I keep reading that only certian heads will work for 86 and before and 87 and up?? can anyone tell me what the difference is and why 86 heads wont work on an 87 block?

 

I do believe it had to do with the bolt patterns and angles.

 

86 is a bastard block. 85 and down all blocks had 2 pc rear main seals and flat tappet lifters. the heads used were the basic SB chevy heads - peremeter bolts, non self aligning rocker arms. In 86 they up dated the block design to a 1 pc rear main seal and added bosses in the lifter valley for a roller lifter retention system but still used flat tappet lifters and the "old" head design. In 87 all blocks got the 1pc rear main seal, a cam retention plate and roller lifters with a lifter retention system. Heads now have center bolt valve covers and self alligning rocker arms and the angle on the center intake manifold bolts was changed. so...

I misread your post (kinda) and typed all that garbage in so I am not deleting it : p. the heads are interchangable but rocker arms and valve covers are not. Also an intake manifold for the wrong style of heads will have to be slightly modified to bolt up.

 

so what im trying to get at is could i put 86 heads on a 87 tpi block

 

Yes you can.

You can't put '87 valve covers on '86 heads. In order to put '87 TPI base on '86 heads (or vice versa), you'll have to modify the center two base bolt holes on each side of the base because those bolts are at a different angle.

Self-aligning rocker arms didn't come until 1988.