2018 YZ250 - No letter on cylinder?

thatswhathappens
Edited Date/Time 4/16/2022 12:38pm
Wanted to do the top end on this YZ I've had for a bit, grew up putting around on 4 strokes and never did any engine work myself. Saw that you need to order a piston that corresponds with the type of cylinder, and when I checked above the intake, I didn't see a letter to denote the size of the cylinder...



Any idea what this means? Am I looking in the wrong spot for this? Googled "yz250 cylinder no letter" and that wasn't much help.
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TheTrackKid
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TheTrackKid 2/14/2022 10:18pm
2/14/2022 10:18pm
That is usually where it is marked.

Not an expert, but I have had a few yz’s and I’ve yet to see anything stamped besides “B”. That also being said, I would run a “D” piston whenever I did a top end and never had any issues. I know there are a few other yz connoisseurs on here including Mr. Robinson, mwilcox349, and cdoggy81 that I bet could provide some more info.
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kb228
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kb228 2/15/2022 5:39am
2/15/2022 5:39am
You could always mic the bore and order the piston accordingly
wiedeman
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wiedeman 2/15/2022 10:46am
2/15/2022 10:46am
As long as you can still see crosshatching and plating you will be fine with any size piston in there. I have had ran an A sized piston in a D sized cylinder before on my 125 and put 30 hours on it no problem and everything looked fine when I took it out.
I personally would use an aftermarket forged piston.
Moto520
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Moto520 2/15/2022 8:17pm
2/15/2022 8:17pm
I think the marking is on the power valve side

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cdoggy81
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cdoggy81 2/16/2022 12:06pm
2/16/2022 12:06pm Edited Date/Time 2/16/2022 1:14pm
Marking is above reed cage typically. If the bike is older or has a bunch of hours (washes basically) the black letter stamp is probably gone.

If you can see crosshatching & no score Lines or plating flaking off then your cylinder will be fine.

The differences between cylinder sizes A-D is like .01” per letter size. Basically on a cast piston & their expansion rate you would be good with any size slug in that hole.
FGR01
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FGR01 2/16/2022 12:29pm
2/16/2022 12:29pm
cdoggy81 wrote:
Marking is above reed cage typically. If the bike is older or has a bunch of hours (washes basically) the black letter stamp is probably gone...
Marking is above reed cage typically. If the bike is older or has a bunch of hours (washes basically) the black letter stamp is probably gone.

If you can see crosshatching & no score Lines or plating flaking off then your cylinder will be fine.

The differences between cylinder sizes A-D is like .01” per letter size. Basically on a cast piston & their expansion rate you would be good with any size slug in that hole.
It's much smaller than .01" per letter. .01" would be pretty huge. You're correct that it is so small it basically doesn't matter that much which letter piston you use.

Here's the actual sizes for each letter piston.

Falcon
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Falcon 2/16/2022 1:14pm
2/16/2022 1:14pm
I understand Team Yamaha used to run "D" pistons in "A" cylinders in the '90s.
cdoggy81
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cdoggy81 2/16/2022 1:19pm
2/16/2022 1:19pm
Ok yes, FGR01 is technically correct in that it isn’t exactly .01” :-) I was going off memory which for an old guy like me probably isn’t the best... At least I had the location correct LOL 😂

Your link above doesn’t work but here is a pic from my collection of yz notes:


Moto520
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Moto520 2/16/2022 1:22pm
2/16/2022 1:22pm
I just went through this. When you buy the Yamaha piston kit......they include the "B" piston. Well....i have a "C" cylinder. I'm so anal that i wanted the same piston as cylinder.....why mess with it?

Anyhoo.....i have a new "B" piston for sale for the 2014 YZ250 if anyone needs it.
thatswhathappens 2/16/2022 7:46pm
2/16/2022 7:46pm
Thanks for all the help fellas, I'll probably just go with a B or C piston and hope for the best. My biggest concern was running the biggest piston in the smallest bore and accidentally causing some damage, but it sounds like that shouldn't be a problem.

Thanks to each of you for the help!
mwilcox349
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mwilcox349 2/16/2022 7:46pm
2/16/2022 7:46pm
For the 250s, I've run Wiseco single ring pistons. They only have the one size for them. So I have stopped even paying attention to the letter on the back of the cylinders. I haven't had any issues at all. I replace the ring at 20-25 hours, then piston at 45-50. I can't remember the last time I had a piston failure while running Wiseco.
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captain.olives 2/17/2022 5:39pm
2/17/2022 5:39pm
Yep, the "stamp" isn't a stamp at all in the casting. Ink or paint of some sort and usually wears off after a few years.

cdoggy81
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cdoggy81 2/17/2022 6:10pm
2/17/2022 6:10pm Edited Date/Time 4/16/2022 12:38pm
Haha, funny story when I got my first yz250 I was so anal I engraved the letter in so I would never forget it after the black ink letter would wash off. It took a few years before I saw the light 😂

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