Losing coolant at manifold heat riser - Why?

I dont think sealer will work, no. I think really it's your porter out the money. When I go too far, it's my dime. Not the customer's. I'd pull the head, have a different shop take the exh valve out of that port, and mag the area well. If it fails, take the parts back to your guy, and (nicely) ask what he can do. If he was the only guy to work on them, it's his fault. No way around it, no hiding from it. It does happen, and the best thing you can do is work thru it. Worst case, give him the heads back, and get all your money back for them(this, will not happen, but that's worst case) Best case, he agrees it's his fault, finds you a repalcment head, ports it, sets it up with your existing parts, and hands it to you. (that's what should be)