So I am finally getting around to doing the top end on my 99 YZ 250. Went last weekend and bought all the parts, new Piston, new cylinder, pin, bearing, etc etc etc. So spent 2 hours swaping over the parts on the cylinder (first time I have done a 2 stroke). So I get ready to put the piston back on the rod and notice the piston is marker with an A and the cylinder is marked with a B. So out comes the Yamaha manual and I find the section dealing with this and is says to "Follow the chart" which says, A piston with A cyl, B piston with B cyl, C piston with C cyl, etc. So now here I am two days before I am supposed to leave and I am not sure if they gave me the wrong parts or what? The piston is pretty snug in the cyl but I don't have a bore gage to measure things and see if they are within specs. So what is the skinny? Do I have mis-matched parts or what? I am going to call them in the morning but I would like to get some outside info before they tell me "Oh yeah it's fine" and I have a bad motor half way thru the season.

Basically Yamaha marks their cylinders A-B-C-D depending on the cylinder machining tolerances. I never had a problem mismatching the piston and cylinder letters because the size is very minute. You will be fine running it but call your shop and double check with a tech.

wow thats not what i always thought....woodster knows alot about motorcycles too.

whatever you find out please post the answer

Maybe alot of Woodsters knowledge got knocked out of him after his first race !

I'd go to Harbor Freight & pick up a digital caliper to measure everything. If with in spec assemble and break it in.
Not much to a 2 stroke piston cylinder and head!

Well after calling 3 different Yamaha shops I was told pretty much "Well yes you could run that but they are supposed to be coded the same. Running an A piston in a B Cyl will work, it will just be ever so slightly looser (minute was the word one used). but if I was racing then I should make sure they were coded the same." So I just went a ahead and had them reorder me a piston coded with a B. I figured better to be safe then sorry. I figured if I rebuilt this bike every year or two then I would just run the A in the B but since the top end being replaced had aout 6 years on it I don't see redoing this anytime soon. (crossing fingers).

I don't get why it would be loose..
Cylinder should be bore & honed to the next size piston size..

That is what the letter code is identifying. The piston diameter and the cyldier bore size.

So in other worders code "A" is the correct diameter piston for code "A" cyl bore dia.

"A" being the smallest and "D" being teh biggest.

I was never able to get an answer on what the actual dimentions each code represented but I assume they would be +.005 each letter you go up or something like that.

The simpilest way that I could wrap my head around it is to think of it like a motor in you car. You can bore the block over .010 and then you have to run pistons that are oversized .010. You could probably run a standard piston in a .010 bored block but you wouldn't really want to.

I figure that in the 2-stroke the tolerances are tighter thus people get away with running the mis match without problems. However I would think if you were to try and run a "C" coded piston in a "A" coded cylinder you would have problems.

Gotcha!! My bro in law has a yz250.. This will make good convo at the dinner table for Thanksgiving.
I'll report back if have something good for ya!

Yamaha does it that way because the cylinders are nikasil ( or whatever they call it now) plated and you cannot bore or hone them out. So lets say you have an A cylinder and an A piston when new, the A piston will eventually wear on the cylinder down slightly. So now the A piston is to small, so you go to a B piston. And so on. Hope that helps.

Makes perfect sense, forgot the bike cylinders are plated! Unlike the quads!

is that the deal? i always heard the plated cylinder does not wear....we need to figure this out

really want to learn this....any ideas where we can find the answer?

i dont trust just calling a service dept some of those kids are new and stupid.

The answer is what I said!

The new piston should be properly matched to the new cylinder. Too tight and you could end up with the bike locking up. Too loose and the piston will rattle and wear out or possibly break prematurely. I would bring the piston and cylinder to a 2 stroke expert and have it measured up before I ran it. Be careful of what expert you pick though. Not all self professed experts are truly knowledgeable.

One question though. How did you know that you needed to replace the cylinder? Was the plating flaking off? was it measured out of tolerance? Just curious.

so why even make 4 different sizes....they should just make a D size if this is the case...man this sucks i need a new top end bad really want to learn this. sorry woodster i totally respect what you say still waiting to hear how you found the og olds jump you never responded in the other thread.

They make four different sizes so you can keep using the same cylinder. If you just have one size, eventually the piston will wear out the cylinder and start to get sloppy. This way you can go up a size and keep the stock cylinder. Or order an aftermarket such as a wiseco. I always like to run stock, so I think Yamaha had a good thing going. As far as the Olds jump, that thing has been around for 10 years or longer. It used to go sideways instead of the way it is now. The HUGE jump is the one in the Crusty 2 video. Where Seth lands on the cr250? That's the jump we found. We were there doing it when Mcgrath, Emig, Hughes and Seth showed up and started filming. Seth didn't even pre run the jump, he just went for it like an idiot. The guy wanted to kick his azz for landing on his bike. I think Seth ended up buying that bike off the guy, because every time we saw him afterwards he was riding a cr250. Sorry to hijack your thread D-n-Tyke!

so get ready to laugh...ive been rockin the same top end for 2 seasons.

have a B cylinder think i should run a C or D....reason i ask is that North County Yamaha is having a sale on yz250 D piston kits.

You would be totally fine with the D piston. Honest! I could take a brand new A cylinder bike and slap a D piston in it and it will run fine. Were talking microscopic differences in the sizes here. Have it measured up by a tech of your not comfortable with that though. 2 seasons on a top end is fine, it all depends on how many hours. But nothing beats the fresh feel of a brand new 2 stroke topend on a bike!

Is a Honda cylinder plated too? Might be coming across a cr250

I was looking through my 99 YZ250's maintenance manual last night.

The difference between an A cylinder and D cylinder is 0.012mm or 0.0004724"

If you need more specs let me know and I'll get you the numbers. I know it has specs for the piston to cylinder clearance, ring end gap, ring to piston groove clearance.

I should just scan in the page...

woodster was 100% correct.

called North County Yamaha today...they ONLY carry D size pistons for my nickel seal cylinder bike.

Well here are some numbers

Piston clearance: STD 0.045-0.050mm or 0.0018-0.0020" Limit 0.1mm or 0.004"

Ring end gap: STD 0.40-0.55mm or 0.016-0.022" Limit 0.95mm or 0.037"

Your 'B' cylinder should measure 66.404-66.406mm or 2.61433-2.61441"

The 'A' piston is 66.352-66.355mm or 2.61228-2.61240" and should have a red mark on it if it truely is an 'A' piston.

Worst case your piston clearance is 0.054mm which just just over the standard spec but well within the limit of wear.

You could run a 'D' piston but things would be under the STD spec by 0.008mm or 0.0003" which is so minute you would probably have trouble getting that kind of measurement tolerance. If you take your time wearing in your motor you'd be fine. I'm probably due on a piston and ring on my bike.

guess who was up late tonight putting in a new piston.....ha....forgot my damn carb at the house im workin at....wakin up in 3 hours to go get carb...clean and install....start bike and break in around the neighborhood at about 4am tom morning....then put on my paddle tire and roll to glamis.....sound about right?