998 Budget Blaster

With most of the 1275 and 1100 motors being flogged to death. It’s about time we did a 998 with a difference, a budget build.

Let’s start with the bottom end with this build, we will not be using a wedge crank or lightened rods, just a standard crank and rods, but we will have them all balanced. With the rods, have them checked for straightness and sizing of both the little end and the big end. If not right have them closed and honed and fit new little end bushes. Then fit the bearings and torque them down to 40 ft/lb. We do this so we can have the journals on each pin ground to get the right clearance, then measure the diameter of the big end bearing. Do all four right and write the size down of all four.

For this we are going to run the clearance a little tighter, one to one and a half thou clearance or .001  to .0015. With the side clearance on the rods, give them at least .005 clearance. Now with the mains we will give them the same clearance .001 to .0015. With the tighter clearance the little motor will last a little longer but we will give the little motor a good run on the Dyno.

For the block take out the gallery plugs and the welch plugs and have the block cleaned. the block and then be bored for this project. I’m going to use my 998 +.060 flat top pistons and I will give them a .0015 clearance. Do a dummy assembly with the pistons to check the deck height to .001 to .0015 inch down the bore. After you have skimmed the top of the block make sure you countersink the stud holes.

Now for the head of this build I am going to use late model heads, the ones with the really small round inlet port. Out here in Oz land we had pollution a head that had ports in the front of the head to pump air into the exhaust ports. I am going to go about this two ways. One head I’m going to leave it stock standard with a different seat cut. Then I will do a budget modified one. The modified one I will use the Mini Cooper 998 inlet valves, 1.218 inch inlets and 1 inch exhaust. Because the minis have very low air speed in the head, we need to get all the air speed we can to try and fill the cylinder. This build we are not going to have a lot of lift as we are going to use the standard Rockers. The late model sintered type that I call duckbill rockers. They are possibly the lowest lift of all the mini rockers, but they are the late model rockers so this build will be what you have is what you get.

Let’s do a little grinding first on the inlet port.


The port will be no wider than standard but the floor and the roof I will taper off from the same point at the front of the port up to where it starts to open up to go left and right. It’s not a very big port but it will help to let the motor breath a little better. 

I have been doing a lot of work with the seat angles and I have developed a very good multi angle cutter. For those of you that live on the other side of the world in the United Kingdom. I have given Keith Calver of Claver Special Tuning one of these cutters so he can do the multi seats over there.

I’m going to keep the standard choke area and just use the multi angle cutter to open up to the 998 cooper valve size. This should give us some good air speed out of the valve as we need to get very good flow from off the seat to maximum lift which will not be a lot, only about .300 thou maximum.

Flow Bench Figures at 25 inches

As you can see this is a lot of flow for a valve of this size in a mini head but sadly we are only going to use about .300 lift with the standard rockers. We will see what it came out with when we get the motor together. One thing I have learned is that the lower the valve lift, you have to have use a smaller valve choke area. Have a look at Project Small Bore Cylinder Heads and you’ll get an idea. You need to get the air to turn around the seat so you need that venturi shape. 

I have made a new cam profile just for these little motors. It will be smaller in duration than my 266SS grind but it will have .280 thou lift on the lobe which is .010 thou more than the 266SS, as we are going to use standard rockers. The valve springs we are going to use will be the Mini Cooper Dual Springs with compression ratio of around 9.5 to 1. As for the carburettor, we will use at 1.5 SU HS4 on an after market alloy inlet manifold and the exhaust will be the free flow maniflow type.

This is a high velocity head and it is not a big port but the size of the port and shape along with the shape of the valve, throat and seat is the important thing. How it leaves the 45deg seat in the chamber is the most important thing.

Flow bench time!

On the flow bench using the Pitot tubes to measure the velocity in the port and around the valve. The air speed around the Throat was very high and using a piece of string and letting It go down the valve it had massive swirl. The important thing about these heads is that you get to get the air to turn around the short turn and use all the diameter of the valve, not just let it flow across the back of the valve.

Pitot tubes and string.

Three things that I have found to achieve this is the shape of the valve pocket, the seat, and the short turn. Now if you have a look at the head I have cut in half you will note the 998 head has a lot of metal on the bottom of the port that allows you to play with the shape of the short turn.

With the inlet port, the standard port is very ugly. The air going in has a lot of trouble trying to turn the corner left and right. With this you have to get a really nice radius here to get it to turn efficiently.

Let’s have a look at the exhaust valve port. The way the exhaust gases leave the cylinder is very efficient so for this project we are going to use the standard exhaust valve, 1 inch with the right valve seat. This little valve will flow a lot of air for its size. The seat profile used for this is a good top cut leading into the full radius. Under the valve is the choke area for the exhaust around the valve guide. I use a 3/8 round carbide to cut a groove each side of the valve guide to give a little more area around the guide boss. This is all you need to do, you don’t have to open the port itself as this is big enough to feed the 1 inch valve, even up to a 1.062 inch exhaust valve. 

Big valves 

Now fitting bigger valves is a good thing sometimes, but there is a few things to look out for putting bigger valves in the head, you can create a problem with shrouding. I see a lot of times people fit bigger valves in the small bore all but touching the walls of the chamber. Here’s where we have two dilemmas we can hack the chamber wall, grinding the chamber wall away, giving the valve a lot of room to breathe. This is good, but then we find we have very low compression ratio. This is why we need the help of a flow bench. Now flow benches are good, but you can make them tell lies. The bigger the holes you make the more and they flow, not good. You need velocity to fill the cylinder because you don’t have much time, now getting back to shrouding of the valves, not only do you have to clear the chamber but you have to get the valve to breathe around the cylinder wall. Here is a photo of the valves in the bore.

Looking up the bore at valve shrouding.

This one is when I was doing the project 68 mm article making sure the valves had plenty of room to breathe, inlet and exhaust.

Looking at valve shrouding with the head bolted to the block.

One other thing I do is flow the head and spray dye into the port from about 1 m away and just let the bench suck the dye into the port. Now this is very interesting as to what the air is doing at different lifts. It really changes how it enters the chamber and the cylinder.

Playing with dye at different lifts

Die testing at different lifts

The tube that I flow the head on is the same bore diameter as a mini cylinder and it’s clear so I can see what it is doing on the cylinder and it makes a mess as you can see from the photo of the chamber. You can see how the air swells right around the chamber these photos were taking at full lift and with all the pumps on on my flow bench that’s 400 CFM. So the air is moving very fast when using low lifts the dye is totally different, wetting a lot more of the chamber wall, also the different valve seat angles changes how the air leaves the seat and enters the chamber. This is why I have developed valve seat cutters for the mini. You can have a three angle seat job one is 45 deg, but the width and the shape of the other two can change the whole thing, I just love to play.

The standard 998 head will be reconditioned, but it will have a very good seat profile on the inlet and exhaust. Plus the compression will be raised to the same as the modified head. One thing to note is I will not skim the small bore heads any more than .080 thou, be it a road head or a race one, any more than this as you will start to have head gasket problems.

Building the bottom end and photo from the top.

Now for the bottom end and as you can see from the photo there is nothing special, a standard crank, balanced, same with the rods, resized and balanced. The main bearings are ACL Conrods are King bearings, both heavy duty. When assembling the bottom end use a good assembly lube on all the bearings, check the end float on the crank, use good lube on the cam and check the end float. I like to keep this tight as any movement back and forth will upset the ignition timing making it’s scatter. For the oil pump please do not use engine oil, you must use a very sticky assembly oil. The reason being if you have the engine on the engine stand and rolled it over to work on you will drain the oil out of the pump and it will not pick up oil on start up. You need to be able to get vacuum in the pump to be able to suck the oil out of the gearbox.

Rockers fitted to the Std head on the motor.
Rockers fitted to the modified head on the motor.

When fitting the cam followers into the block use a light oil as you want the followers to turn when turning the motor over by hand. When dialing in the cam I do not dial in off the lobe, the reason being you have all that lube on the cam and there is not enough load on the cam. I always dial it in off the valve cap with tappets set. This way you have all the pressure on the valve springs and all the slack is out of the chain which is much more accurate. The cam I made is on 106 lobe centers. I dial it in at 103 ATDC full lift as the chain will stretch a little after running the motor in and doing some testing, this will leave a little advance to help the torque down low. The lift at the valve was .326 thou but it did vary a little. 0.326 what is the maximum the lowest was 0.321 thou giving a rock a ratio of about 1.15.

Free Flow Maniflow exhaust.

On the Dyno

Let’s go to the Dyno, after setting the motor up on the Dyno we run it for 45 minutes. First for 20 minutes at 2500 to 2800 RPM with around 40 to 50 foot pounds of torque. Then we increased the RPM to 3500 to 4000 RPM with a little more torque. After that and while hot, I pull the head down and reset the tappets to .012 Thou with the timing set at 32° for initial start up. The SU is running a number six needle.

Putting Keith to work while I make a cuppa 🙂

We give the string a pull down low at 2000, 3000, 3500 and the air fuel ratio it was showing 17 to 1 up to 20 to 1. It was a little lean to say the least. I started to linish the needle down to a very thin needle and even cut 3/16″ off the bottom of the needle to no avail, it was still really lean. I then decided to put a .100 thou jet in the one and a half inch SU. From then on we could get some good readings. The part throttle readings were excellent, all up around 14 to 14.5 to 1. The needle we wound up with was a BCV and the jet down .07″ from flush. The reason we had to go to the .100 thou jet was that the head was working really well and pulling very hard on the jet. The .090 thou jet which could not supply enough fuel, even with the needle hanging out of the jet.

Next came the exhaust, we took off the free flow and put on a set of maniflow LCB medium bore. They lost just a little down low, same through the mid range and made 3 to 4 hp in the top end. An interesting point, It wanted 37° total timing. Have a look at the Dyno graph that’s very good torque and horsepower for a little 998+ .060 bore. 

We reset and try the standard head by removing the modified head. It was very interesting to see the swerl pattern on the top of the pistons.

All set up with the standard head and back to the free flow exhaust with the RC 40 set up. The standard head gave very good results. The only thing it had was a very good multi angle seat job. 

Dyno Sheet
Dyno Comparison Graph

Summing up we were very happy with the results and just goes to show that the little motors can be made to work really well without having to spend a lot of money, just spending time and money in the areas.

Ultimately kits will be available based on the results of this R&D program, featuring the specially develop cam for this project.

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