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Hi Jon,

Great stuff, I' ve studied all you replies, etc. Could you tell me if the piston clearance on a '94 JT 25 is the same as on the latest of the 250's? I've had to put a new piston and barrel on recently, but it still seems to have more piston noise than the newer 250's. There are plenty of other noises there, it is after all no spring chicken, but the piston one bothers me the most, especially if a new, updated piston could convince it to whisper a little more. Thanks for your time.



Most of the piston clearances are generally the same, which is based on cylinder material/plating and piston alloy. A general rule of thumb is .001 inches of clearance for each inch of cylinder bore. The Nikasil plated cylinder can't be overbored, however, Gas-Gas has "oversize" pistons available to exactly size the piston to the cylinder. They come in A,B,C and sometimes D sizes with each one being .05mm larger.

You or your Gas-Gas dealer could measure the cylinder bore with a dialbore indicator and see if an "oversize" piston might bring it up to exact tolerance. Was the topend bearing changed? This can sometimes cause noise from the piston area. All this is assuming that the noise is actually caused by too much piston clearance. If the new cylinder and piston was matched ("A" cylinder and "A" piston for instance) I doubt that this is what is causing the noise. You might want your dealer to listen to it and give you some advice.


Professional tuner and 'Trials Competition' writer Jon Stoodley has very kindly stepped up to the plate to answer your questions. If you're having problems, or need some advice just send an email to Jon. If it's something that could be useful to others, we'll post it on the site.
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