Just got an 86 ty250r and had the top end off for a look. It's not bad but has some scratches. It's got a piston with 50 stamped on the crown. Also has a shaped crown rather than even curves. Only have ordinary vernier calipers so only measred the bore at the very top. Seems to be 68mm there. Can anyone tell from any of this whether it's already been rebored. Does the 50 mean 0.5 oversize?

Also any one know if Birketts will still do the 270 kit & shorten the inlet tract? Last posts i've seen were about 4 or 5 years ago.

Cheers.

From memory 68 is standard. Ty trials does s big bore kit which takes it to 270cc. Involves a bore out to 70 (again from memory)

Just looked it up...68 is standard

It's usual for metric pistons to be stamped in mm, so

.25

.50

.75

1.00

and so on.

It's more than likely it's a 68.5mm piston.

 

I'm not certain as I've never done one, but the 270cc might not be striaght forward rebore.

All the past bikes I've tried to take out like this needed the mouth of the crankcase opening up, but I could be wrong!

 

 

 

its commonplace nowadays to be able to buy (Wossner) pistons up to 2.0mm bigger than standard and generally do not need a bigger OD sleeve

 

Thanks! Sounds like I could pick up a 270 piston kit and get it rebored then. Does anyone have the specs for the inlet or port mods or know if Birks will still do them?

The shortened inlet doesn't make a lot of difference. You can make more difference by reducing the flywheel weight, porting or using a bigger piston.

Your piston seems to be a 68.5mm. Birketts supplied me with a 69mm woosner piston last year which worked perfectly. Please be aware that some suppliers sell 70mm pistons from other yam models but they tend to have the wrong gudgeon pin height so the base of the barrel has to be skimmed. Just check the gudgeon pin distance from the piston crown before buying. The woosner was the correct height.