I've just been trying to renew the rings on my 97 JTX 250 and found that I can't get rings on their own so spoke to GG uk who advised that its a full piston kit, mine has a B piston in it so on their advice ordered another B piston.

It landed on Friday and I got ready to attack it today but found that the new piston is bigger that my old one - new one is 72.47mm when the old is 69.14mm

anyone any ideas why these don't match???

Edited March 5, 201214 yr by HoggyF

Further info today, GGUK don't recognise my piston size, they say that its between a 200cc and 238cc piston size yet the barrell has 250cc written on it and they don't use the engine no. only the frame no. but i have a feeling the engine isn't the original.

Got to ring them back tomorrow with the frame no. but I don't hold much hope.

Are you measuring the piston or the bore? Remeber there needs to be some clearance but of course a barger piston will not fit in a smaller bore without a very large hammer!

I did some digging and in 1997 these sizes were produced:

JTX200 66mm bore

JTX270 74 mm bore

JTR250 71 mm bore.

JTX370 61mm bore !

I have heard that the jugs are interchangeable among the TXT engines (93 to '01) so someone may have swapped a different jug and piston. The bores are sputter plated with Nikasil, not regular hard chrome. When I seized my JT25 I had to send the jug to Rising Sun (US Importer) and they sent replacement jug with matching piston and rings. Not cheap but it runs like a new bike now.

I am no expert. I found these by using Clymers Blue Book on line. It is a bit clunky to select year, and model but then it shows bore and stroke for all.

http://www.clymerval...ake=Gas+Gas+USA

Good luck.

Edited March 6, 201214 yr by Thats_a_Five

Gas Gas had sent a 2001 250 piston which is why it was 72.47mm but -

I've re-measured it last night and I was measuring above the top ring originally (69.14mm) when apparently I should be measuring just above the skirt so.... I now get it to around 70.8mm which is close enough to the 97 250 piston so its been ordered and we'll see if it fits when it arrives.