Been looking around at cylinder heads, and a lot of them say, "good for 1961-1986 sbc." What did they change that won't allow older heads on a newer block? Does this have something to do with roller/flat tappet cams? Did the change happen all across the board in 1987 or did some model lines retain the older block?

 

the differance is in and after 87 some heads have a differant intake bolt pattern on the center 2 bolts., the ehads would still bolt up to a post 87 block as they would a pre87 block. as for actuall differances in the blocks, provisions for roller cam, one piece rear oil seal, and some have no provisions for a mechanical fuel pump.

 

And along with the other differences Ed mentioned, most '87 and later SBC heads have no guided push rods, so self-aligning rocker arms are used instead. Most of those same heads had center bolt valve covers instead of the older perimeter bolt style. Almost all of the later engines used cast cranks, whereas the older engines had a mix of cast and forged. The change in the erar main seal design in '86 also necessitated a change in the flywheels to accomodate the seal carrier and correctly balance the crank. Almost all the later blocks used the right-side dipstick tube, and used the bolt-on oil filter adapter.