Piston rings

[declan]

@calum the box says 56.90 then under that it says .50

[declan]

@declan I should add the box also says it's a tzr 125/250 piston and doesn't state that it fits the dtr but i have run it without issue

[markus.w]

@declan if it's any help to you I know there was a difference in the early and later dt125r bores. I think the 88 model had a 50mm stroke and was later changed to 50.3mm so to keep the cc the same there was a slight difference in bore size.

[declan]

@markus-w yeah I know there's 2 sizes now, it's just a little annoying that pjs doesen't seem to stock the size i need

[markus.w]

@declan I just checked the Haynes manual and it says that only the 3db 1988 round slide carb model was 56.4 then the 1989 3rn1 onwards is 56.0 so seems like you got the rare early model or at least the early engine/barrel.

[declan]

@markus-w maybe, my barrel is stamped 3mb though I'm not too knowledgeable on engine codes I'll just have to keep looking around I'll probably just have to get a whole piston kit

[declan]

My theory is the shop that handled my barrel work or a previous owner thought it was a tzr barrel and pressed in a tzr cylinder liner (providing the ports match) and that's why the dtr pistons don't match but the tzr ones do

[markus.w]

@declan they all say 3MB it's the first digits of the engine/frame number you need for ordering parts. It will start with 3DB if it is the early 56.4mm bore or 3MD from 89-96 and 96 onwards is 4BL. That's presuming you got the original barrel but if it 56.4 then it is a 3DB.

[declan]

@markus-w my frame number starts with 3md

[declan]

@declan and that's supposed a 1990 or atleast it's registered as such

[markus.w]

@declan sounds right as 3MD is 89-96 so your barrel has probably been changed somewhere along the line.

[SpookDog]

@declan
Have you talked to PJME? If you have a 3mb barrel it was never fitted with a 56.5 piston. A 3bn oversize will fit them both as the piston skirt and porting is the same. As far as I know

[declan]

@spookdog nah I didn't talk to them, I've ordered a piston from yam bits to suit my barrel size it's for pre 90s bikes it should work I'll compare them before fitting just in case it arrives tomorrow so I'll keep the thread updated over the weekend

[declan]

The piston works it was a piston for pre 89 dt125rs from yam bits, the bike has a lot of felt compression now seems way more than it had when it originally had a rebore and piston making me wonder if the rings were defective from new on the original piston, i did tighten the squish up just a tad with a slightly thinner gasket but I highly doubt that's the reason for the increase in compression before the kickstarter had litteraly no fight back on your foot or hand and it was like that the whole time I rode it.

[declan]

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