First off hello everyone new to the forum got a problem with my sons 2000 model dtr125 here goes

noticed a lot of blowby on standard piston when fitting a dep pipe, whipped the top end off and sent to pjme for a rebore new piston and a shave of the power valve as its now going to be 2.00 oversize

got the parts back checked the ring end gaps etc all bang on put it together checked its turning free then  tried to start it and 6 kicks later very erratic idles like the slide is stuck slightly open then tried to stall then Rev up again, I  killed the bike took the plug out turned the kickstart quite hard and notice a metal to metal knock now the piston box is marked tzr125/250 me thinking pjme obviously know it will fit else they would not of supplied it 

I'm about to tear the bike down and check the deck height but thought I would post here incase anyone else had any ideas I'm thinking wrong piston maybe slightly taller but surely they wouldn't of made a mistake like that ?? I'm gonna give em a call after I check the deck height 

You need to tear it down to find the problem, the tzr 125 piston is a little disconcerting, I thought they were slightly different sizes. Depends on year/model etc though.

Hi cynic just been into the top end and seems as though the piston is hitting the centre metal band of the Athena head gasket,pjme advised us  to grind it away for bigger clearance so the piston would not touch it and done that bolted it back up and she went first kick and is running great. So anyone having similar when running a 2mm oversize piston check that head gasket 

Check the wrist pin height,  against the old piston

Glad it was simple, did they not tell you this when you picked up the parts?

Been on grampian motors Liverpool who sell mitaka pistons. If your going too far oversized for they recommend a yzr piston.  Looks same dome height etc.