putting engine back together after getting forged pistons and the guy noticed a hair line crack on one of the camshaft caps, can I use JB weld on this?

 

You CAN use JBWeld, but it will fail spectacularly.

 

Needs replacement.

 

A bit of advice, just post things like this in one thread. You have 2 going so it will be confusing to read through all the replies. But no, you cannot use that cam cap like that.

You need a full set of them from the same head as they are all line honed to match, so you cannot just replace the one cap. May be easier to find a used head that is in good shape.