Yamaha cylinders A, B, C, D

[MRKRacing125]

Yes, it is going to matter which letter you have on the back of your cylinder and you have to match it up with that letter of piston. The D cylinder is the biggest cylinder and the A is the smallest. As for aftermarket piston goes, they all should have letters.... but I normally go OEM, because I hear you dont find that much of a difference with aftermarket piston's unless you have motor modifications, or race gas, so on so on

 

[JANDERSON]

aftermarket

What's funny is that Wiseco, who makes a great piston, only has a one-size-fits-all. I installed one in my sons 97 YZ125 and the bike runs great, no problem. I asked Wiseco about the sizing with no good answer from them. Of course Yamaha defaults to OEM. I have the A cylinder and again, NP with the Wiseco. My 2 centavos...

 

[spanky250]

The A, B, C, and D sizes are used for the manufacturing plant, to allow for production tolerances. When the parts are ready for assembly, they measure each cylinder and stamp it A, B, C, or D. They do the same for each piston. During assembly, they match an A piston to an A cylinder, a B to a B, and so on. There is only about .001" difference between sizes. The Wiseco is a one size piston that will work with any cylinder, because there is so little difference in size.

 

[MikeT]

Spank, so can you order A,B,C,D pistons from Yamaha? And what about Fritos question about putting, say a C piston in a B cylinder? If it's only .001" difference, it sounds like that would be OK. What do you think???

PLUS Does anyone know for sure what the sizes of the A B C and D pistons are??

 

[KXKen]

I took a digital caliper to the dealer and couldn't measure any difference between an A and a D. Maybe with a micrometer you could (I don't know).

 

[MikeT]

Hmmmm....:think

 

[MX823]

I have run a couple of different combos,

The ring is the same for each bike.

My last 2 YZ's have had a "c" cylinder, There have been times when I have needed a top end and run a B piston in it , as well as times when I use a "d" piston.



I recently got a motoworld kit and all they sell was D size pistons, ......


I would not worry about the piston since it is a such a small difference, I would be more concerned about having a fresh piston/ring in it.

 

[MikeT]

Originally posted by MX823

My last 2 YZ's have had a "c" cylinder, There have been times when I have needed a top end and run a B piston in it , as well as times when I use a "d" piston.

So you have run the D piston in your C cylinder.... No scoring problems or anything?? We still don't know what the actual difference in sizes is do we?

 

[MXP1MP]

I just put a new wiseco in my bike the tolerance issue I can't say if it really matters right now. Comparing the differences to what the manual said the piston I got from wiseco would be perfect for an "A" cylinder my bike has a "B" cylinder. I just took it out on sunday and I can't tell any difference in the performance with the slightly smaller piston. It fit in tightly and there is no excess vibration from it being slightly under tolerance. My friend has been running wiseco top ends in his yz's for years without an problems and he's alot harder on his top ends than I am. I can't see it causing any problems but the jury is still out right now cause I just broke it in.

 

[MikeT]

Originally posted by FritoBandito
It might be a few weeks until I get to do this, since it is snowing like crazy outside.

Because it's snowing, or are you looking for those Taco flavor Doritos??;) Yes get back to us.

 

[MX823]

I have not had any scoring problems, just allow bike to break in properly

I used to use a cycle for pistons

I had a C cylinder so I ran it through 2 C pistons, then on the third ran the d, but I needed a piston bad once and all the shop had was B, I took it since I figured I would just change it out ....... but I left it for the normal service interval, no problems..

After I ran it through a couple D's I usually had a new bike so I never got to the what if point......

On my current YZ , had a c cylinder and went directly to a D on first piston.

A local shop on the fish finder has the info.......listed as optional.......

 

[MikeT]

OK this might sound stupid

I just cam home and was looking at my cylinder.... WHERE is the ABC or D stamp??

 

[MikeT]

Is it embedded into the metal or is it an ink/paint type stamp?

 

[MXP1MP]

Its an ink/paint on my '02.

 

[MikeT]

Ya, 96whyzee125 sent me a picture. I looked in that area, back center about midway up, for the designation but didn't see it. I'll wash it off with a tooth brush tonight and take a better look tonight.

 

[JANDERSON]

A-B-C-D PISTONS/CYLINDERS

hope this helps you all:

PISTON
A-RED MARK-2.1243-2.1244in
B-ORANGE MARK-2.1244-2.1246in
C-GREEN MARK-2.1246-2.1247in
D-PURPLE MARK-2.1248-2.1249in

CYLINDERS
A-2.1260-2.1261
B-2.1261-2.1262
C-2.1263-2.1264
D-2.1265-2.1265

THESE ARE THE SIZES YOU HAVE BEEN ASKING ABOUT.

YAMAHA ONLY RECOMMENDS A-A, B-B, C-C, AND D-D COMBINATIONS.

THIS STUFF IS ALL LOCATED IN THE FACTORY SERVICE MANUALS. THE POSTED INFO IN QUESTION IS FORYZ125J1. MY KIDS IS A 97. IT'S RACED, BUT ONLY WITH WISECO PISTONS, NO PROBLEM EVER.

THE CYLINDER MARK ON HIS IS STAMPED IN.

 

[MikeT]

Well, I've got a "B" cylinder. I wonder if I can throw a "C" piston in it and freshen it up a little. Any opinions?

 

[JANDERSON]

C in B

Mike T...put in a wiseco, the price is lower (at least that's what I've found) and the results are as good as OEM.