Small Bore Cylinder Heads

Three Head Inlets
Click to enlarge

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