In general terms, what is the risk of running too much, vs too little bore : piston clearance. Lets say normal OEM limits of a given yamaha are 55.95mm piston (smallest normal) and max bore wear is 56.05. Thats a .1mm max allowable difference. Now lets say I have some nice tastey light pistons that are 59.84mm, and by some long chain of events, I end up with a 60.05 mm bore. That's a .19mm difference. Thats roughly twice OEM. Am I really in the danger zone? Thanks, Rich
the recommended clearance is usually more with aftermarket forged pistons than with oem cast pistons, so you might be ok if your trick pistons are forged
.19mm is .0075". Wiseco recommends a clearance of .0025 on their oversize pistons. So, you would be asking the rings to do a lot of work, and the ring lands on the pistons might not be too happy about that. Are these 1mm pistons for an FZR, by chance? If so, that is a lot of slop.
Way out of spec for a cast piston. Out of spec for a forged as well. Clearance is based on diameter and metalurgy. Cast is around .002 inches for most cycle pistons. Almost never under .002 and no more than .003 is pretty standard. The rings are not the problem, although they will not be happy-happy. Oil control on the down-stroke will be poor. It's the skirt rocking/slapping around and not allowing heat transfer to the cyl wall that will cause crunchy bits to accumulate in the oil pan.
They are +4 for an FZR400 The JE charts last night specified .0025" Min clearance. BUT, they also noted up to .003 additional clearance for endurance. I ran the motor through a couple heat cycles. On startup, I could hear piston slap. It went away, after reaching operating temp. Well, one practice to make sure she does not blow, and two sprints I suppose. Then back to the drawing board. Thanks for the input. Rich
You could re-sleeve and re-bore to the correct size, if you do it BEFORE the pistons and rings are trashed from slapping around.
Generally for a racing engine you would use the higher acceptable clearance of ring gap to allow a little lower friction (though less life). Running a small piston makes it more dangerous to allow looser rings no?
Ring gap on the top ring (cold piston) is adjusted so that almost total elimination of the gap occurs when at full op-temp and full load. Second ring (oil scraper) gap has now changed to a larger gap than in previous years to allow blow-by to help push the oil film off the cyl wall, which lowers internal friction just a tad. Running too little top ring gap is catastrophic as the ends will buttweld and rip the top off the piston. Stock rings are "pre-gapped" and always leave a little room for tolerance variations, but if you CAN buy "file to fit" rings and take the time to do it right, there is some power to be had.
I may be reading that quote wrong, but the .003 was NOT supposed to be in addition to the .0025, it was INSTEAD of...
UGH I read it as in addition. Regardless, I got my hands on an alternate set of pistons and cylinders that I can run. Just not the biggies I wanted to run. No sense in destroying good pistons for the sake of one weekend. I guess TWF2. Rich
I have crqate after crate, just getting low on the good stuff like cranks, con rods, and pistons. Thanks for the offer though. Rich
