Hi 

I've just took my head off and cylinder to clean up so have decided to change the piston and rings as the gap on the rings is quite big. 

Question is I can't find what size the piston is I have measured the bore and it is 69.775 will a 70.00mm piston kit be fine for that? Any advise will be great 

thanks 

It depend on the state off the sleeve

Guy

70mm is nominal standard bore size, if there is no step at the top of the bore or damage to the liner you are lucky. Lots of standard size piston kits around still, just make sure you don't get a 311 piston as they don't fit without modification. 

Easiest way to tell if the piston is correct is that the 434 piston has a taper on the skirt out to the transfer cut outs, the 311 doesn't & is thicker as well. The 311 piston hits the crank where the taper should be & will not allow the piston to get full travel

You are right about the 311 piston, and yes they are a little heavier than the 434, but the modification is easy to do and I have not notice any difference in performance in 2 bikes that I use them on. I posted a picture a while back of the 2 piston side by side.

Guy

Be careful if you don't rebore it because some (most) new pistons have an L shaped top ring which means that the top edge of the top ring travels right to the top of the cylinder. If you have the original TY250 piston in it, it will have "square" rings and being set down the piston a bit, these rings don't travel right to the top of the cylinder. If there is even a microscopic lip in the bore at the top end of the travel of the old rings, this lip will cause problems for a piston that has a top L ring.

If it didn't rattle much before you pulled it apart, it might be worth just fitting new rings to that piston. If you do, make sure that you are getting A model rings for an A model piston. B model and later TY250s came with L ring pistons so their rings will not fit your piston (if yours is the type with square rings)

Comparison photo

A guy I know has a really nice 434 Ty that he has nikasiled the bore, when he had it rebored. Would this be a good recommendation, his bike does run very quietly and and very well. I would love to have my 434 pepped a little but would like opinions before I decide which way to go.  

Cheers

 

Hi thanks for the replies 

when you say an L ring piston ring do you mean the top ring is in the shape on an L and the bottom is Normal, as this is what is on the piston. There is no play so would be good to just change the rings I've brought these since which I'm hoping are the correct rings for it 

 

I also have a piston kit which I was assured would fit even though it says it's for a d e model but wasn't sure still so brought the genuine rings after too 

Top ring has a L shaped section & the piston is stepped in above the ring land, L shaped ring goes right to the top of the piston. Sounds like that's what you have & bought 

The piston kit you have shown doesn't have a good reputation, can be noisy very quickly even when bored to correct size

On 8/2/2018 at 1:17 PM, tony27 said:

Top ring has a L shaped section & the piston is stepped in above the ring land, L shaped ring goes right to the top of the piston. Sounds like that's what you have & bought 

The piston kit you have shown doesn't have a good reputation, can be noisy very quickly even when bored to correct size

Last sentence, +1, concur - i was warned about these too for the mono. At a third the price of the real McCoy it tells you summat....