427 smog hole threads

[Roger G.]

What size are the threads in the smog hole 427 exhaust manifolds, they don't match the thread chasers I have. Thanks

[Dick W.]

Re: 427 smog hole threads

They are tubing thread, either 5/16 or 3/8. It is a straight thread, not tapered like IPT (pipe)

[Roger G.]

Re: 427 smog hole threads

They are tubing thread, either 5/16 or 3/8. It is a straight thread, not tapered like IPT (pipe)

thanks, they're not NPT?

[Dick W.]

Re: 427 smog hole threads

thanks, they're not NPT?

Nope, not IPT or NPT but either 3/8X24 or 7/16X20 thread. You will need a bottoming or plug tap to clean the threads, a regular tap will not get all the threads.

[Roger G.]

Re: 427 smog hole threads

Thanks Dick

[John H.]

Re: 427 smog hole threads

thanks, they're not NPT?

Roger -

No, they're not. They're 1/4"-18 NPSF (National Straight Pipe Thread).

[Dick W.]

Re: 427 smog hole threads

Roger -

No, they're not. They're 1/4"-18 NPSF (National Straight Pipe Thread).

Oops! Yer right, got off my a** and went and looked at the tap pin. Sorry 'bout that

[Roger G.]

Re: 427 smog hole threads

Roger -

No, they're not. They're 1/4"-18 NPSF (National Straight Pipe Thread).

Oops! Yer right, got off my a** and went and looked at the tap pin. Sorry 'bout that

Thanks guys, I ordered a tap today. I picked up a set of big block manifolds that look almost perfect, no pitting, but the holes were plugged. After removing the plugs which only went about 1/2 way in, the threads below the plugs are either burned out or full of carbon, or both.

I don't know if running the manifold with plugs that are short will cause the threads to disintegrate, sure hope not

[James G.]

Re: 427 smog hole threads

What year manifold? 67's are rare as only California Corvettes had to have a Smog pump. My 390hp 427 Powerglide, air conditioned convertible car was on of 38 with K-19.

[Roger G.]

Re: 427 smog hole threads

What year manifold? 67's are rare as only California Corvettes had to have a Smog pump. My 390hp 427 Powerglide, air conditioned convertible car was on of 38 with K-19.

Jim, they are 69s,although the car was purchased in California... Ventura to be exact... Bet you know it

[Terry M.]

Re: 427 smog hole threads

Thanks guys, I ordered a tap today. I picked up a set of big block manifolds that look almost perfect, no pitting, but the holes were plugged. After removing the plugs which only went about 1/2 way in, the threads below the plugs are either burned out or full of carbon, or both.

I don't know if running the manifold with plugs that are short will cause the threads to disintegrate, sure hope not

The plugs going only a couple of threads in is exactly what happens when one tries to put NPT (National Pipe Thread) plugs into those straight NSPT tapped holes. NSPT plugs are not so commonly available and many don't know the difference. The exhaust gasses usually don't come out, so who cares. Then there are the tubes that are inside that hole to direct the injected air at the exhaust port. Are yours there, or do you care?

[Roger G.]

Re: 427 smog hole threads

The plugs going only a couple of threads in is exactly what happens when one tries to put NPT (National Pipe Thread) plugs into those straight NSPT tapped holes. NSPT plugs are not so commonly available and many don't know the difference. The exhaust gasses usually don't come out, so who cares. Then there are the tubes that are inside that hole to direct the injected air at the exhaust port. Are yours there, or do you care?

Terry...my current setup is plugged manifolds, with no smog. The manifolds on the car are badly pitted. I picked up a nice set to put back on the car. I also found a complete date correct smog system.

The manifolds I picked up also have plugs. Those are the ones I plan to retap and restore.

[Wayne M.]

Re: 427 smog hole threads

The plugs going only a couple of threads in is exactly what happens when one tries to put NPT (National Pipe Thread) plugs into those straight NSPT tapped holes. NSPT plugs are not so commonly available and many don't know the difference. The exhaust gasses usually don't come out, so who cares. Then there are the tubes that are inside that hole to direct the injected air at the exhaust port. Are yours there, or do you care?

Terry -- your post encouraged me to drag out a RH K19 manifold (off an early '68, I believe; G_9_7) that I've been tripping over for about 20 years, and to remove the plugs. What a job; they were brass, and (yes ) NPT, so they just engaged the first 2 threads. The others are filled with carbon, but I hope a 1/4"-18 NSPT thread chaser might clean them up, if I can find one. As I understand the GM catalogs, the A.I.R. pipe "assembly" was exactly that (LH & RH), so the fittings (or NSPT plugs) are not available separately, and probably not reproduced.

About the tubes (nozzles): Seems to be a section of tubing that protrudes into the exhaust gases. Notice in pics that the wall thickness has eroded somewhat compared to the upstream end (pressed into the manifold casting ?).

[Joe L.]

Re: 427 smog hole threads

Terry -- your post encouraged me to drag out a RH K19 manifold (off an early '68, I believe; G_9_7) that I've been tripping over for about 20 years, and to remove the plugs. What a job; they were brass, and (yes ) NPT, so they just engaged the first 2 threads. The others are filled with carbon, but I hope a 1/4"-18 NSPT thread chaser might clean them up, if I can find one. As I understand the GM catalogs, the A.I.R. pipe "assembly" was exactly that (LH & RH), so the fittings (or NSPT plugs) are not available separately, and probably not reproduced.

About the tubes (nozzles): Seems to be a section of tubing that protrudes into the exhaust gases. Notice in pics that the wall thickness has eroded somewhat compared to the upstream end (pressed into the manifold casting ?).

Wayne-----


An NSPT tap is available from McMaster-Carr and is quite modest in cost.

Internal square drive NSPT plugs (like those originally used for some 1970 big block applications) are also available from McMaster-Carr (or, at least, used to be available as I once purchased some from them). Internal hex drive NSPT plugs are definitely available.

I don't know if the NSPT tubing nuts are available but it does not really matter. Here's why:If one tries to repair a single tube on one of the AIR pipe assemblies, it is necessary to slightly shorten that tube (i.e. remove flare and re-flare). When this is done, it will be very difficult, if not impossible, to connect, with the other 3 tubes connected, this tube to the manifold. In order for this to work, one would have to remove all of the tubes, shorten all of them equally, install the tubing nuts, and re-flare. I might also add that the type of flare utilized for these tubes is necessary and VERY difficult to duplicate. The reason that one shortened tube won't work is that all of the tubes must be FULLY SEATED on the extensions (i.e. the inner tubes) or there will be an exhaust leak. The NSPT tubing nuts will NOT seal by themselves.

As far as the tubing nuts go, even if they are not available, one could use tubing nuts salvaged from scrap tube assemblies. All of the tube assemblies, big block and small block, Corvette and otherwise, use the same tubing nuts.

[Terry M.]

Re: 427 smog hole threads

Terry -- your post encouraged me to drag out a RH K19 manifold (off an early '68, I believe; G_9_7) that I've been tripping over for about 20 years, and to remove the plugs. What a job; they were brass, and (yes ) NPT, so they just engaged the first 2 threads. The others are filled with carbon, but I hope a 1/4"-18 NSPT thread chaser might clean them up, if I can find one. As I understand the GM catalogs, the A.I.R. pipe "assembly" was exactly that (LH & RH), so the fittings (or NSPT plugs) are not available separately, and probably not reproduced.

About the tubes (nozzles): Seems to be a section of tubing that protrudes into the exhaust gases. Notice in pics that the wall thickness has eroded somewhat compared to the upstream end (pressed into the manifold casting ?).

Wayne,
The exhaust extension (Thank you Joe for the proper terminology.) shown in your second photo appears to be shorter than original (although the angel of the photo makes this assessment deceiving). I would expect that because long term operation without the benefit of cooling injected air will deteriorate the extensions. These extensions were originally lose in the tapped holes and the tubing nuts and AIR pipe held them in place. When new those extensions would just fall out when the tubing nut was removed. I can not tell you how I know without receiving hoots & jeers, but it has something to do with exhaust headers on a almost brand new LT1. Carbon and heat induced corrosion have no doubt locked what remains of yours in place. I believe those extensions are made of a heat resistant material, but there are limits to the effectiveness of that material.