The engine on this TY250 is well-worn, with a good .3mm piston to bore clearance at its worst point, so a rebore needed! But am curious if this is the original, Yamaha, piston. It measures up as standard size, just under 68mm so plenty of rebore options left.

The big-end is also on its last legs so need to start saving, but in the meantime I

Looks like a std piston to me but if you are reboring the barrel why not take it out to 270cc and have the inlet manifold shortened. Makes for a MUCH better engine. Birkets will oblige.

Looks like a std piston to me but if you are reboring the barrel why not take it out to 270cc and have the inlet manifold shortened. Makes for a MUCH better engine. Birkets will oblige.

As OTF says, the piston looks to be a Yamaha one, and I've got to say that I also love the 270 engine. Nigel Birkett (Birkett Motosport) is the man for this, whether you just have a rebore or fully mod the engine - he has a lot of pistons in stock and also does the rebore himself. He was even a development rider for this bike back in the early '80s!

Are you certain the big end has gone? Whilst it might have done it is very rare. The main bearings do wear over time though and it would be worth checking and probably replacing these, and obviously if you split the crank cases do the crank seals as they are not much and do start blowing eventually.

Once the engine is done, they run for years. Best wishes - Alan

Thanks for the replies, food for thought.

Bit of a dilemma about the big-bore kit as I will be letting my boys loose on the bike so need the power nice and soft, whereas I LOVED the power on my Gas Gas 300 txt pro that unfortunately had to go when I lost my job a while back.

Point being that maybe throwing lots of money at this bike is not the right thing to do, when, in an ideal world, I would like a modern w/c bike again once finances improve, and the boys can use the TY until they are old enough to buy their own!!

Saying that, I need to ride the bike properly a few times and see how it performs, I might be surprised. But the gasser

Thanks for the replies, food for thought.

Bit of a dilemma about the big-bore kit as I will be letting my boys loose on the bike so need the power nice and soft, whereas I LOVED the power on my Gas Gas 300 txt pro that unfortunately had to go when I lost my job a while back.

Point being that maybe throwing lots of money at this bike is not the right thing to do, when, in an ideal world, I would like a modern w/c bike again once finances improve, and the boys can use the TY until they are old enough to buy their own!!

Saying that, I need to ride the bike properly a few times and see how it performs, I might be surprised. But the gasser

Suggested the 270 conv as it is about the same price as having it rebored anyway and the power IS dead soft.

I had a few of these and the 270 is soft, when its done correctly they are really very tractable... superb, but not for launching up big things... but hey most people don't want to do this anyway.

The main benefit of a proper Andrews (or Bikett) conversion is the shortened inlet... makes it a bit more responsive.

The other thing to do to make it rev better is to but a thicker head gasket on it. Yes! not to soften it as such but we found that it made the bikes run cleaner and rev better with the slightly lower compression... honest!

Have bored quite a few of these to 270 but what know one has said is you need to shorten the barrelas well. The conversion uses a TY250 piston. The big end is built like a Brick sh** house.

OK lots of opinions, slightly confusing but all on the same line I guess!

What does this mean though??? :

I have just bought a Ty Mono and its done very little work around 2000kms on the trails from new.  The engine runs well but it whistles, and it seems to come from around the crankcases. Is this a sympton of bearing issues big or small .  The bike has stood around for many years and can this cause bearing to become noisy. The oil seals seem fine , but is a lower engine rebuild on the cards.. Thanks.