factory, a flattop 4valve relief piston with the 416 heads is 9.5:1, but since it's been rebuilt, there's no telling, cause the heads could have been cut and the wrong thickness head gasket could have been put in. anywhere from 9:1 to over 10.5:1 depending on what's going on....

 

Actually a LG4 car in 1983 would have about 12cc dished pistons in it. The L69 got flat-tops with 6 valve reliefs. My LE9 truck 305 came from GM with the L69 style pistons.

 

yes, fast355 is correct, and the factory lg4's had 8.6:1 c/r, but i figured since your engine had been rebuilt, that it truely had a flattop four-valve relief piston in it.