I'm shortly getting the top end done on my TY 250 Mono. Its currently on a .25 oversize piston and I've had two opinions on where to go next.

One guy reckoned moving up to a .50 would be fine but another said I would need to go to at least .75 to make sure it was over any previous wear.

Does anyone have experience or any recommendations they would like to share?

Ta.

Edited May 4, 200818 yr by tt5th

I'm shortly getting the top end done on my TY 250 Mono. Its currently on a .25 oversize piston and I've had two opinions on where to go next.

One guy reckoned moving up to a .50 would be fine but another said I would need to go to at least .75 to make sure it was over any previous wear.

Does anyone have experience or any recommendations they would like to share?

Ta.

how bad does it rattle ? It would need to be horrendous for a .25 rebore not to be enough.

have you got feeler gauges just to give an idea of the amount of wear ?

It's not bad at all, its quiet when its cold but starts to tap a bit as it warms up.

I do have feeler gauges, slip them down the barrel after I've take the head off?

Edited May 4, 200818 yr by tt5th

If its quiet when its cold 1 oversize at .25mm will be fine. I had one that wasn't that noisey years ago but you could feel it rattle under load and I could get a 6 thou feeler gauger between the piston skirt and the bore. Got it rebored and went up one piston size and it was perfect. So unless you can get an 8 thou gauge in I'd say you will be fine.

Rebore it to 350cc put a pair of yz carbs on it. new exhaust, custom peco system should do the trick.

If its quiet when its cold 1 oversize at .25mm will be fine. I had one that wasn't that noisy years ago but you could feel it rattle under load and I could get a 6 thou feeler gauger between the piston skirt and the bore. Got it rebored and went up one piston size and it was perfect. So unless you can get an 8 thou gauge in I'd say you will be fine.

The piston will have been made to be tapered and so the only correct position to measure the O/D for signs of wear is 10mm up from the bottom of the piston skirt, opposite the gudgeon pin.

Putting a feeler gauge between the piston skirt and the bore, (presumably at the top) is nonsense!

You need to measure the piston and clock the bore as well as measuring the gap between the rings when they are slipped into the bore while off the piston to fully determine which parts are worn and need replacing.

I still have the standard bore on my TY250. I recently fitted a new piston and rings as they were worn, but did not need to re-bore the barrel as that was still within manufactures tolerences.

All Yam's rattle, it's part of their charm. Don't always automatically assume a re-bore is the answer, it may be more simple than that!

Nice, cheers for the advice! I actually heard about a guy who had a full rebuild done at no expense spared, it still tapped on the overrun apparently! Good old TY's!

Folks, like any 2-stroke bike, the TYs are sensitive to carbon build up in the exhaust. When there is too much, it affects the exhaust pressure waves which affects the scavenging process and may make the piston rattle if not corrected. The TY sounds rattlier than others because it is air-cooled and hence no water jacket to muffle sound. Burn it out. It won't cost you much at all and you'll find the need for engine o'haul may not really be necessary.