When i called arc i talked to jody & after explaining that i ordered a 90mm stroker piston he told me they don't carry 90mm just the 88mm
Sounds like a headache....i would call ARC and get my money back for the piston, then find another 90mm piston somewhere else that is correct for your build, there are other suppliers out there like Vegaskarts, Havoc Racing, and others. Also, there are motorcycle shops that sell 90mm pistons that can and do work in these engines as long as you order the correct compression height, make sure it has the correct wrist pin size that your rod uses, and get the rings to go with the piston
I have a customer wanting a GX390-460 If someone has a good price on one let me know