i put some ls2 rods and pistons in my lq4 block with a 5.3 crank are the pistons suppose to hit the head with out the head gasket

 

Yes, ls motors all have pistons that protrude from the deck surface of the block. That's why you don't want to run too thin of a head gasket as you risk the piston hitting the head at high RPM's as the rod stretches slightly.

 

so stock head gasket .051 should be fine

 

Measure your piston to deck height, then choose your gasket from there for optimal quench/compression.