Project 68mm – Part 1

Before reading this, I suggest the reader go back and read Project Small Bore Pt1, Pt2, Pt3 as lot of things in that project are relevant to this project.

The beginning of the process of building the 68mm project motor starts with boring the block. Looking at the block from cylinders 1 to 4 (right to left) or water pump to oil pump, the block needs to be offset bored

no1 cylinder needs to be offset towards the water pump by 0.025″

no2 cylinder needs to be offset towards the oil pump by 0.025″

no3 cylinder needs to be offset towards the water pump by 0.025″

no4 cylinder needs to be offset towards the oil pump by 0.015″

The reason for the no4 cylinder having a smaller offset is that you can run the risk of breaking through to the oil way at that end of the block. If this happens, it can be fixed by sleeving the oil way.

Once the block is bored, before honing, chamfer the bottom of the bores as the pistons will come out the bottoms of the bores at BDC and the bottom of bores, if not chamfered, become razor sharp and can shave the bottom of the piston skirt away.

When boring a motor this size, it is necessary to use a 1275 head gasket. Some people have put these straight on without any modification to the block and head. But the head gasket firing ring only just covers the water holes at the back of the block and could leak water into the cylinders. IMG_0005On cylinders one and four of the manifold side of the block, these have to be plugged and redrilled to match the head gasket. On the spark plug side of the block, cylinder four needs to be blocked off, but not redrilled.  IMG_0007
Another modification I like to make to the block is modifying the oil way behind the oil pump. This helps take the load off the pump and gives much better oil flow through to the block. I also do the one from the oil filter into the block.

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This is not something you have to do, but every little thing helps. Once this is all done, honing can be completed and the block cleaned and prepared for mock up.

Main Bearing Bolts

In both the small and the 1275 motors, but not the Cooper “S” motors, they used two types of main bearing bolts. One is just like any other type of bolt, but the other is quite special. If you have a look under the head of the bolts, one has a washer face like most bolts, but the other one is quite different. Under the head of the castleated one, you will notice that it is undercut where the shank meets the head. There is a reason for this, when you tension the bolt down, it pulls down on the outer edge of the bolt. You may ask so what? Now if you use the STD bolt, have a look at how it pulls down on the bearing cap, you will notice that it does not have very much bearing area because of the small washer face and the hole in the cap being a clearance hole. It tends to break the edge of the bearing cap away and looses tension. This however can be overcome by using an ARP 7/16″ high tensile washers under the head of the bolt. I have seen these STD bolts used in quick road motors and have gotten away with it. I don’t like doing this, for the sake of a few washers you can make it right. However, by using the castleated bolts, you don’t have this problem.IMG_0022IMG_0023


The crankshaft has been wedged and balanced as this helps with the harmonics of the motor.

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One thing that does help 1100cc cranks is to have all the radii shot peened. This helps to prevent cracking around the radius of the pins and mains.


Con Rods

The con rods have been checked for straightness and bore size


This is the jig for straightening rods. It is very simple and not hard to make, but it is very accurate. From the centre of the rod to where the dial indicator is, is about 3″. At this point, it magnifies the bend in the rod, so you can get the rod straightened very accurately.

I have also machined .150″ from the sides of the rods and balanced them, just to get rid of a little weight. My preference is to use 3/8″ high tensile cap screws or ARP bolts, rather than cheap bolts made from rubbish steel.



The camshaft I am starting with is the new 266 that I have designed. It has .266 duration. The duration can be altered by different tappet settings, because of the opening ramp I have used, you can set the tappets from .012 – .015″ which effect the seat duration of the cam.
The lift is 0.270″ on both inlet and exhaust, But I am running more duration on the exhaust than the inlet. The rate of acceleration is a lot greater that your normal 266 type grinds.

Another cam we shall try is my new 274 grind, which has .274 duration on the inlet and .270 on the exhaust. It has 0.312″ lift on both inlet and exhaust and also has a faster acceleration. This one may need some good springs to make it work well.

When a new cam goes in, it is important to use new cam followers or have the old ones reground. When I fit a new cam, I always regrind my followers weather they are new or old as a lot of followers do no have the correct radius. (See cam followers) The radius on the followers can vary from 34in radius up to 76in radius. A lot of followers today have very little radius and some are just completely flat.


Now that we have all the components for the short motor, assembly can be completed.

Cylinder Head 

The first head I am going to use is an 12G202 head with 1.260″ inlet and 1.070″ exhaust.

As per the block, the water holes in the head need to be modified to suit the 1275 head gasket.

On the manifold side of the head, the number one and four holes at the ends of the head need to be blocked and redrilled to suit. Whilst on the sparkplug side, the number one and four will need to be blocked off and not redrilled.




The choke area I am using under the valve is very small at 80% of the valve size. I am trying to get more torque into the little motor. The exhaust has an unleaded insert fitted in it. It’s running a full radius seat with a big top cut. In regards to the chamber, don’t try to relieve the exhaust valve to much as a slightly smaller area around the exhaust valve will help funnel the exhaust gas towards the valve, rather than have the gas running around the chamber trying to find a way out.


The chambers have been modified to suite the valves and to reduce compression as flat top pistons have been used. What I am going to do is use a STD inlet port and a STD exhaust port, then take some readings on the dyno. I’ll then take the head off and modify the inlet ports and redyno. This will give us an idea of what modifications make to the performance of the engine.

To fit the exhaust seats I use a purpose made cuter to cut the head. The insert is generally 0.005″ to 0.007″ bigger than the hole in the head, depending on the seat or the type of head being used. I use a Sunnen seat machine to cut all my seats and fitting inserts. There is a range of hardness in the inserts with the hardest of all being the ones used in LPG powered cars. They wear extremely well, but can be very hard on carbide cutters when cutting  the seats.


The rockers we are going to test with are the OLD type Keith Dodd 1.5s

Now that some of these rockers are getting older and some may need to be refaced. Not a lot of people have the jig to regrind them anymore, but they are still around. When I bought my valve cutting machine, it was optioned with this extra feature, so I bought it. This machine was the first one they had ever sold with this option, as not many cars use rockers as used in older cars like the mini do.IMG_0025

On thing to make sure of is the setting up of the rockers. When rebuilding a STD motor or building a performance motor, you deck the block, face the head, grind the cam and cam followers. They all affect the angle the rockers work across the tip of the valve. If it is not setup up correctly to work at the correct angle, you will be wearing out valve guides very quickly. So lets look at setting them up correctly. If you have a valve lift of lets say 0.400″, at half lift 0.200″, the centre of the pad would be in the centre of the valve. To achieve this, you may have to pack the rockers up or maybe machine down the bottom of the posts.

Now the head and rockers are ready, they can be added to the short motor to complete the long motor. The long motor was added to a gearbox and clutch housing ready to run on the dyno.



The extractors I am going to use on this motor are the maniflow medium bore as people can relate to these extractors and are easy to get hold of, and a lot cheaper than mine….


The Carburettors that will be run on the motor will be a single 1 3/4″ SU on a Mini Spares manifold. The other will be new toy, a 44mm IDF downdraft weber on my new manifold.



The dissy used will be a STD 29d with vacuum advance.


The long motor was set to a gearbox and clutch housing and readied for the dyno.

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Again, the reader should go back and read the Small Bore Project articles, as they will go into more detail as to what else was done through this build.

In the next article, we will run it up on the dyno, in it first configuration to see where this setup is at.

See Part 2 here