Author Topic: Plugging Big Block Smog Exhaust Manifolds  (Read 2566 times)

sixt9x33rs

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Plugging Big Block Smog Exhaust Manifolds
« on: September 08, 2013, 01:56:19 PM »
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
'69 RS Z/28 Olympic Gold, 711
Flat hood no spoiler, black top, endura, 4:10
POP 39,000
69 X77 Z/28 69 711 Original Paint Unrestored
'69 X66 Convertible Cortez Silver 712 black top Endura, auto, bumper guards, am/fm rear speaker 44k miles

JohnZ

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #1 on: September 08, 2013, 05:29:29 PM »
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).
'69 Z/28
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x66 714

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #2 on: September 08, 2013, 06:04:10 PM »
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
See America's First, Chevrolet

1968 Z/28 Corvette Bronze. Black Hounds Tooth. 02E Los Angeles born 3/13/1968
1969 SS396 Yellow/Yellow 08E Norwood born 8/28/1969

sixt9x33rs

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #3 on: September 08, 2013, 10:53:13 PM »
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 :)
'69 RS Z/28 Olympic Gold, 711
Flat hood no spoiler, black top, endura, 4:10
POP 39,000
69 X77 Z/28 69 711 Original Paint Unrestored
'69 X66 Convertible Cortez Silver 712 black top Endura, auto, bumper guards, am/fm rear speaker 44k miles

sixt9x33rs

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #4 on: September 09, 2013, 10:13:40 PM »
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
'69 RS Z/28 Olympic Gold, 711
Flat hood no spoiler, black top, endura, 4:10
POP 39,000
69 X77 Z/28 69 711 Original Paint Unrestored
'69 X66 Convertible Cortez Silver 712 black top Endura, auto, bumper guards, am/fm rear speaker 44k miles

vtfb68

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #5 on: September 09, 2013, 11:13:07 PM »
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
   
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08E LA RS Y2 749 L30 M35
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sixt9x33rs

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #6 on: September 10, 2013, 01:14:43 AM »
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?
'69 RS Z/28 Olympic Gold, 711
Flat hood no spoiler, black top, endura, 4:10
POP 39,000
69 X77 Z/28 69 711 Original Paint Unrestored
'69 X66 Convertible Cortez Silver 712 black top Endura, auto, bumper guards, am/fm rear speaker 44k miles

vtfb68

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #7 on: September 10, 2013, 02:01:42 PM »
They should already be tapped, the smog tubes came out of them. Standard 1/4" size pipe plugs.

   VT
05C LA RS/SS U2 712 L34 M21 BR
08E LA RS Y2 749 L30 M35
"In the pursuit of accuracy"

1968 Z28

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #8 on: September 10, 2013, 02:15:35 PM »
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.
Jerry G.

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Charley

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #9 on: September 10, 2013, 02:59:29 PM »
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

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #10 on: September 10, 2013, 03:30:06 PM »
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.
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sixt9x33rs

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #11 on: September 10, 2013, 08:36:01 PM »
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.
'69 RS Z/28 Olympic Gold, 711
Flat hood no spoiler, black top, endura, 4:10
POP 39,000
69 X77 Z/28 69 711 Original Paint Unrestored
'69 X66 Convertible Cortez Silver 712 black top Endura, auto, bumper guards, am/fm rear speaker 44k miles

169INDY

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #12 on: September 13, 2013, 03:33:27 AM »
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
Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS

169INDY

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Re: Plugging Big Block Smog Exhaust Manifolds
« Reply #13 on: September 13, 2013, 03:58:07 AM »
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
Jim
68 SS/RS L35 Th-400 LOS
69 Pace Car L48 Th-350 LOS