are sold as 1.5:1, both roller rockers and forged rockers, are actually measuring 1.6:1 and higher. This creates a lot of problems with valve timing and lift.

Some cams I am aware of are giving 330 duration, instead of the nominated 280, and instead of the rated 0.480” lift have 0.520” – 0.530” lift. This is because the rockers are a higher ratio than stated, so are accelerating faster onto the valve, which in turn gives longer duration and higher lift. This can be corrected by opening up the tappet clearance.

I have found that with 1300cc – 1480cc A-series race engines you don’t need more than about 0.480” – 0.500” lift, and with small-bore engines, like the one we are looking at, 0.400” – 0.450” is adequate.

So, now we have the head on the engine, the rocker gear and valve timing sorted, and we’ve decided on the inlet manifold and carburettor.

In the next post we will be putting the engine on the dyno and running a series of tests with a variety of exhaust extractors, inlet manifolds and carburettors, and should be able to prove most of the things I’ve mentioned this time around. Following that will be the post on the 2014 Small bore build to see what has changed and developed over the years.


Project Small Bore

A few years back Russell Engineering built a small bore project motor and wrote an article for The Mini Experience Magazine.
Below is part one of the article and in the following weeks we will be posting the next two parts. The reason for bringing these articles up  again is that Russell Engineering is in the process of building a new and some may say improved small bore which will feature a few of the new products, such as the 68mm Small Bore Pistons and threee new Camshaft to try. These previous articles will be referenced to, for the new build as some things have changed and some have stayed the same. Stay tuned, read on and enjoy.

Small Bore Project Part 1

We all long for the power and torque available from a 1275cc engine, but with these becoming more expensive and harder to find, we’ll take a look at the alternative of getting some decent power from the small bore engines.

For the price of a good 1275 crankshaft you could buy two or three small-bore motors. The cost of reconditioning a small-bore block isn’t going to be a lot less than doing a 1275cc, but the cost savings on parts like the crank alone would go a long way towards modifying a 998cc or 1098cc motor. Over the next few issues I’ll look at how to get the best results from an 1098cc engine. The bore for the 998cc and 1098cc engines are the same, but the 1098cc, with its longer stroke, will produce a better spread of torque across the rev range. If you wanted to do a 998cc the process is much the same. The engine will not give as good a torque figure, but it will rev more freely at the top end. In this issue we will take a look at what we can do with the block, crank, and camshaft, pistons and rods. I’m going to assume that if you are intending to do this type of work, then you probably already know your way around the motor fairly well, and you will already have the engine stripped down. However, before removing the old main bearings, check that the oil holes in the block line up with the holes in the main bearing shells.


If not, the block will have to be ground out to match its oil passages to the bearings. Starting with the block, it should be chemically cleaned to remove as much of the rust and rubbish from the water ways and oil ways as possible. But, don’t forget to remove the two oil gallery plugs first to ensure everything that can be is cleaned out. Now, there is a cost-effective way to get rid of all the rust out of the block (and the cylinder head too) without any special high-tech tools. Simply mix up a solution of molasses and water (one part molasses to three parts water) in a plastic tub. If the mix is too thick to get into the water ways easily, you may need to thin it out a little with more water. I used to use this method and it worked very well, but when the aluminium bung in the tub disintegrated – molasses eats aluminium the result was very messy! Nowadays I send my blocks and heads to Redi Strip in Blacktown (Sydney). It costs over $100 to have a block chemically stripped, but the results are fantastic.


This block has been cleaned with the molasses bath.Photo 3: Line the hole in the crank up with the groove from the bearing shell. Photo 1: Here the oil hole in the bearing shell doesn’t line up with that in the block. Just submerge the head or block in the solution for about three or four days. Take it out and hose it off. If all the rust isn’t gone, repeat the process. One thing to remember is that it will not remove paint, grease, carbon, dirt or anything else, but the rust. With our project motor I’m going to bore the block +0.040” (40 thou’) oversize. As we are not going down the forced-induction path, we will be running high compression pistons I like to run around 10.5:1 for more power without sacrificing reliability. Now, you could go out and buy some special flat-top pistons, but part of the point of doing this motor is to save money. If you use the standard 998cc pistons, you can machine off the 0.140” crown, to get flat top pistons much more economically. With the block cleaned and bored, and the pistons sorted, the next step is the crankshaft. Remember how those oil holes in the block didn’t line up (as is usually the case)? Well, neither do the holes in the crankshaft. It is important for the holes to be moved over so they line up with the grooves in the bearing shells, to ensure proper lubrication under the extra power we will be feeding into the engine. The standard arrangement is fine for a standard engine, but not up to scratch for what we are doing. To align these holes I use a little dremel, or die grinder, to lead the oil from the bearing groove directly into the oil hole in the crank.


With this modification, it is not necessary to cross-drill the crank if the bearings have a full-circle groove around them. In saying that though, I always cross-drill my cranks that are going to be used in race motors. You may have heard the term wedging the crank. Well, there is a right way and a wrong way, and simply grinding off metal to form a blade is the wrong way. The metal removed from the web must only be removed back as far as the crank pin.


Only remove metal from around the pin area, as shown at the bottom part of the photo. This will improve the harmonic balance of the crank. Do not blade the crank, as we need as much weight as we can get on the outer end of the web to counter-balance the weight of the piston, rod and crank pin.


(Left to right) a Russell Engineering billet crank, a correctly wedged and balanced standard crank and an untouched standard crank.

When balancing the crank, only remove metal from the pin area, not off the web. We are trying to improve the harmonics of the motor, and there is a big difference between dynamic balance and harmonic balance. A good example of this is in an article by the manufacturers of Schenke balancing machines, where they took a Mercedes crank, cut all the counterweight off and balanced it dynamically. They then put it in the engine and ran it. The noise levels were way up, but power was way down, and it nearly shook the motor to pieces, because the vibration level was so high. This was an extreme case, but illustrated the importance of balancing harmonically. BMC were one of the worst offenders. I’ve seen Cooper S cranks that have had about 3/4 of the centre counterweight removed to get the thing balanced. And they did it with standard 998cc and 1098cc cranks as well. In a case like this, you need to weld up the counterweight and regrind it, then remove the weight from the other end. Next are the connecting rods. These can be a real problem, due to inconsistencies in their heat-treatment. When I started Mini 1000 racing we had a few rod failures, breaking just above the big-end. I measured all the broken rods and found they all measured around 18 to 20 Rockwell C hardness, while good ones measured around 23-25 Rockwell C. I then had all my rods treated to 30 Rockwell C and no more problems! Don’t have the caps heat-treated though, as they tend to open up with the heat and then don’t give a good fit. Also, make sure to remove the little end bushes from the rods before heat-treatment. After treatment, the rods can be lightened by machining the sides. You can take 0.150” off both sides (photo 6). Now to the camshaft. There are many cam grinders in Australia and overseas. They all have their favourite cam, and they all believe their cams are the best. I’m no different, and in this motor I’m going to use my favourite cam – the RE13. This is a sports cam and a good all-round cam for performance in a road car, but not what I use in a standard road engine. And it is not a scatter cam. Just an aside if you think you know who first came up with the scatter cam, you are probably wrong. It was first developed right here in Australia, by probably our greatest race engineer, Phil Irving. In his book Automobile Engine Tuning (first printed in 1962 by Pitman) Irving refers to scattering lobes in motors with Siamese ports. I’ve been told that he was playing with this type of cam back in the late 1950s with BMC engines and Holden Grey motors.


Big-end of conrod showing where it has been machined 0.150” George Wade of Wade Cams was grinding scatter cams for Peter Manton in the early ’60s. When Clive Stenlake joined George Wade in about ’65 or ’66, he was grinding the ever-popular 285-CO and 176-0 cams. Clive left Wade Cams in 2001 to start his own business, Clive Cams, and is still grinding these cams today. Getting back to the RE13. This cam pulls strongly from 2500-2700 RPM, and runs through to 7000-7500 RPM, making strong torque and horsepower. So, as I said, it is a good all-round cam.


RE13 cam on the grinder.

Now it is time to assemble the block. First, do a dummy assembly and check the deck height, making sure the pistons are flush with the top of the block at Top-Dead-Centre. Check the main-bearing clearance with a Plastigauge. It should be between 0.0015” and 0.0025” (one and a half to two and a half thousands of an inch) for a performance engine. Ensuring all parts are nice and clean, we can now go ahead and do the final assembly. When fitting the oil gallery plugs, use a little bit of Locktite – just to be sure. Two items that I consider very important in a Mini engine are the crankshaft end float and connecting-rod side clearance. The old thrust bearings can often be cleaned up with some wet-and-dry paper on a sheet of glass, to get 0.005” to 0.006” end float. This will give enough clearance to get the oil flowing in and out, to avoid overheating the oil, which would damage the main-bearing. We also need enough clearance so that when the crank flexes on that accidental over-rev and believe me, they do flex it doesn’t grab the thrust bearing. I have found that where the numbers are stamped on the back of the thrust bearing, they can raise up around 0.003”. This would give 0.006” over the total width, and this is enough to lock up the crank. So, make sure you check the width of the thrust bearings over the numbers as well as at each end. This can be rectified by rubbing on wet-and-dry paper to remove the raised edges of the numbers.


Raised stamp on thrust bearings.

Getting enough oil into the thrust bearings is very important too, and there is another trick here. I like to put a slight chamfer, about 1mm on a 45 degree angle, on one edge of one main-bearing shell.


Where to chamfer main bearing.

Do this on the thrust side (the clutch side) of the main bearing, so it can squirt oil onto the thrust bearing. Give the conrod side clearance at least 0.010” for the same reason – to keep the oil flowing and prevent the crank from grabbing the rods. With the Plastigauge, check the conrod bearings have 0.001” to 0.002” clearance. I like to use Vanderval/AE Heavy Duty or ACL Duraglide bearings. If you are going to use aluminium-tin bearings, make sure your clearances are on the bigger end of the scale. Use a good quality assembly lube on the bearings and crankshaft. Don’t use lock-tabs on either the mains or rod bolts (as are used on the original motors). These are made of mild steel (very soft) and with high revs they compress, and lose tension. The result is spun bearings and a BIG bang! Always prime the oil pump with a thick assembly lube, or it won’t pick up oil when you first start the engine. Now the piston rings. If you are lucky enough to get a set of rings you have to gap (they are often made too small) aim for a gap of about 0.010”. When fitting circlips to the gudgeon pins on small-bore motors make sure they go in the right way around. Don’t laugh, it is a common problem and easy to get wrong. You’ll notice that the circlip has one rounded edge and one square edge – this happens when they are stamped out. Put the square edge facing out. With the wire-type clip, make sure they are a tight fit. If you prefer, you can replace the circlip with some Teflon buttons. When fitting the pistons into the bore, I only use a light oil – WD40 or similar – on the bores as you want the rings to bed in quickly. Never use friction-modified oil in assembly or running in, as the rings won’t bed in properly. I never use friction-modified or synthetic oils in Mini engines, because the oil doesn’t stick to the rockers, or the camshaft, when in motion, which causes wear. When fitting the camshaft, my rule is “new cam, new cam-followers”. Be careful to use only good quality cam-followers. For timing the cam, I use the “full-lift after topdead-centre (ATDC) method. This is more accurate, and I always have the head fitted to put tension on the chain or belt. If you have a camshaft but don’t have any setup figures, just split the overlap at Top Dead Centre (ie: inlet and exhaust valves open the same amount at TDC). To do this, use either full vernier gear or offset cam sprocket keys. That should be it for the block.


Part Two-We will look at the head, valve timing, manifolds and carburettors.

2014 Phillip Island qualifying in the RAIN!

This years Phillip Island saw a wet start to proceedings and qualifying in the rain makes for some fun times.

Graham qualified first by more than three sconds to the nearest competitor, but unfortunatly racing was not an option after breaking a rocker during qualiying.



Mini Racing

2014 has arrived and like every year, the workshop is busy with preparations for the racing season across a broad range of categories. I race currently in an Nb car and have been fettling both the car and motor, and I am still playing with coming into the first race meeting of the year. I’ll be updating my page soon with a run down on some of the projects I am working on. Bare with me, I will get there.

In the mean time have a look at the videos page for a link over to youtube of me running about in my at the time Nc car.

It was filmed at the mini 50 celebrations that were put on at Wakefield Park Raceway back in 2011. The car had a problem on the first lap so I pulled off to let the field go by before sputtering back to the pits. As I pulled back on the car cleared it throat and roared back to life. So I set about chasing the field down. Now that was fun!

Check back and hopefully I’ll have some more musings for you soon!

11 Stud/Bolt Small Bore Head

One of the problems with the small bore head when a lot is machines off the face, is the lead or blow head gasket out the ends. So, this is how to convert a small bore head to an 11 stud/bolt setup.

Just a note on machining the face of the head. I never take any more than 60″ – 80″ off the face of the head as it flexes and warps to much and you end up losing more than you gain with gasket seal.

Back to the 11 Stud/Bolt set-up.

I take an old Cooper ‘S’ head, sit it on top of the small bore head and locate it in place with drills down the stud holes. Now take 7/16″ drill bit and drill down through the end of the head at the no.4 end. Remove the Cooper ‘S’ head and take a larger drill bit or a counter sink tool and put a large chamfer on the top and bottom of the 7/16th hole you had just drilled. We will come come back to this hole to finish it off after we drill the other end.

sb11stud2  sb11stud3

Now, take the Cooper ‘S’ head and place it gasket face to gasket face onto the small bore head, then locate it with drill bits again. Now take a drill that fits the hole in the Cooper ‘S’ head at the no.1 end and drill into the face just enough to mark it. Remove the Cooper ‘S’ head and drill a 1/4″ hole through the gasket face only. NOT RIGHT THROUGH the head. We will hold this end down with  1/4″ capscrew underneath the thermostat in the water way.


Ok, now back to the no.4 end. We have to make a tube 7/16″ outside diameter and 5/16″ inside diameter to go down inside the hole we drilled.

Make the tube the same length as the thickness of the head as it has to  be welded in. I brazed my tube in. To do this the head has to be pre-heated, so I just placed it on the BBQ with the lid down and warmed it up to 500-600 deg Fahrenheit or 260-315 deg Celsius.


The head will go a soft blue and with a very soft flame on an oxy torch, I filled the chamfer with brass with the tube in place. Turn the head over (It’s hot, so be careful) and repeat on the other side. NOTE. Use cast iron flux, not normal brazing flux. Once done, turn off the BBQ and put the lid back on and let the head cool down slowly.


After the head has cooled down, you can then machine both faces, top and bottom to clean them up.


Lets move on to the block.

Take the head and place it onto the block and locate it with studs or bolts, which ever you are using, and drill the block enough to mark the face. Remove the head and drill and tap the block with a 5/16″ UNC thread at the no.4 end and at the no.1 end drill and tap 1/4″ UNC thread.


When it is time to assemble the head to the block, you can use cap screws for both the new holes. The no.4 5/16″ cap screw can be tightened down to 15ft/lb, while the no.1 end I use a ball end allen key with a spanner on the end. This I tightened by feel, until the allen key starts to twist. because you are pulling down directly onto the gasket , you don’t need a lot of pressure.


There you have it. A small bore 11 stud/bolt head.


68mm Flat Top Pistons – Suit 998/1100

New product

68mm flattop piston to suit 1100/998 motors



These pistons are made of 11/12% silicon.These are a very strong piston and will suit race application. The weight comes in at 286g with pin and rings. The pistons are drilled behind the oil ring so there are no slots. the gudgeon pin is fed by oil from the top and the bottom. Being a flat top piston, dishes can be machined in if required.



Small Bore Cylinder Heads

Three Head Inlets

Ok lets take a look at the cylinder head. Here we have three small bore cylinder heads.
From left to right we have a AEG295 head (left). This is the performance head for small bores. The second one is the AEG202 (middle) and the third one is the 998 head (right). Now what i’ve done is cut them all through the inlet port to show you the difference in port size of all three heads.
Now the AEG295 head has the largest port of all which is really quite a big port for the size valve it uses. It measures 2.220″ high by 1.030″ wide the valve size is 1.218″.

The second one is the AEG202 it has an inlet port that measures 1.000 high by 0.937″ wide and the valve size is 1.156″.

The third one is the smallest one of all, the 998 head, of which there are several casting numbers. This has a round port (0.937″ in diameter), with a big chamfer at the manifold face which tapers from 1.187″ down to the 0.937″. You will notice that all three ports are a different core (the outside diameter).
Now because there are more 998 heads around than the 295 or 202 this is the one we will modify because it’s the smallest of all three. There is a little more work to get it to flow really well, but because there is more metal to remove there is more metal to get the shape right for better flow.
Now if you look at the 295 and the 202 inlet port you will notice that they have lowered the bottom of the port to open alot further than the 998, in doing this they have done away with the short side radius which is bad for flow at low lift and for high lift.
Now if we cut the port the right way it will be a really good flowing head.
Starting with the top of the port we have to angle the roof of the port down 1 degree from the manifold face towards the bottom of the port. Going square across the top of the port you can go as high as 1 3/16th diameter on the manifold face or 0.880″ from the rocker cover face to the top of the port. If you are doing this on a mill you can use a 7/16 diameter end mill long series going in to the depth of the flutes. In doing this it is going to leave a step where the end mill finishes. This is where you will have to blend it in to the roof of the port. Have a look at the photo.

Port Milled out

Milled out ready blending

Modified 998 Port

The second photo shows the port after it has been blended in.

Now for the width of the port you can go to within 1 millimetre or 0.040 from the pushrod holes. As for the bottom of the port, that has to be angled up at 3 degrees. Now with this you cannot go to the diameter of the 1 3/16th chamfer. From the rocker cover face to the bottom of the port measures 1.750” These dimensions are a guide only of most of the heads i’ve measured. They are not quite all the same but are close to the same so be careful.

Hold back a little if you are worried about going through to the water jacket,as you can see in the photo the 998 head has a lot more metal on the bottom than the others but the smaller port will still work very well with these dimensions. The port is still bigger than the 202 head but not quite as big as the 295 head. This size port will give a very good velocity as well as volume for good filling of the cylinder.

Std 998 Port

Standard 998 Inlet Port

Manifold Face

Modified 998 Inlet Port

Re-Seating the Valve and Throat Area

Ok lets cut some seats now. If you have a look at the 295 head you will notice that it has a very narrow seat 0.040” wide or 1mm, that’s fine but the bad part is they just made a hole straight down from the seat. They have a valve of 1.218″ diameter and a throat diameter of 1.170″. This is 96% of the valve diameter, which is a little bit big for a good flow at low lifts and high lifts. I’m going to use a 1.260” diameter valve and a throat diameter of 1.008”. This gives us a throat diameter of 80% of the valve. I sometimes go down to 78% of the valve, by doing this it helps accelerate the air out and around the valve as we are creating a venturi shape under the valve. Have a look at the photo and you get the idea and it’s very easy to blend the throat into the port area.

Valve open

Valve Closed

Now on the short side radius you can get a really nice short side radius because you have a lot more metal to play with, unlike the 202 head and the 295 head. This gives you a really good throttle response out of corners and on part throttle openings. Now to get this throat diameter I use a full radius cutter that I have developed because of the small valve diameter and the siamese port they need a different type of radius to other types of cylinder heads. I also have 3, 4 and 5 angle seats that work very well.

Five Angle Seat

But I find it easier to get a better short turn with a radius cutter.
Now because the mini has a port 90 degrees to the valve it is very important to get the air turned to come down onto the back of the valve and flow around the whole diameter of the valve and not just flow across the back of the valve. The difference between a good short turn and a bad one is huge. Now with the divider, because of the small choke area you can get a really good fat radius around it to help with flow.

The Combustion Chambers 

The combustion chamber you must get rid of some of that peaks it shrouds the exhaust valve causing it to overheat and burn out, which also leads to cracking the exhaust seats and chambers. The shape that I have come up with works really well.

Chamber Shapre

This chamber has been CNC’d


Photo of an exhaust insert fitted to a 295 head. Valve size 1.070″ Throat diameter is 83% of valve diameter. Note, the full radius and top cut.

You need to take next to nothing away from the beak-side once you have cleared the inlet valve as the air is being fed from the centre of the cylinder. Have a look at how the air is leaving the cylinder and at the angle of the port to the chamber.

Minis have a very efficient exhaust port, you can grind a little away on the spark plug side to help angle the air towards the port. As for the seat itself, I like to use a good top cut and leading into a good full radius seat with at least a 83-85% choke area of the exhaust valve. On the small bore heads they have quite a large area compared to the choke area, you don’t need to grind much out of the exhaust port itself just give it a good clean up. Most of the time I use a 1.070″ diameter size valve. On the centre exhaust you can grind the guide boss away but be very careful when grinding it away you will notice to the left and right of the valve guides there are 2 hollows. Do not try to grind the centre of the boss down to their level, because you will find water! Be warned!
Here is a photo of the exhaust port number 4.

Exhaust no4

Now this is using a standard 1.00 valve and note the choke area is not under the valve but around the guide boss area which is still smaller than the area of the exhaust port. Also note how close to water is getting, don’t grind too much or you’ll get a little thin. 1.070” is a very good size, as you can get a lot of air through a valve that size. For an all out race head I never go bigger than 1.125″.

Here are some photos of Standard exhaust ports. Note, 295 exhaust ports and valves are the same as a 202.

998 end

Std 998 End Port


Std 998

202 end

AEG 202 End Port


AEG 202 Port

Below are two photos of the centre port in a 1275 12g940 head that has been modified for full race. These will give you a good comparison between small and large bore porting.



Inlet Ports

Three Heads

The photo above features a 295 head showing how they dropped the bottom of the port to get the port volume. In doing so they sacrificed the short turn radius and the air is going to flow across the back of the valve and not the whole diameter of the valve head. Note how they machined the throat area, the valve size is 1.218” diameter, the throat diameter is 1.170”. That leaves 0.024” to cut the seat on which gives you about a 0.040” or 1 millimetre seat. Note the throat diameter is 96% of the valve which is a bit big. But note how they have done it, they dropped the bull nose cutter down to blend into the radius where the guide is, then they have dropped another cutter down to get the throat diameter.


Bull nose cut on throat

295 Valve

295 head with valve fitted standard

But they have a gone down 90 degrees to the chamber roof, which is not going to help the gas get out, it is going to crash into the back of the valve. Now have a look at the throat in the head I did.

Valve open

Valve Closed

Valve Seat 2

Three angles on the valve
Have a look at the valve at low opening and you will see how the air is helped to get out by the shape of the radius.

With the 202 head they use a 1.156” diameter inlet valve and a throat size of 1.000” which gives it a throat diameter of 86% of the valve and the same percentage for the 998 valve. With this you can start to get a better seat shape which would be better than just a 45 degree that they had as standard.

With these small bore heads it is really necessary to use the longer valves, 1275 type length, as you have a better choice of springs and less chance with coil binding with sports and race camshafts. But in doing so it will be necessary to pack under the rockers to get the rocker angle correct. The way I do this is to have a rocker pad or roller in the middle of the valve tip at half valve lift. To achieve this, you possibly need about .0.100” or 2.5mm thick spaces. This is a good starting point to get you close.

To summarise, the 295 head is possibly the easiest and cheapest way to get better performance out of a small bore motor. But for an ultimate performance small bore head, it turns out the 202 and std 998 head is the best to modify. For an all out race head in the small bore class you can go to 1.280″ to 1.300″ inlet size. Any bigger than this and you start to run out of metal to create the correct shape.

Something that was quite interesting is that all three 998 heads that were cut still had casting sand present inside the heads. This is something that is difficult to get rid of after the casting wires are pulled and is also difficult to detect and rectify.

Casting Sand

In a future article will see some dyno figures for the various heads and combinations of inlet and exhaust port sizes.

Graham Russell