In 1973 Pontiac started stamping extra numbers (1/4" tall) on their heads.These head numbers represented chamber sizes.They are usually stamped on the machined pads by the valve covers.
Head ID Location
The following are 73-74 heads
4X-1H uses 4.881/4.870" valves
4X-1H are 455 head with 114cc, 2.11/1.66" valves and pressed studs.
4X-3H are 400 4bbl head with manual transmission, 98cc, 2.11/1.66" valves and screw-in studs.
4X-4H are 400 2bbl head, 1.96/1.66" valves and pressed studs.
4X-7H are 400 4bbl with auto trans, 98cc, 2.11/1.66" and screw-in studs.
46 are 350 2bbl/4bbl heads with about 86cc, 1.96/1.66" valves and pressed studs.
4C-5 are 350 with AIR, 1.96/1.66" 86cc, pressed stud.
4C-9 are 400 with AIR, 1.96/1.66", 98 cc, pressed stud.
*All heads cast on or about April, 1973 received threaded studs.
So, take the head above and if they were cast after or about April 73,
the "pressed" applications will be "screw-in".
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5C heads were installed on engines until around mid 1975 then dropped
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The 6X castings started being installed on engines after about 2/21/75
6X-4 350(and 77-79 T/A 6.6), 93cc, 2.11/1.66"
6X-8 400, 101cc, 2.11/1.66"
6H-6 455, 124cc, 2.11/1.66"
6S-7 350 w/AIR, 93cc, 2.11/1.66"
6S-9 400 w/AIR, 101cc 2.11/1.66"
The 6X-4 and 6X-8 were the only heads used after 1976.
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(Let me know if you find any more information on these codes or any not listed.)