Hello, i have a question about the piston on the dt 125r, its a 1999 model and engine is marked 3MB00 it has a +1.25 piston in it and its worn out. I did search the web and could not find pistons of  +1.25 anywhere except the expensive wössner and other, but i found a lot of pistons marked 3RME for dt 125r 1999 at a decent price, so what is the difference  ?

Where did you get 1.25 from? Generally pistons go up by .5. Anything less doesent generally clean the bore.

Mitaka do piston kits for your bike up to 2.0mm oversize for around £50.

I bought the bike it had a +1.25 in it, its a prox piston  and it has 1.25 stamped on it, the diameter of the cylinder is 57.25mm and the standard is 56mm so it is + 1.25. Now i did some more searching on the web and found a piston at a good price      http://www.ebay.fr/itm/Meteor-Kolben-YAMAHA-TDR-TZR-DT-125-Typ-3MB00-57-25mm-Piston-kit-/131554566952?pt=LH_DefaultDomain_77&hash=item1ea1436328 any ideas about it ?

Why do you want to change the piston?

This is done when its worn past its limits or has been damaged. On a 2stroke when its replaced you go up a piston size. Generally .5mm sometimes more. I had to go a full mm on my TDR when the powervalve and piston came together with the piston ring. What is the problem you have, I'm guessing you may just have excessive wear even then you will need to rebore. You may get a bit extra with a set of rings but, again. My TDR was brought low by the age of the piston and the ring peg failing to stop the ring rotating.

If the bore is clean and not out or round you may be able to use a .25 increase but it is tiny, barely more than the piston tolerance of .125mm either side. You may be able to use it but the full 2.0mm bore may be better for long term reliability. As will using GENUINE gaskets, don't believe the bull, anything other than genuine yam or Athena head gaskets will fail.

The only names repeated for these bikes for replacement pistons that will stand the use are Mitaka (best price), Woosner and Genuine OE Yamaha, (gold plated I think).

I fitted mitaka in my TDR250 on the recommendation of everybody I asked pretty much.

 

Hi kezz " how do you know its worn out. is it slapping .  did you gap the rings ,  try them in the bore  .  I would go up .50  if a rebore is needed, .

And Wiseco & wosner are  expensive, 

Hey, yeah it was slapping, took the cylinder open and the piston was moving to sides, the rings gap was more than 0.25. i think i dont need a rebore because the cylinder diameter was 57,25mm , i cheked from 5 spot and there are no scrapes or anything, its smooth as glass.  

I would measure your cyl carefully.

It should be done at 3 or 4 points down the bore.

If you have piston slap the bore will be worn. Most likely not properly round any more. Maybe by only 0.3mm but that is enough.