der_nanno said:
Good philosophy on a race engine. From memory (I'm doomed now!) 20cc volume on this engine sounds catastrophically small (apologies in advance, I'm not calculating, just feebly remembering - needs 26cc - 23cc (for the daring) or so?).
There is great value in considering the two things as disparate tasks. Without making a big engineering discussion, consider setting your squish as described. The ideal thing to do would be to first, ignoring everything else, machine the gasket surface of the head(s) to fit the cylinder tops with no gasket. That means cutting a step in the head that will use the stock copper gasket recess to center the head. This will screw up both the squish and combustion chamber volume (compression ratio), but ignore that for now. Add a groove for an o-ring. Measure the squish gap, and re-machine accordingly to get a bit less than 1 mm, or a bit more than .038". Your squish is now done. For part 2, decide the compression ratio you want (6.5/1 - 7.0/1?) and calculate that volume. Measure the volume you have now (as previously described) and then carve out as much more as you need from the chamber to get it. If you leave the depth of the spark plug threads constant and keep a more or less hemispherical chamber shape (in other words, obtain the needed chamber volume by increasing the chamber diameter rather than increasing the depth) you will reduce the squish band width to a reasonably appropriate size and end up with some very good performing streetable heads that don't leak. You can do a much more detailed analysis, but you will get very, very close to the same thing. Keep your thoughts on the important goals. 1) You need to have reliable sealing of the heads. This is not a problem now, but it WILL be if you get your engine running well - hence the head step and o-ring. 2) the squish distance is very important. Fix it! There is a lot of complicated science and opinion(!) here, keep it simple, get it in the range. The distance between the piston and head at the perimeter of the bore is more important than the width of the squish band or the taper between the piston crown and the band. (not that hose things are UN-important). 3) Compression ratio (when running - pipes greatly impact this) is life threatening for your engine. Get the other things dealt with, and select a ratio you can work with and implement it. Be conservative on a street motor. You can always increase it.
None of this is hard, but it will involve a little cut-and-try. Turning the heads to step them and o-ring seal them will very adversely increase the compression ratio. Bringing the squish clearance in to the the right number will make this problem worse. (there is a definite variation in heads, so consider any statement accordingly). Do those two things alone, and you will have a grenade (usually). However, it is a simple matter to remove enough material from the chamber to bring it into a reasonable volume. Just takes a willingness to bolt it all together, make some measurements, and act accordingly (wash, rinse, repeat as they say). Once it is done, it is DONE! (and you will never buy (or anneal)) another head gasket again! Your bike will be fast and reliable. I promise.
