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For my '80 MHR900, the manual says piston to bore clearance should be 0.02 to 0.04mm new with a wear limit of 0.14mm. To us Americans that new clearance is 0.0008" to 0.0015". My machine shop guy thought this clearance was extremely tight for an air cooled motor. So do I.

What do you guys say? Is that really the correct spec? I found it here: http://www.ducatimeccanica.com/darmah_9 ... /index.htm Page36.

I'm trying to decide if I need to rebore and put in the 87mm Wiseco pistons or maybe just hone it and keep going. 25k miles. The shop will measure where I'm at currently.

If it needs new pistons, my plan was to first get them, have the shop accurately measure each, and cut the bores to match, given the clearances above.

Any help is much appreciated as always.

Hi Jim,

Yup, those are the correct tolerances as given in the MHR workshop manual, BUT it may be worthwhile remarking that they're for Ducati pistons (actually made by AE Borgo, I seem to recall), not Wisecos. So if you do decide to fit new ones, it would be best to check with Wiseco what tolerances they specify for their pistons.

P.s I quote:
"Wiseco “M Series” pistons are designed to run at a specific piston to cylinder clearance, and are manufactured under controlled conditions for a specific bore size. Finish hone the cylinder to achieve the recommended piston to cylinder clearance, which can be found on the label on the piston box."

HTH

Craig

Craig,
Thanks for the advice. That's exactly what I needed to know. I'll see how the measurements turn out and now will know how to handle the new pistons if it comes to that.

Cheers,
Jim in New York

I run mine at .003" and have had no problems. The Wisecos are forged and grow a bit more than the cast pistons.

I`ve been running Omega pistons in an old GTS with .003" clearance and they seem happy enough. I`d be inclined to agree with HailwoodJim`s machine shop guy that .0008-.0015" to be a little snug on an air cooled donk, particularly if you live in a warmer climate.

I would agree that if you're using pistons other than factory-style replacements, the close clearance needs to be evaluated. However, I've installed new oversized pistons with stock factory clearances, and all was well. The designers worked hard and spec'd the pistons to allow close clearances, with beneficial results.

If you're using factory-style parts the proper factory clearance works, and its what the bike came with. Do make sure the guy boring/honing the cylinders knows what he's doing - achieving a properly finished cylinder within a 7/10ths bore tolerance isn't child's play.