Thanks checked for air leaks, also this is 3 pistons on 2 different bikes. After rebuilds we going to switch to cam2 and maxima 927.
I used to love bel ray products... Went through 3 pistons in one year on my CR250 using H1R. Same story mixed properly, good air seal, name brand pistons. All machine work done by LA Sleeve (They are good).
I switched to Castor 927 3 years ago, and haven't siezed a piston since.
My current bikes are a 2010 YZ250, and a 2004 Service Honda CR500AF. Both of them have many hours with no problems.
I would still check the jetting though.