can you make a 13b header fit on a 81 12a . And if so how thanks

 

You can, but the ports will not line up entirely and you will lose the benefit of the header.

 

Sounds like you've tried it before. Does it bolt up or does it need an adapter plate/modification? I understand it isn't ideal, but would it be better than a stock manifold?

 

the mounting studs are spaced slightly farther apart front to back on the 13b compared to the 12a, making the ports not line up exactly between header and rotor housings. don't rig it, buy a 12a header.

 

thats what i did i just heard of some people cutting it thanks for info

 

that depends: dual exhaust or single exhaust.
if its dual, chop the flange in the rear, and then cut the engine flange in the middle (it requires trimming also) to fit a12-A motor. pretty much, use the 12-A exhaust gasket as a template.

for single exhaust, same as above but it gets a little bit trickier on where you cut off the "wye" at the end of header.