Hi Guys

I’ve posted a couple of times here regarding my 1980 DT 175MX which I’ve converted down to a 125 by swapping the barrel, head and piston. Hopefully you can help me with this one! After running the 125 engine for a few miles I decided that the honing I had done hadn’t been enough and it sounded like I had a bit of piston slap so I decided it needed re-boring and a new piston. Job done and I picked it up yesterday. At home I looked at the piston and realised it was different from the original. It has no open ports on the reed valve side of it, and is solid all the way round other than the hope for the gudgeon pin. The make is Mitaka. Duly took it back and the chap at the workshop said he would call their suppliers and call me back. Just had a call back and he tells me that the supplier recommends this type of piston for my engine because by closing off the reed valves it increases the crankcase pressure and gives better performance and is a far superior piston. But they do have the type that I was expecting and would change it if I wanted.

This is a bit beyond my knowledge of the Yamaha 2 stroke engine. I assumed (mistakenly?) that the ports were necessary to help lubricate the crank.

Can anyone help on this? Will this piston be okay in my bike? The box it came in does actually have DT 125R on it, so do you think they aren’t listening to how old this bike is and just lumping all DTs together?

Any help much appreciated

Thanks

Paul

He is talking complete coblers more ports more power simple, trouble is without those windows there is nowhere for the fuel mix to actually get into the crank case with the design of your barrel. I can't see it working at all.

The piston you have is for piston ported engines dating back to the early seventies as without the reeds you needed the full skirt with no holes to keep the mixture in the crank till the right time.

You really do need the piston with holes in (tech speak) i've ridden the 175 and the 125 and the 125 needs all the help it can get as its best when reved and closing those ports will surely rob it of the ability to do just that.

If closing those ports was worthwhile why did my powervalve have honking great big holes in it, (i found after it siezed it was a race engine) surely according to this fellows info they would have been closed up.

Edit, made a boo boo .

Like you said he was not listening, the DT125R induction is direct into the crankcase unlike your DTMX, get it swapped

Like you said he was not listening, the DT125R induction is direct into the crankcase unlike your DTMX, get it swapped

Ahhh didn't know yours went straight into the cases OG.

Ok thats another reason for not having the holes in the piston. Neither suit the DTMX though.

Thanks for you help OG and Cynic. This explanation just didn't seem right to me either.

Paul