Rocket Racer,
I agree, the JPs are ok for road use, but I would not use them for a racing engine. There's nothing better than a good FORGED piston for high performance use due to the higher material strength.
I had problems similar to yours with the Italian GPMs in the A10, I think I told the story in another thread already. Many others use them without any troubles.
I don't really know what the reasons for this discrepancy are, maybe its large manufacturing tolerances, insufficient quality assurance or finally just the riding style, no idea.
But like the crack in your JPs the problem with my GPMs definitely had been a mistake somewhere in the manufacturing process, the solution for me were the Wisecos, too.
But unless you are able to find a set of good Hepolites, there aren't any alternatives for the A7 I would know of. All the JPs I have fitted to several bikes so far actually never caused any sort of problems.
What I always do now before fitting new pistons is to temper them prior to fitting. I measure the diameter of the new pistons on several positions and directions and then slowly heat them up to 250°C (this is the maximum temp. the kitchen oven can deliver, but I suggest to wait 'til the better half is absent to avoid discussions similar to the one that occurs when you get caught cleaning cycle parts in the dish washer
) and let them cool down, I do this 3 or 4 times. When finally cooled down I measure again and check for any irregularities (residual deformations, visible cracks, etc.).
This way you have a chance of seeing the results of geometric irregularities or material imperfections and reduce residual stresses from the casting process.
Speaking of the piston weight: All pistons for the A7/A10 range I have seen so far are significantly heavier than the Hepolites, which in turn were slightly heavier than the original BSA pistons as well. The Wisecos I fitted to the A10 aren't really light-weight either, I think they they were around 4 or 5 grams heavier than the GPMs, if my memory serves me right.
Cheers, Markus
