69 A.I.R manifold thread size

[Greg L.]

I'm trying to clean out the holes in my exhaust manifolds so that the new A.I.R pipes will thread into them. They have been run for years with pipe thread plugs filling the holes so the threads are quite full of rust, carbon or whatever.

I had planned on just running a tap through them but I'm having a heck of a time finding a tap. The pitch is 18 and I thought it would be a regular tap but today when I went to purchase a tap I was told it was pipe thread. I had the actual tube assy with me and the guy selling the tap said you could clearly see the "B" nuts had a pipe thread taper to them...I sure couldn't see it though!

I'm pretty sure that it is not pipe thread so I didn't buy the tap but now I'm wondering if the exhaust manifold is in fact pipe thread. If it is a "normal" thread then where would I be able to get a proper tap to chase the threads OR what is the best way to clean out these holes?

Thanks guys

[Terry M.]

Re: 69 A.I.R manifold thread size

Gerg,
You got it right -- they are NOT pipe thread. I think you will need what is called a bottoming tap. It does not have the usual taper at the beginning. As to the size -- others will have to help with that.

[Lyle24961]

Re: 69 A.I.R manifold thread size

Terry
I have used a pipe tap on those threads and have had no poblems. GM may have used a pipe tap with no taper (straight) originaly if 69 is the same as a 72-73 trees.

Lyle

[Dick W.]

Re: 69 A.I.R manifold thread size

Greg, it is a standard straight thread. Compare it to a bolt of the same size, find you a bottoming or plug tap, and proceed to clean out the threads.

[Terry M.]

Re: 69 A.I.R manifold thread size

I know pipe threads will go in a turn or two -- that is why one usually finds pipe plugs in them when the trees are removed, but the proper thread has no taper, and if one can find that kind of tap it will go in farther. There is also an extension, held in by the fitting on the tree, in the hole. If those were not removed when the trees were removed, they might block the tap from going full depth. Removing those extensions, or more likely their remains, with the manifold installed on the engine might be problematic if the extensions were left in when the trees were removed.

[John H.]

Re: 69 A.I.R manifold thread size

Those holes are 1/4" STRAIGHT pipe threads; although they show as 1/4"-18 BPSF threads on the drawings, they're the same as a conventional 1/2"-20 thread. You can get rid of the SS injection tubes and plug the holes by using a #10 x 1/2" flat head machine screw in place of the tube, then screw in the air manifolds and the tube nuts will torque down to exactly the same depth as they did with the tubes in place (assuming you've gutted the pump).

[Greg L.]

Re: 69 A.I.R manifold thread size

Thanks guys. I didn't think they were pipe thread but when a "reputable" source tells you otherwise it made me wonder.

I'm still at a loss on what the best way to clean out these threads are seeing as there doesn't seem to be such a thing as a straight tap of this size with an 18 pitch. The manifolds are off so I might try soaking them in carb cleaner to loosen up any carbon. I was at Pick a Part today and grabbed some A.I.R tube "B" nuts to try and work them into the old threads. At least I won't ruin my new trees if this doesn't work out too well.