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I have a 1994 Mk1 250 Pampera which recently "seized". The piston and cylinder are scored and I will now need an oversized piston and rebore. The O rings in the head gasket were obviously ruined. I had not been riding the bike hard but mainly trail riding. I use synthetic two stoke oil at 40:1. My problem is not knowing what caused the problem, I am not aware of hearing the fan on the cooling system coming on. Is there any way or checking this out?

Dave, I'm not aware of any oversize pistons available here in the States, as the bore is treated by the Nikisil process, a tough ceramic type coating, and cannot be rebored. Your Gas-Gas dealer can help you with the cylinder repair or replacement (depending on the amount of damage) and test the fan operation. The fan on my Gas-Gas is pretty quiet, so I put some white spots on the fan blades so I can check easily to see if it's working. A good Gas-Gas technician can diagnose the cause of the problem which needs to be done first. A seizure can have many causes ranging from overheating to a poorly fitted airfilter. In the States, we usually run a premix ratio of around 64 up to 100 to 1, with 80/1 being the general average. I don't think the premix ratio caused it, provided the jetting was correct to begin with. Jon.

NOTE. JIM SNELL, of Rising Sun Imports/Hebo USA, says that oversize pistons are available in A, B, C, and sometimes D sizes (increments of 0.05mm) so that the piston can be sized exactly when replacing in a stock bore.

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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