Does anyone know for sure the thread size for the AIR (Air pump system) tubes that screw into the stock exhaust manifolds on a 1987 Formula 350 engine? It looks very close to a US thread 1/2x20 but it might be metric.
I am going to remove the AIR pipes and want to install plugs in the exhaust manifolds for obvious reasons Thanks.

 

1/4" NPT , any hardware store . good luck .

 

Thanks for your reply Tom. I tried a 1/4" NPT and it does engage a few threads and may work if I really want to crank them into the cast manifold. My concerns is that I don't want to risk cracking the manifolds (at least not right now) due to the extreame heat/cool cycles. The AIR tube system has a flared end on the tube that goes into the manifold, like a brake line fitting. I also measured the diameter of the nut at each end of the threaded section and there is no taper. These 2 things lead me to the conclusion it is not a tapered pipe fitting. If I find the proper size, I will post it here.

 

 

They're 5/16 inverted flare, which has 1/2-20 straight threads. 1/4 NPT also has 1/2-20 threads but they're tapered.

Unless you could find a 5/6 I.F. plug or machine a bolt to put the seat in it, a pipe plug is probably the quick and easy way to go. Plugs to exist, though. Dorman has one, I believe.

 

NOT THIS, IT IS WRONG

This is correct:

Yes plugs do indeed exist. I have a car sitting in my driveway with 8 of them installed in it. Of course they've been in there for about 30 years now, and inverted flare stuff in general isn't as commonplace as it once was; so it may take a bit of effort to track em down.