will p30 oversized pistons fit in the stock bore of a B18C1? Does it just give closer clearance measures?

 

Nope. You need to have it bored to 81.5 mm.

 

whats the benefit? why not just go with regular sized pistons?

 

Bigger pistions = more displacement,
More displacement = more power hp/tq.
all = more $$$

 

do stock pistons work for ctr cams in gsr? then I wouldn't need different pistons. I guess i want to bump compression from 10.1 to 11.1. can i get a smaller head gasket?

 

The difference in bore and subsequent torque numbers from using oversized pistons compared to standard pistons is going to be very negligible. Unless you sleeve and bore out a substantial amount like 84mm, you aren't going to notice any difference.

The main reason people use oversized pistons is so they can slightly bore out the cylinders to correct out-of-round, large scratches that cannot be buffed out with a light hone, and tapered cylinders.

Why not just get a standard bore OEM piston with higher compression ratio?

CTR cams will work fine at the stock 10:1 compression ratio, but a higher compression setup will benefit more from them, of course.

A thinner head gasket isn't the best idea. It will work but it won't increase compression very much to make it worth it and it will be thinner, making it more prone to blowouts. If you are going to do the head gasket, take another hour, drop the oil pan, and swap the pistons instead.