427 smog hole threads

[Joe L.]

Re: 427 smog hole threads

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.

Terry------


Yes, the extension tubes were made of a high nickel steel alloy. When they were last available from GM they carried a GM list price of almost 10 bucks, each. The reproductions are much less but I do not know if they are made from the same material.

The ones in the photo do appear short to me, too. However, these things usually do survive pretty well in the extremely hot environment but they usually become thickly encrusted with some "ceramic-like" exhaust product.

[Roger G.]

Re: 427 smog hole threads

Terry------


Yes, the extension tubes were made of a high nickel steel alloy. When they were last available from GM they carried a GM list price of almost 10 bucks, each. The reproductions are much less but I do not know if they are made from the same material.

The ones in the photo do appear short to me, too. However, these things usually do survive pretty well in the extremely hot environment but they usually become thickly encrusted with some "ceramic-like" exhaust product.

Joe, do you know who repros these? I had good success with the tap, but apparently these tubes need to be in for the smog tube flange to seat properly. My manifolds don't have the tubes, and the smog flange doesn't fully seat, even with the nut screwed down flush

[Dick W.]

Re: 427 smog hole threads

Joe, do you know who repros these? I had good success with the tap, but apparently these tubes need to be in for the smog tube flange to seat properly. My manifolds don't have the tubes, and the smog flange doesn't fully seat, even with the nut screwed down flush

Paragon 1234K $65.00

[Roger G.]

Re: 427 smog hole threads

Paragon 1234K $65.00

Awesome, thanks!

[Joe L.]

Re: 427 smog hole threads

Joe, do you know who repros these? I had good success with the tap, but apparently these tubes need to be in for the smog tube flange to seat properly. My manifolds don't have the tubes, and the smog flange doesn't fully seat, even with the nut screwed down flush

Roger------


Yes, as you've found, the extension tubes need to be installed for the tubing nuts on the AIR pipe assemblies to seal properly. No sealing is provided by the NSPT threads. The sealing is produced by the seating of the tubing nut against the face of the extension flare.

Of course, if one uses NPT plugs, that's a different story. These just jam into the threads in the manifold and, thus, seal.

[Terry M.]

Re: 427 smog hole threads

Roger------


Yes, as you've found, the extension tubes need to be installed for the tubing nuts on the AIR pipe assemblies to seal properly. No sealing is provided by the NSPT threads. The sealing is produced by the seating of the tubing nut against the face of the extension flare.

Of course, if one uses NPT plugs, that's a different story. These just jam into the threads in the manifold and, thus, seal.

And if the NPT plugs are jambed in strongly enough the NSPT threads will be damaged enough that renewing them will be necessary. Fortunately I haven't had to do it, but I think the chances of finding a NSPT Heli-coil or Timesert are pretty slim. I think one could find themselves in a deep hole quickly.

Edit Add: A photo fron Ralph Spears:
air TUBE.png

[Wayne M.]

Re: 427 smog hole threads

Edit Add: A photo fron Ralph Spears:
[ATTACH=CONFIG]51858[/ATTACH]

Per P&A30, this part # 3877484 is the small block "Pipe assy, exh. gas comb.end and ctr. pipe" $0.72 [in Oct.'67]

I'll try and extract the "pipe assy." ( Gr. 3.675, #3880887) from my '68 BB manifold to compare.

[Terry M.]

Re: 427 smog hole threads

Per P&A30, this part # 3877484 is the small block "Pipe assy, exh. gas comb.end and ctr. pipe" $0.72 [in Oct.'67]

I'll try and extract the "pipe assy." ( Gr. 3.675, #3880887) from my '68 BB manifold to compare.

Sorry Wayne, I was unaware there was a difference in the pipes. That difference adds more fun to the hunt.

[Wayne M.]

Re: 427 smog hole threads

Interesting how this thread has evolved, on a subject rarely touched.

Well I extracted the (only remaining) two from my 828 manifold (early '68 MY dates). They are really thin wall, even the 1/4" below the flare that the cast iron shields from the exhaust gases. Have no idea how long these originally were.

[Joe L.]

Re: 427 smog hole threads

Interesting how this thread has evolved, on a subject rarely touched.

Well I extracted the (only remaining) two from my 828 manifold (early '68 MY dates). They are really thin wall, even the 1/4" below the flare that the cast iron shields from the exhaust gases. Have no idea how long these originally were.

Wayne-----


The big block tubes, GM #3882805, were 2-3/4" long. The small block, GM #3877484, 3" long, as pictured above. The 3882805 was discontinued in April, 1993 and replaced by the 3877484 which was also discontinued several years ago.

[Mike T.]

Re: 427 smog hole threads

If using a pair of A.I.R. exhaust manifolds 'without' the smog tubes, and the NSPT plugs won't seal well (by themselves) and the NPT plugs will only seal when the threads jam up, what's the best way to plug the holes? I thought about possibly cutting short a set of the extension tubes and tighten them down with the NSPT plugs but when we recently moved from Northern California to Prescott Arizona, I tossed all the old parts I 'thought' I wouldn't need any more :-(.Mike T.

[Terry M.]

[Mike T.]

Re: 427 smog hole threads

Terry, that's a great idea, hadn't thought about using brake line for the flare. When I get around to adding the 427 to my smallblock 65 Coupe, I'll most likely buy a pair of the Dorman BB exhaust manifolds and I'm pretty sure the only configuration they offer are the ones with the AIR fittings.Thanks, Mike T.

[Joe L.]

Re: 427 smog hole threads

If using a pair of A.I.R. exhaust manifolds 'without' the smog tubes, and the NSPT plugs won't seal well (by themselves) and the NPT plugs will only seal when the threads jam up, what's the best way to plug the holes? I thought about possibly cutting short a set of the extension tubes and tighten them down with the NSPT plugs but when we recently moved from Northern California to Prescott Arizona, I tossed all the old parts I 'thought' I wouldn't need any more :-(.Mike T.

Mike-------


I think if you were to insert a small washer that just fits into the orifice on the manifolds (i.e. just simulates the end of the extension tubes), that should work well. The important thing is that the OD of the washers is right. The ID does not really matter. I would use stainless steel washers.

[John H.]

Re: 427 smog hole threads

If using a pair of A.I.R. exhaust manifolds 'without' the smog tubes, and the NSPT plugs won't seal well (by themselves) and the NPT plugs will only seal when the threads jam up, what's the best way to plug the holes?

Michael -

I've "de-commissioned" a number of these systems over the years (while maintaining their original appearance), and after removing the stainless "extensions", I put a #10 stainless flat-head machine screw in its place and install the air manifold; when you tighten the tube nut, the tapered bottom of the screw head seals to the same flare surface the extension did, and there's no visual clue that it's anything but original (same number of threads exposed, etc.). Of course, the A.I.R. pump is gutted (so it's nothing more than an idler pulley) and there are no hot exhaust gases to damage the finish on the air manifolds or the check valves.