I just installed a new weisco in a 03 yzf450 and the piston knocked loudly on fire up, I took the piston back out and measured the piston to cyl clearance, which I should have done to begin with,and it was at .004" which the maximum wear limit in tne yamaha manuel, I called weisco and they wanted the piston and cylinder to measure themselves. I already put the yamaha pston back in so I could ride this weekend., so be sure and check new pistons befor you insall them and save a headache.

thats not suprising to me, I put a set in an older FZR1000 and it was plaged with blow-by

The pistons were to small as well

The Wiseco YZF pistons are made to run .003-4" clearance. How did you measure? We run our GNC racebikes with Wisecos at .004". Tdub

Thanks for the heads up. I'm getting ready to put a new piston in.

I used feeler gauges, I do not have a cylinder bore gauge. my yamaha piston I took out and put back in measured at .025". The weisco knocked right on the frist start up, I put the yamaha piston back in and everything was back to normal. I used weisco in two-strokes and have never had a problem with them. My cylinder is in good shape and within spec. I am at a lost what to do. I am not happy about spending $160.00 on a piston I cannot use. I understand about weisco wanting to see the piston and cylinder, but the racing season is here and of course I have to pay to ship it back. I am having the owner of the shop where I bought the piston from call weisco with the meas. that he verified with me, hopefully somethig can be worked out.

I meant .0025" on the yamaha piston.

Mine knocks too. Wiseco acted like I had not a clue about building a motor. I will never buy wiseco again. Mine knocks alot too, I rode it for the first time the other day and much to my surprise it ran fine. I know I will buy a Yamaha OEM piston next time.

Guys you are fretting over nothing! The Wiseco was meant to run with more clearance. The stocker is a cast piston and the Wiseco is Forged. Jeesh, don't make something out of nothing! Like I said before, the Wiseco is meant to run with .003-4" clearance. :):)

The knock is so loud that people comment on it, I can believe that weisco runs their pistons at.003" to .004" but I do not believe that that knock can be good for the longevity on the clyinder, it does only knock at idle and does run fine, but that knock has me worried that the piston will hammer itself apart. Anyone want to buy a piston with about an hour and ahalf on it, make an offer.

Jeesh :)

FFR, do your pistons Knock. :)

If it were me, I would take a real good look at the top of that piston and the deck of the head. You may be surprised to find that you have some contact there (is it the high compression piston?). I really doubt you'll hear an audible knock with a piston @ .004 versus .0015/.003. Especially after it warms up (that piston will seek out the cylinder wall). But you will hear the contact of a piston contacting the head (even lightly) or if you have the high compression piston and the air/fuel/spark is incorrect and it's detonating. I'd look beyond the piston a little bit further. Just my opinion :) amd some experience, "been there done that". :)

Also check the clearance between the pin boss and crank at BDC. I have had issues in the past with as little as .008" clearance(not just with Wiseco, but others as well). The stock piston has .035" clearance. It can be checked thru the case window with the clutch cover removed. Tdub

Thanks FFR and YZDAD1 for the tips, if the piston is hittiing the head what do you suggest, two base gaskets? or coat the head gasket, the knock is real loud and that sounds like it could be the case, I will check the piston tonight.

You can try or do a couple of things. Yes you could "double up" the base gasket, or better yet get an aluminum shim style base gasket of the correct/needed thickness. Are you running the OEM Yamaha gaskets or Cometic? I've seen a difference with them "squishing". Or, you can trim the lid of the piston the necessary amount and run the base gasket you have (that's what I do). If it's only knocking at idele and runs well, I'd be surprised if you don't find light contact between the head and piston. By the way, you may find it only hits on a side (left or right) which could warrant an inspection of crankcase to cylinder and crankshaft all being "true to the world". Good luck and keep looking, I'm pretty confident with Wiseco pistons (JEs Too!). :)

I have a question about this also, I bought a 96.5mm 440 for my bike, got the cylinder used but it is in excellent shape and though the piston had quite a bit of rock front to back inside the cylinder, a local shop said it is just because of the pancake style and I brought it to the yamaha dealer they measuered it and said it was perfect, I still didn't believe them and brought it back again they still said it was good to go (before they dropped the piston and f#*$ it all up and had to overnight me a new one) I put the engine together and it seems to run well but still scares me also people have said there are alot of cracked pistons from these kits any input guys

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