The shape and volume of the combustion chamber have a huge effect on the performance of all 2T engines. The so-called “clearance volume” affects the trapped compression ratio and thus not only performance but also how much effort it takes to kickstart.
The squish band affects the trapped end-gas and thus the propensity towards detonation as well as the squish velocity.
The squish clearance has a mechanical minimal limit on the order of 1% of stroke in order to prevent piston/head contact under worst-case conditions.
The blend radius affects the squish velocity. The blend radius is typically 3 to 8% of the bore diameter.
Squish velocity affects the rate of combustion. A high squish velocity produces more aggressive combustion, but also higher thermal losses.
Stock 280 head with blend radius of only 2mm
Left side 250 head, right side 280 head
Left side 280 head, right side 300 head
Standard versus high-compression head
The adjacent dyno curves come from the OSSA Special Products Catalog. It supposedly shows the power difference between the standard TR280i head and their “racing” head (sometimes called the Red Head). The head is described as having an 11.1:1 compression ratio and quoting OSSA, “provides a power and torque boost in the entire PRM range.” Clearly, the stated compression ratio is geometric - it is much too high to be trapped volume. But I am not sure I believe those curves. It is difficult to find dyno curves for any trials bike, but those are very flat power curves. They look much more like trials torque curves. OSSA was careful not to divulge RPM or power but, presumably, the graph's origin is zero speed and zero power. The assumption is that such graphs are scaled linearly (not logarithmic or produced by some trickery.)
TR280i standard head dimensional drawing
Although not what OSSA used, I prefer silicone for the inner O-ring. Nitrile (Buna-N) is fine for the outer O-ring.
All head O-rings have a 1.78mm (0.070") cross-section.
All engine displacements (250, 280, 300) use that same outer O-ring.
Outer: Parco P/N 0568-045 (4.0" ID, 4.125" OD, 1/16" CS) Nitrile. Just slightly short. With a tiny bit of stretching, it fits.
Inner O-rings are different due to different bore sizes.
Inner: Parco P/N 0568-042 (3.25" ID, 3.375" OD, 1/16" CS) Silicone. Fits 300 perfectly. Can be made to fit 250 and 280 by shortening a small amount and using Cyanoacrylate (Super Glue) to join the ends.
Because gravity is working against the process when reinstalling the head, I use a dab of silicone sealer in multiple spots to hold the O-rings in place. Grease is often used instead.