Other Piston sizes?

[Bubz692]

Alright lads, I'm new to the forum so cut me some slack!

I have been riding for about a year now, and slowly working my way into the technical stuff, I've just bought myself a 2 stroke (Yamaha DT125re 2006), and I'm thinking of getting a new piston kit and replacing it.

Can someone please explain why there are two or three different piston sizes for the same engine? I.e. Oversized piston.

Surely only one size fits?

 

[Mihylo33]

Alright lads, I'm new to the forum so cut me some slack!

I have been riding for about a year now, and slowly working my way into the technical stuff, I've just bought myself a 2 stroke (Yamaha DT125re 2006), and I'm thinking of getting a new piston kit and replacing it.

Can someone please explain why there are two or three different piston sizes for the same engine? I.e. Oversized piston.

Surely only one size fits?

It's done that way for cylinder wear on the yammi. A-B and C pistons I believe. Your cylinder will need to be measured and I recommend taking the cylinder to a shop and watch them measure it and then order the piston from them. It's easier for them to show you and explain in person than us trying to give you directions on how.

 

[Redordead89]

Welcome to DBA Josh, we Brits are a bit scarce on here but the boys don't give me too much stick for it

He's from Scotland.

so it's all good
Stick around Josh, lots of helpful people on here!
I had a black silver DT125R 5or 6 years ago, great bikes
Unfortunately the DT125 R never made it to the states but being a yam 2T it's all relative state side too

 

[Bubz692]

It's done that way for cylinder wear on the yammi. A-B and C pistons I believe. Your cylinder will need to be measured and I recommend taking the cylinder to a shop and watch them measure it and then order the piston from them. It's easier for them to show you and explain in person than us trying to give you directions on how.

I thought that was what it was for, I wasn't certain though. Thanks for the reply.

I think I'm going to let a shop do the rebuild this time around, I'll do it myself next time.

I don't want to ruin the f*cker and have to take it to a shop, it would be a real pride killer!

 

[Bubz692]

Welcome to DBA Josh, we Brits are a bit scarce on here but the boys don't give me too much stick for it

so it's all good
Stick around Josh, lots of helpful people on here!
I had a black silver DT125R 5or 6 years ago, great bikes
Unfortunately the DT125 R never made it to the states but being a yam 2T it's all relative state side too

They're great bikes, a little underpowered, but built to last, there's obviously going to be some performance lost.

It's my first two stroke so didn't want to jump straight into the deep end on a YZ

 

[Redordead89]

That DT is fast enough for an introduction to 2T, quite tall too, I'm a short ass
A YZ 125 is a lot quicker and much more fun tbh but the DT has its merits too for roads, trails and field use if your not out on the track
If you have a decent budget then send the barrel and piston to Stan Stephens

 

[Bubz692]

That DT is fast enough for an introduction to 2T, quite tall too, I'm a short ass
A YZ 125 is a lot quicker and much more fun tbh but the DT has its merits too for roads, trails and field use if your not out on the track
If you have a decent budget then send the barrel and piston to Stan Stephens

Aye it's a great little bike, only got a thousand on the clock as well so shouldn't go bang on me any time soon. The reason for wanting a rebuild is I suspect it has been abused by the last owner by the scuffs on the plascits, bent gear selecter etc. i don't want to be stranded in the woods!

I will look into Stan Stephens, where is he based?

 

[2Wheels]

Yamaha has abcd pistons to achieve exacting tolerances with mfg anomalies. The cyl comes off the line + - .0000" and they match with the piston size most closely suited. Personally, if Im getting the cylinder replated, I send a new piston or have them supply. If honing only, just get a D size. The difference between a brand new A cyl (smallest) and new D piston (biggest) is .0012" which is .0004" below min spec. But, if your honing a used cyl you can't go wrong with a D. And I believe that's a stock cast. after market like weisco just have the one size. Yamaha oem is just way anal about it. Theyre the only ones who do this if I'm not mistaken

 

[Bubz692]

Yamaha has abcd pistons to achieve exacting tolerances with mfg anomalies. The cyl comes off the line + - .0000" and they match with the piston size most closely suited. Personally, if Im getting the cylinder replated, I send a new piston or have them supply. If honing only, just get a D size. The difference between a brand new A cyl (smallest) and new D piston (biggest) is .0016" which is .0004" below min spec. But, if your honing a used cyl you can't go wrong with a D. And I believe that's a stock cast. after market like weisco just have the one size. Yamaha oem is just way anal about it. Theyre the only ones who do this if I'm not mistaken

Thanks dude, I did have a look around at other manufacturers/aftermarket suppliers, and couldn't find anything, I had a feeling it was just a Yamaha thing. Being kinda new to engine internals I didn't want to mess things up and have to pay for a full rebuild.

So correct me if I'm wrong, but as a recommendation I should go for the 'D' size? (The bike is around 10 years old and has done 1000ish miles)
Thanks

 

[Redordead89]

Thanks dude, I did have a look around at other manufacturers/aftermarket suppliers, and couldn't find anything, I had a feeling it was just a Yamaha thing. Being kinda new to engine internals I didn't want to mess things up and have to pay for a full rebuild.

So correct me if I'm wrong, but as a recommendation I should go for the 'D' size? (The bike is around 10 years old and has done 1000ish miles)
Thanks

No!!!!! You r bike has very Little miles on it and is very unlikely to need a D piston already, Like Was said by Mike! M33, you need to get the barrel spec'ed by a shop/bike mechanic before you can choose a piston size, it's more likely that on a thou miles your bike still needs only a A or at worst B piston and rings, My dad has a 2000 dt125r and my bro has 2 98-2003!
Any local bike dealer or bike workshop can do this for you if you can't due to tools etc but
Stan's the man for cylinder work over here and will gladly port and polish any cylinder for extra sheckles!

 

[PALMER84ONE]

Someone needs a lesson?

 

[Bubz692]

No!!!!! You r bike has very Little miles on it and is very unlikely to need a D piston already, Like Was said by Mike! M33, you need to get the barrel spec'ed by a shop/bike mechanic before you can choose a piston size, it's more likely that on a thou miles your bike still needs only a A or at worst B piston and rings, My dad has a 2000 dt125r and my bro has 2 98-2003!
Any local bike dealer or bike workshop can do this for you if you can't due to tools etc but
Stan's the man for cylinder work over here and will gladly port and polish any cylinder for extra sheckles!

Right mate I'm with you now. As I said, I'm kind of new to the technical stuff! I've only done minor repairs/tuning so far, never disassembled an engine, yet.

I've had a read up on him and he sounds like he knows his stuff. I'm going to call them tomorrow and send it down next weekend.

 

[Bubz692]

Someone needs a lesson?

I did say I was new to engine rebuilds! Haha

 

[PALMER84ONE]

I did say I was new to engine rebuilds! Haha

Well, If you find the Godden boys (James and Eli) smack them around for me. Damn pussies are not coming out to US until August when it's hot as hell. but then, anything to get out of the pissing rain and spotted dick everyday can be nice.

 

[2Wheels]

My opinion is yes, a D piston. Again IF your bike has an A cyl a D size piston would be tight by .0004" which is a nothing burger. That amt of cyl wall is surely worn away with minimal use. And that is IF you have an A. Any other series and your in spec. Better than having an A piston slapping in a D cyl. Just my opinion.