core which makes it ideal for saltwater marine use. Compressed thickness is 0.039". ... core which makes it ideal for saltwater marine use. Compressed thickness is 0.039". B ... stems, single groove design, hardened tips, used on ZZ454. 12366987 2.25" 11/32" ... stems, single groove design, hardened tips, used on ZZ502 and ZZ572. 3879618 2.30" ... need to be reworked for this valve. Exhaust Valves 3946077 1.88" 3/8" Stock ... stems, single groove design, hardened tips, used on ZZ454. 88963128 1.88" 11/32" ... stems, single groove design, hardened tips, used on ZZ502 and ZZ572. CHEVY BIG- ...