...Oh, and another question about another option I was considering.....
Cool heads. Those seem kinda universal. How big are the chambers' diameter cut? How does one used the same head like this for different bore sizes?
When I make domes or machine combustion chambers I do not like to make the step in the squish band larger than the bore. Making the squish OD larger than the bore can cause detonation issues on highly developed engines and or slightly miss-tuned engines. Unfortunately many shops or dome manufactures will cut the squish OD in the head for the last aftermarket oversize piston that is available for that model.
Honda usually machines the squish OD to 66.5 mm on the 250Rs but uses so much piston to head clearance the piston will never hit the head even when the head is not centered over the bore. Often aftermarket pistons that are larger than the gasket will hit the gasket and then the gasket will hit the head
Most recreational engines do not have dowels or means to exactly center the head over the bore but use the head studs to approximate centering the head over the bore. High performance engines need close piston to head clearances to promote good combustion, prevent detonation and keep the piston crown cool. Depending upon the engine package, the RPM range the engine is developed to operate and the condition of the lower end, we typically set up the piston to head clearances on the 250Rs in the .025" to .050" range.
Using the studs to center the head over the bore will usually allow a 0.5 to 1.0mm of uncertainty, requiring the squish OD to be .5 to 1.0 mm larger than the bore to prevent the piston from hitting any part of the head that is hanging out into the bore.
