As far as I know, the 485428 were cast starting in late 1970 (December maybe) and were used for all 455s (HOs or otherwise) up through 1974, other than SDs. I know the 483677 castings started in summer of '70 sometime and were definitely used for HOs, but can't find any other casting for that timeframe for non-HOs. I am sure some standard 455s were built prior to the 485428 and after the 1970 carryovers ran out. The reason why I ask is that I found a 483677 block with 4-bolt main&caps but with a YC code (definitely YC, not WC). Seemed odd to me so I figured I would check with others here.
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1968 Firebird 400HO convertible
1971 Trans Am 4-speed, white/blue
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