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Hi Gang, Another quick question for you, I am just fitting 8 to 1 compression ratio Omega pistons to my twin, and want to know how much clearance is required between the valves and piston cut outs. cheers
And be particularly careful of the area just outside the cut out towards the outside of the piston. Years ago it was this area of the piston which used to foul the valve head as the cut outs on top of the piston did not have a large enough diameter.
Something else to watch out for here are the later die cast muffs!!
They tend to compress on one side than on the other.
Not a problem if you bore and hone the cylyder using the top lip of the liner as a datum, but in a moment of (not uncommen) wisdom, i got my guy to use the bottom of the muff as a datum thinking this would bring the bore back to 90 degrees with the crankcase.
This it did, but moved the axis of the bore within the liner and caused clearence problems between the (in my case) exhaust valve and piston cut out.
The change in bore axis was so large that you could see it on the taper at the bottom of the inside of the liner. (the taper was smaller on one side than on the other)
I told your Dad years ago, that diecast muffs are to be thrown away, and never put anywhere near your bike.
That assumes that the deck is square with the mainshaft, there are one or two like that !Regarding the correct datum from which to bore a cylinder. At one time it had not occured to me however I believe that I read in MPH maybe 4 years ago that the datum should be the base flange of the cylinder as this then makes the axis of the cylinder perpendicular to the crankcase mouth / deck. Armed with this information I asked my re-borer which cylinder end he intended to bore from. The look was withering and he proceeded to tell me that he had been re-boring for the last 50,000 years; using the base as a datum so in conclusion I don't think using top lip of the liner is correct.