RM 250 - Pro X vs. Wiseco Piston Size

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1/18/2019 12:46 PM

I have 30 hours on a sleeved 2005 RM 250 (cast iron sleeve, not nickasil). When I had the sleeve bored it was bored to a "standard" sized Wiseco piston. I came to find out when it was time to put in another piston that a Wiseco "standard" piston size is 66.40, but I bought a Pro-X "standard" piston which I discovered is 66.35. Any ideas why Pro X would make their standard size piston smaller? Is it safe to run? The clearances seem ok, but I am always leery of running a piston that is too loose in the cylinder. I did not know if this was related to the Wiseco being forged versus cast. Anyone else run across this? Thanks for the help.

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1/18/2019 1:29 PM
Edited Date/Time: 1/18/2019 1:38 PM

I believe wiseco advertises bore size not piston size... also i believe the pro-x 66.35mm is the (A) Piston depending on piston to wall clearance in a used bore you may need a (B.) or (C) piston...

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1/18/2019 1:59 PM

Paul_Pitzonka wrote:

I believe wiseco advertises bore size not piston size... also i believe the pro-x 66.35mm is the (A) Piston depending on piston to wall clearance in a used bore you may need a (B.) or (C) piston...

Thanks Paul. Your right, Wiseco only makes 1 size, they don't do the A, B, C thing. So I assumed the "standard" Pro X piston would be the same size as the Wiseco. The Wiseco piston I ran it in looked great, so I assumed little wear in the cylinder, honed it up and ordered another "standard" piston. Yes, the standard Pro X piston does have an A on it.

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