FoaTech wrote: That clearance for a forged piston is right, .003 was too tight. .004 will be just fine. Ferrari was covering an engine rebuild years ago and delayed me in tearing it down. Broken valve had cracked a head and coolant was in every cylinder. by the time they agreed and let me take it apart, the coolant had killed every sleeve. It doesn't take long.
Way back in my H-D racing days I set my bike up at .004 (nearly the same bore as this engine) as the manufacturer suggested. That's the way I ran it hillclimbing (on dirt, not roadracing) but when I went roadracing with the same 4 it blew smoke at the end of the straights when I snapped the throttle shut and I was having a terrible time with air pressure in the oil system. My H-D buddy asked what I'd done with the piston clearance and was horrified by the 4 number....no, no, no, 1-1.5. I was like really? That was magic, no more oil issue.
Since then I've been scared of going too loo loose. Thinking it through I guess the H-D cylinders get a whole lot hotter than a water cooler cylinder so it will expand more with the piston I guess and can't really be compared to a water cooled engine.....so 4 it is with the cylinder stone cold.
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