Decided to delete the cat today. I have a 91 with a 305 Tbi. I went to take off the cat because it’s clogged and I was just going to straight pipe it. ( I know the emissions law and everything) but still going to do it. Anyways I noticed there’s a small tube going into the cat as well coming from the header it looks like. What can I do with that tube? Would it effect anything if I block it off or just leave it open? Thanks! I’ll get some pics up in a bit.

 

It injects clean air into the exhaust. If you cap it off, or leave it open, it won't do anything bad.

 

I usually have it welded into the exhaust. I Don't think plugging it would be good if your smog pump is pumping air through it.

 

Sounds good thanks guys. I left it open for now until I removed the smog pump/ a.i.r. box components. Does anyone know what size the threads are on the holes on the headers? I want to plug them up.

 

I WANT to say that its 3/8" NPT. Hard to remember exactly since I did mine 12 years ago lol.
What I'd do is take it all off, then cut off one of the fittings and bring it up to the hardware store to match up to be sure. I just used brass plugs on mine and it worked fine.

 

1/2-20 in. inverted flare plug is the correct part for plugging the AIR threads in the manifold. Some people have used 1/4 NPT plugs and it worked since they are also 1/2-20 threads but on a tapered (pipe thread) body. But for the cost of the correct fittings I went ahead and spent the extra couple of dollars. Got mine at the local auto parts store (they had to order them in for the next day since they only had 3 on hand in store). I think they were a like $1.10 a piece.

Rock auto has them for $.65 but unless you are ordering something else, shipping might kill the deal.

https://www.rockauto.com/en/parts/ed...flare+plug,267

 

As stated by others, the fittings are NOT pipe thread. They are 1/2"-20 bolt thread. The tubes use flare fittings, but if you look in the holes, there is no flare to mate with the flare fittings, which is why I just used 1/2"-20 allen head bolts...

 

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