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Need help with piston choice...

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I'm helping my friend do the top-end on his 05 YZ 250. As far as we know, the motor has never been rebuilt. Do we just order the standard bore wiseco (66.4 mm)? Or is it like a KTM where you normally go with the #2 piston that is slightly oversize? Thanks.

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Look on your cylinder and you will see a letter (should be in front of the carb, but its on the cylinder), It should say A.B.C, or D. That is the size of the piston from the factory.

But i went by the letter that was on my bike but i alway measure my cylinder to make sure i get the correct sized piston.

And as for pistons i ran pro-x they are really good pistons in my opinion. I never had a problem. Wiseco and vertex, etc are alo good as well.

thanks man and god bless

Alex

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Thanks. The cylinder does have an "A" on it, and I see that Pro X has A,B,C,and D sizes. We want to use a wiseco however. So would it just be the straight 66.4 mm piston? If I'm reading their website correctly, the next size is a 67mm, which must be an overbore.

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You're right on the money.

Wiseco doesn't offer any tolerence sizes for the OEM YZ cylinders. Just check that your bore is within the service limit in the service manual and use the regular 66.4 Wiseco.

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yeah plus theres not much difference between those oem sizes

A = 66.353mm (+-)0.003mm

B = 66.357mm (+-)0.003mm

C = 66.361mm (+-)0.003mm

D = 66.365mm (+-)0.003mm

its increments are about .004mm which is roughly 0.000157inches, i think wisecos would be comparable to a A or B

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I recall reading somewhere that D is the standard size for a worn in cylinder, which is any cylinder getting a replacement piston.

Wiseco does not have sizes, so their standard bore piston would be equivalent to a C.

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if you don't wanna bother with the letters on the clyinders , go with wiesco nxt size bigger pistons, wiseco are one of the most reliable pistons, in my opinion better than pro-x

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Well here the debate could begin. It shouldn't because the OP said he wants to use a Wiseco. That being said, I think the Wiseco is a single ring piston for the 250 and their could be a slight power loss. But most importantly, MAKE SURE YOU WARM THE BIKE UP VERY GOOD. The forgd pistons need to be warmed up really good or could cold sieze. Pro-x are made by Wiseco, ot at least they sell them. It is suposed to be the OEM cast equivelent. I usually go with the OEM or Pro-X.

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Wisecos and cold seizures went away back in 1980. Always warm up a motorcycle, it makes good sense. But the wiseco forged is possibly more temp stable than a cast piston...no lie.

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I didn't know that. Some things do change I guess. Well then there is now reason that I should stay away from them.

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They are good now, and have been for a long time. You woudl be amazed at how many motorcycles come stock with forged pistons. Street, dirt, you name it. Never any special advise on warm up or break in. But when it comes to the Wiseco forged piston...... look out! Makes no sense.

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Well here the debate could begin. It shouldn't because the OP said he wants to use a Wiseco. That being said, I think the Wiseco is a single ring piston for the 250 and their could be a slight power loss. But most importantly, MAKE SURE YOU WARM THE BIKE UP VERY GOOD. The forgd pistons need to be warmed up really good or could cold sieze. Pro-x are made by Wiseco, ot at least they sell them. It is suposed to be the OEM cast equivelent. I usually go with the OEM or Pro-X.

not to offend you but, wiseco is a piston manufacturer and only makes wiseco pistons. they do not make PROX.

going from double ring to single ring doesnt make power loss. it actually is the opposite.

and to the OP.

if its an A then go with an A by wiseco/forged pistons. wiseco doesnt supply 0.01mm in increments. its one size fits all until you start over boring the cylidner.

if cast were desire then B size is more appropriate since the bike is used and never been rebuilt since new. if the bike was used and had alot of hours meaning alot of piston changes, and was to use cast, then C to D size on a USED A size cylinder would be prefered.

nikasil doesnt wear that much/faster than most ppl think. most ppl think that they need to hone the cylinder or get it replated whenever its time to change a piston. not all are top level racers.

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Hmmm which bikes come stock with forged pistons? I have never seen one so I'm just curious.

Yamaha four strokes use a "liquid forging"

Most, if not all of the current 250f and 450f bikes use forged pistons. Kawasaki's 2012 zx14 is running forged pistons. The high-end four stroke pistons are forged.

WR, this next parts is not towards you.

I swear, if I have to read the "warm it up good" comment again, I'll be ill. Somehow if the bike comes with a forged piston, no special warm up is needed. But if one is added later, look out. Its silly. An engine shoudl always be warmed up correctly before riding as it should extend the life of the motor.

If you go to the car forums you'll see the same argument..over and over again.

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My 426 has a cast piston or it at least looks like one. A CRF450 I rebuilt for a friend looked cast also.

Is this OE forging made to look cast?

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not to offend you but, wiseco is a piston manufacturer and only makes wiseco pistons. they do not make PROX.

going from double ring to single ring doesnt make power loss. it actually is the opposite.

and to the OP.

if its an A then go with an A by wiseco/forged pistons. wiseco doesnt supply 0.01mm in increments. its one size fits all until you start over boring the cylidner.

if cast were desire then B size is more appropriate since the bike is used and never been rebuilt since new. if the bike was used and had alot of hours meaning alot of piston changes, and was to use cast, then C to D size on a USED A size cylinder would be prefered.

nikasil doesnt wear that much/faster than most ppl think. most ppl think that they need to hone the cylinder or get it replated whenever its time to change a piston. not all are top level racers.

Perhaps you know better but I have read over and over that the Wiseco piston makes slightly less horsepower. And as far as Wiseco not making the ProX piston, perhaps you are correct as well but when you bring up the web site for Wiseco, you get the Wiseco products and the ProX products. I understand that it doesn't mean that they make them but they at least supply them. Did I state they made them or did I say I think they made them? I can't remember. Either way, if you are correct then I stand corrected.: banana:

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i really dont know if wiseco produce less power but i bet any pro wouldnt feel it anyways. could be their clutches are worn or they put metal plates instead of aluminium and therefore would be less snappy.

wiseco was a few grams heavier than my ART piston on my cr80 when i weighted them.

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Perhaps you know better but I have read over and over that the Wiseco piston makes slightly less horsepower. And as far as Wiseco not making the ProX piston, perhaps you are correct as well but when you bring up the web site for Wiseco, you get the Wiseco products and the ProX products. I understand that it doesn't mean that they make them but they at least supply them. Did I state they made them or did I say I think they made them? I can't remember. Either way, if you are correct then I stand corrected.: banana:

Pro-x is a sister company to Wiseco and they resell OEM "ART" pistons in varying sizes, often in much smaller increments and many more sizes than what OEM offers. Wiseco does not produce cast pistons.

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