hello, i rebuilt top end of a 91 350l. bored .20 over, wisco piston rings, gasket set. started it up let it run for about 2 mins.rev’d it about 1/4 way and it backfired inside motor and blew the crank seal (on clutch side, didnt have the clutch on it yet) out about 10 feet away. i know i put piston in right way with arrow on piston facing exhaust side. what could have caused this? im at a lose on this and getting highly aggitated with it….