I'm working on a 2001 CR125. My cylinder is marked "B" and the piston isn't stock so the mark isn't there (B AR1T060). I looked on a few websites and I can't figure out the deal. Can you use an A piston with a B cylinder??? I was told that the diameter is different and it won't work BUT Wiseco only makes one size???? I want a stock Honda piston anyway but everyone has an A and noone has a B. What can/can't I use and what is the difference between the two? Thanks!

your better off running a after market piston in this bike. The stock piston has to much clearance. Thats just the basic reason why.

The aftermarket piston that is in there now is a Wiseco, at least that was what I was told. It has some funny marks on it and I thought that stock would have been better as I have run in my other Hondas. You guys may be right about the aftermarket piston though...give me more details before I deciede on one. The Wiseco put a few light vertical lines in the bore and had some wear marks on the side of the piston near the wrist pin indentation. I assumed that this was from needing rings???? The marks were slight enough that a ball hone took them out. What about Pro X pistons?? I use them in my CR500 with good success. I would still like to know...what is the difference between the A and B piston.

I agree with MXP1MP, a friend of mine had an '01 CR 125 with the stock piston and the top end sounded like it was coming apart! After using a Wiseco piston it not only ran better but was a lot quieter. Tolerances between piston and cylinder wall quite a bit tighter.
Darringer

To answer your question about the scuffing marks on the piston, They are on the skirt area? around the exhaust port bridge? on the cylinder area and the wrist pin. Sure Sounds like lack of lubrication during warm up, ya gotta make sure those forged pistons are warm before you scream the bike. Its like those fools you see who start thier bikes and from a dead cold wring the piss out of it. Or maybe just maybe it sucked alittle bit of dirt? How bad is the piston colored does it look like it had lots of blow by?

Looking at the piston, it looks brand new, the top of it has zero carbon on it. The wear marks are near the wrist pin on the side, not on the skirt at all. The exhaust port was loaded with 1/4 of carbon in the pipe flange when I tore it down, obviously from the last piston.

What the hell did they do install a new piston before selling you the bike? The difference between A & B pistons is the cylinder size - piston size. Its like the YZ 125 which has an A, B, C, D pistons and cylinders. The difference in size is a few thousands of an inch its minor one is just a tiny bit larger than the other. I forget the exact measurements I don't have my CR anymore.

I don't know MX, the previous owner raced it 12 times and he told me that he ringed it often. He also told me that they put in a new piston last time and it "is probably due to be rebuilt". This thing still has the original front tire and the second rear tire on it. It seems like everything should be solid but the carbonless piston and "due to be rebuilt" has me in a quandry!! I have a digital pic if I could figure out how to post the darn thing!

The scoring you have on the piston sounds like a little dirt got through the filter. How's the crank? Maybe you have excessive play in the big end bearing and maybe that's why the previous owner said it needed to be rebuilt? The difference between the A and B cylinders is that the A is approx. .003 in. smaller than the the B. Too bad you have the B. A common practice is to run the factory Honda B piston in the slightly worn A cylinder to take up the excess clearance in the stock setup. I have used wiseco pistons in my 99 cr250 with good results.

I checked the rod for excessive play and all indications are that the lower bearing is good. I don't think that the previous owner was too swift, he may not even know what the deal was. The low speed jet was so clogged up, I'm surprised that this thing even ran. I had to run a piece of wire through it to clear it out. Here's my thoughts on the aftermarket piston at this point. If the aftermarket only makes 1 sized piston to fit an A or B, it would have to be small enough to fit the A size, thus making it loose for a B even when the tollerances are tight. Now that this cylinder has some wear on it, I'm thinking that a stock B size would more closely fit the bore size. Am I laying awake at night thinking about this stuff for nothing???? What do you think, about the size not my insomnia?