Yamaha OEM Piston (A) vs Piston (B)?

[YZThumper]

A while back I was able to pick up a brand new OEM Yamaha Piston on eBay for $19. It was a pretty good deal, considering this would normally run about $56 from a dealer. The piston is for my 1994 YZ 250. I was looking at an online parts fiche and found that Yamaha made several versions of this piston differentiated by a letter on the top of the piston that ranged from A to D. After looking at the top of the piston I bought from eBay, I noticed my piston has the letter B on the top. Can anyone tell me what these different letters indicate?

 

[Durt Cycler]

Different bore size.

 

[RJH]

Years ago a number was stamped on the OD of the liner to determine stock clearance. These days the barrel comes in 4 sizes, for example on the 125 – A is red max 53.960; B is orange max 53.964. C is green max 53.968 and D is purple 53.972. The codes are stamped on the barrel on piston top.

 

[yzeater]

You should measure your cylinder to see if the one you have will fit

 

[YZThumper]

O.K., now I'm a little confused. The cylinder on this model is made of Nikasil (sp?) and I didn't think you could bore that type of cylinder. My cylinder has never needed any adjustments, and as far as I know is within spec for stock from the factory. Did I screw myself by buying a piston that will be too large to use for my cylinder?

 

[yzeater]

You can't bore your cylinder, but it might wear a little. And as for screwing yourself, it's only $19

 

[EricGorr]

The cylinder is stamped with a letter designation on the upper backside. A is the smallest and D is the largest. Its hard to say what size your cylinder is, considering wear. The letter designations are an indication of the bore size. The factory service manual provides size ranges for the letter designations. The piston should work in your cylinder without any trauma.

 

[Durt Cycler]

Hey Mr. Gorr I just have one question...Why do the bike manufactures make different size cylinders and pistons? Is there a purpose to this besides they being like .05mm bigger then each other? I couldn't see a performance gain by this method.

 

[Durt Cycler]

Originally posted by yzeater
You can't bore your cylinder, but it might wear a little. And as for screwing yourself, it's only $19

As far as only losing $19 you need to consider if the piston doesn't fit to spec. then their might be excessive piston slap and cause a blown topend. That could trash the piston, cylinder, cylinder head, powervavles, rod, and crank very easily. That $19 could turn into $1,900 very quickly :eek: :scream: .

 

[H/Sracer500]

Durt, the reason for different sizes are manufacturing tolerances. When a cylinder comes out, they measure it then stamp it so they know what size it is, they then match it to a corresponding piston. I have the same bike as you and when the time comes to change the piston, I measure everything and find the best (see tightest without beeing too tight) piston to match. But I wouldn't be scared to put that B piston in as the differences are very minimal.