Wiseco piston clearance

I am sending my YZ426 cylinder off to Powerseal for a replate and to fit bore to piston, I just picked up a Wiseco high comp piston and on the box it recommends .003" piston to wall clearance. Is this much clearance really neccessary? Has anyone tried running more like .0015"? Powerseal said they could do .0015" but would not warranty against piston seizure.

That's Wiseco, and that's one reason they make more noise than others.  If it were me, as I already told you, I'd use another piston, CP, Vertex, etc.

Yeah, just wondering if anyone has tried running a tighter clearance than recommended wit a wiseco. I wouldn't have bought it had I realized this but I can't return it now and don't really want to toss a new $150 piston in the trash. I ran a Wiseco hi comp in my CRF450r with excellent results, followed by a JE which was just as well but the piston itself was heavier and made noticeable engine vivration.

Yeah, just wondering if anyone has tried running a tighter clearance than recommended wit a wiseco. I wouldn't have bought it had I realized this but I can't return it now and don't really want to toss a new $150 piston in the trash. I ran a Wiseco hi comp in my CRF450r with excellent results, followed by a JE which was just as well but the piston itself was heavier and made noticeable engine vivration.

Your best opportunity would be to call Weisco and tell them who is doing your plating and how tight can you go. .003 seems on the loose side.

For a contemporary engine with a plated aluminum bore and a modern forged piston, .003" IS loose. 

.003 seems on the very loose to worn out side. I have run J. E. pistons under .001 but a simple call with Weisco will get you squared away.

I called Wiseco this afternoon and explained my situation. The first tech I talked to immediately questoned what piston I had as he agreed that .003" was excessive. I gave him the part number and he put me on hold to get more info from the higher-ups. He came back and said right away not to run any less than .003" clearance as the piston would immediately expand once up to temp. And I don't remember his exact words, but once up to temp, that there would only be enough room left to provide the neccesary clearance between the piston and cylinder bore to provide a bearing surface of oil between the two, and that any less would not provide sufficient lubricity.

So, I went ahead and shipped the pair to Powerseal today, specifying to fit to the .003" per Wiseco's instructions.

I figure at the very least, if I had issues, since the Wiseco mic's at 3.737, .003 clearance would put the bore at 3.740, which is almost dead nuts (within .00015) of the stock 95mm bore. So ideally I could throw in diff piston designed for the stock bore and everything is peachy.

I will post results when assembled, though it will be some time as I am rebuilding the bottom end as well and throwing in a WR tranny. I scored this bike for $550, and get this, it mysteriously came with a full NY MC title and 17" DID supermoto wheels. Gonna be fun!

Figured I'de throw in a pic of my work in progress to liven up the thread, I know everyone likes pictures!

20150603_224307.jpg

And yes this is in the middle of my living room, ahhh the single life!

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