Home of  my Yamaha RD400  Drag racer.



Ora et Labora



Now we can start with cutting the Head. This is what we intend to do to the heads.
1…We will skim the head to take the combustion chamber volume down to 20cc. It was 26cc original
2…Recut the squish band.
3…Also we will cut an O-ring groove in the head. That way we can eliminate the head gasket.

So this is how we skimmed the head.
First we make up a mandrel that has a thread cut on one end, that screws into the spark plug hole. The other end fits into the lathe chuck. So now we can just use a standard facing tool to cut the head surface down the required amount. Cannot tell you how much to take off because I forgot to write it down. But after cutting it down we then checked the combustion chamber volume and it measured 20cc.You will have to decide how many cc you want to have.
To cut the o-ring groove we ground up a tool from high speed steel. Here is a very bad pic of the O-ring tool and the Mandrel that we used.

When cutting the o-ring groove just remember too have it about half way between the stud holes and the liner.

Now we recut the squish band.
I wanted the squish band to be about 11mm wide and have a clearance of 1mm. Now there are heaps of theory's about squish band size and clearance. But this motor is just being put together to get the whole project moving. So we might try some different squish band sizes and clearances. We will use these techniques to cut the squish band to what ever sizes we want to try.
Now how deep and what angle do you cut. You must first find your deck height and add or subtract that from your required clearance.

The angle, of the squish band, was about 14 degrees from memory…just measure you piston and take an average.

The Barrel
Once again you will have to sort out what porting spec's you want on your barrels. The spec's that we are using are for Drag racing only and so far unproven. But this will show you how we made our mod's.
We opened to exhaust port up from 38mm chord to 43.6mm chord. Chord width is measured across the port not around the cylinder.
Also we raised the exhaust port from 35mm from TDC to 29.7mm from TDC. That changed the exhaust port duration from 181 degrees as standard to 200 degrees .We used MOTA to test all of our porting changes.
I highly recommend MOTA as a great engine simulator and a must have if you are going to get into porting your own motors
We used a piece of stiff card and drew the shape of the
Exhaust port, in full size, on it. Then coated the area around the port with engineers blue. Felt pen works just as well. Then using a scriber you can scribe around your template and once you remove it you will have a "cut to" line scribed inside your barrel.
Then it's out with your favorite die grinder and into it.