Plugging Big Block Smog Exhaust Manifolds

[sixt9x33rs]

Will the following work on Big Block Smog Exhaust Manifolds too?

#10 x 1/2" flat head stainless machine screws work perfectly as substitutes for the extension tubes; they seal off the hot gases from discoloring the finish on the air manifolds, and are the correct thickness to allow the tube nuts to tighten to the correct dimension, and can't be seen.

Thanks

Lawrence

[JohnZ]

Will the following work on Big Block Smog Exhaust Manifolds too?

#10 x 1/2" flat head stainless machine screws work perfectly as substitutes for the extension tubes; they seal off the hot gases from discoloring the finish on the air manifolds, and are the correct thickness to allow the tube nuts to tighten to the correct dimension, and can't be seen.

Thanks

Lawrence

Yes, it will - the machining of the holes and threads for the A.I.R. tube nuts is the same on both SB and BB exhaust manifolds (I wrote the quote on the machine screws in your post).

[x66 714]

I used clevis pins in mine & put a BB in the vacuum line to the diverter valve. Then gutted the smog pump. Been like that for years no. No problems....Joe

[sixt9x33rs]

Will the following work on Big Block Smog Exhaust Manifolds too?

#10 x 1/2" flat head stainless machine screws work perfectly as substitutes for the extension tubes; they seal off the hot gases from discoloring the finish on the air manifolds, and are the correct thickness to allow the tube nuts to tighten to the correct dimension, and can't be seen.

Thanks

Lawrence

Yes, it will - the machining of the holes and threads for the A.I.R. tube nuts is the same on both SB and BB exhaust manifolds (I wrote the quote on the machine screws in your post).

Yes sir I know that was your quote. That why I posted it

[sixt9x33rs]

John,

I bought the screws as you suggested but they dont screw into any threads. The screws are too narrow. Seems like I need a number  6 or 8 screw? Are they suppose to screw in or are they just suppose to be there to block the gases?

Thanks

Lawrence

[vtfb68]

I have always used brass plugs for the tube holes. ( they come out years later)  I don't think 33 rs is talking about with the smog system in place.
    VT
   

[sixt9x33rs]

I have always used brass plugs for the tube holes. ( they come out years later)  I don't think 33 rs is talking about with the smog system in place.
    VT
   

So you cut threads in the manifolds and screwed them in?

[vtfb68]

They should already be tapped, the smog tubes came out of them. Standard 1/4" size pipe plugs.

   VT

[1968 Z28]

Could be wrong but I think we are talking about filling the inside hole (where the extension seats) with a screw and then screwing the air manifolds into the threaded hole (the flare on the air manifold seats on the screw and the pressure on the screw seals off the exhaust air from coming into the air manifold.) .  If you are not planning on using your air manifold, then using brass plugs to fill the vacant hole is a good choice.

[Charley]

The machine screw John mentioned is just dropped in and then you re install the smog tube. Or put a plug in but the thread is not pipe thread. I forget the thread size but putting a pipe threaded plug in could screw up the threads. Someone will chime in with the correct thread.

[JohnZ]

The machine screw John mentioned is just dropped in and then you re install the smog tube. Or put a plug in but the thread is not pipe thread. I forget the thread size but putting a pipe threaded plug in could screw up the threads. Someone will chime in with the correct thread.

The threads in the manifold for the tube nuts is 1/4"-18 NPSF (National STRAIGHT Pipe Thread), NOT 1/4" NPT (tapered pipe thread). Rick's used to carry the correct straight pipe thread plugs if you're not using the air manifold tubes, or Grainger's, Fastenal, or one of the other industrial suppliers will have them.

The flat head machine screws don't thread into anything - they just replace the extension tubes and seal the opening when you tighten the tube nuts.

[sixt9x33rs]

The machine screw John mentioned is just dropped in and then you re install the smog tube. Or put a plug in but the thread is not pipe thread. I forget the thread size but putting a pipe threaded plug in could screw up the threads. Someone will chime in with the correct thread.

The threads in the manifold for the tube nuts is 1/4"-18 NPSF (National STRAIGHT Pipe Thread), NOT 1/4" NPT (tapered pipe thread). Rick's used to carry the correct straight pipe thread plugs if you're not using the air manifold tubes, or Grainger's, Fastenal, or one of the other industrial suppliers will have them.

The flat head machine screws don't thread into anything - they just replace the extension tubes and seal the opening when you tighten the tube nuts.

I will be using the air manifold tubes so I now understand the use for the flat machine screws.

[169INDY]

Same Idea but custom cut on a lathe.
I have had NO issues with these.

Chop the (a portion of the threaded shank of) screw and after removing the A.I.R. Tube place the remnants of the screw into the counter sunk portion of the hole. The AIR tube tree nuts should capture the screw in place.

Jim

[169INDY]

another series of pics:

As Others have said. The thread is a Straight tap size. I special ordered-purchased a new tap from Fastenal.

The Old nuts can twist right clean off and then you have some real work to do to extract.
Use a flare nut wrench if possible. My tubes were rusted 80k mile orig. They were rusted out shells. I chose to cut the tubes flush with the nut and use a 6 point socket and lots of penetrating fluid and heat to no avail. Twisted clean off. That required me to set up to drill out the nuts and carefully remove the remaining thread. Worked up to just shy of the tap size and fine work with dremel and picks. Easy-outs need not apply. It can be a DIY if you take some time and have access to some special tooling.

Jim