'72 smog pump elbow gaskets?

[Tom L.]

I got my smog pump rebuilt by Bill Hodell, beautiful job as others have noted. When I asked about gaskets he said that there were 2 different ones. The elbow to diverter valve #3942534 and the elbow to the pump gasket. He only has reproductions of the 3942534. Anyone have an NOS elbow to pump gasket (Or both) that they could send me a picture of with dimensions? Bills repro's appear to have been made using a laser cutter which I use at work and should be able to make these. Would be nice to be able to reproduce these since no one else seems to. Thanks!

[Joe L.]

Re: '72 smog pump elbow gaskets?

I got my smog pump rebuilt by Bill Hodell, beautiful job as others have noted. When I asked about gaskets he said that there were 2 different ones. The elbow to diverter valve #3942534 and the elbow to the pump gasket. He only has reproductions of the 3942534. Anyone have an NOS elbow to pump gasket (Or both) that they could send me a picture of with dimensions? Bills repro's appear to have been made using a laser cutter which I use at work and should be able to make these. Would be nice to be able to reproduce these since no one else seems to. Thanks!

Tom-------


The reason why the "other" gasket is not reproduced is because it never existed. Parts that never existed are rarely reproduced.

The GM # 3942534 gasket was used at both ends of the diverter valve elbow. That is, to the pump and to the diverter valve. I can tell you this, for certain: the elbow used for 1969 and 1972 Corvettes was the same. On my 1969 both original and later SERVICE examples of the gasket were identical for both positions.

[Tom L.]

Re: '72 smog pump elbow gaskets?

I have no reason to dispute this aside from what Bill H. told me. He told me that for '72 -'74 big blocks he usually supplied customers with 2 of the 3942534 gaskets if the system is inoperable and 1- 3942534 gasket and one used gasket for customers with operable systems. I had asked for a picture but he said he could send a tracing of one, not sure if that will materialize.

I searched the AIM for references to the gasket and only found a reference on a sheet for the LT-1 option. Not the base, LS5 (What I'm referencing and should have mentioned it) or the "Cancelled" LS6 pages. perhaps there is a difference for LS5? Here's a pic of the elbow on my system, also not noted in the AIM that I could find.

[Leonard M.]

Re: '72 smog pump elbow gaskets?

I would take what Joe said to the bank. Tom, I would not hesitate to use the same gaskets on both ends of the elbow.
Saying that 72-74 BB smog pumps used different gaskets is hard to imagine.
That would mean that the worker assembling the elbow to pump would have to know if it were a BB or small block to use the correct gaskets. BB, SB used the same pump, elbow and gaskets.
I examined my original working '72 BB system and both gaskets show to be the same.
I have taken apart quite a few systems and found the gaskets to be universal. I have old gaskets that I managed to save. One might think the bottom gaskets might be those "special" ones, but looking at the elbow imprint, 2 were from round elbows and one from the '72 and later elbow.
The one picture of a round elbow with gasket shows how they may be deformed over time and look different.
From what I have found is that most '72 pumps have square elbows. Also, late '72 pumps will have the black square edge fan.
In most cases, when someone buys one of those "original" systems they most likely get a re-stamped pump, repro fan, repro pulley, repro clamps, repro hoses, repro tubes, repro brackets. So that being the case, a gasket shape that is almost entirely hidden should not be a concern.
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[Joe L.]

Re: '72 smog pump elbow gaskets?

Leonard and Tom------


I appreciate Leonard's expression of confidence in me. However, in this case I was wrong. Sort of. It took me a LONG time and a lot of "digging" to sort this one out

There were THREE different gaskets (in fact, there were FOUR but I'm not going to get into the fourth one here since it's completely irrelevant to the question here).

Here's what I have found: 1968 Corvettes used gasket GM #3918954. This gasket was discontinued in January, 1970 and replaced by the GM #3942534. All 1969-71 Corvettes with AIR used gasket GM #3942534. This gasket was used on both sides of the diverter valve elbow (or, at the diverter valve-to-pump on 1969 big blocks and 1971 LS-6). I do not know, for sure, what gasket was used for 1972. However, I'm very confident that the same gasket was used for both ends of the elbow. I have verified all of the foregoing from both PRODUCTION and SERVICE information and both are consistent.

For 1973-1979 the PRODUCTION gasket used was GM #3917232. This gasket was to be used on both ends of the diverter valve elbow. This gasket was once available in SERVICE but it was never catalogued in SERVICE for a Corvette application. While it was discontinued from SERVICE in June, 1973 and replaced by the preexisting GM #3942534, according to GM it was used in PRODUCTION through 1979. Personally, I kind of doubt it. I expect that the 3942534 was actually used in PRODUCTION for most, if not all, of the 1973-79 period. I do not know what the configuration was of the 3917232 but, since it was replaced by the 3942534, I expect it was very similar and is certainly functionally equivalent. The GM #3942534 was the only gasket EVER cataloged in SERVICE for all 1969-1979 Corvette applications.

A curious thing is that while the GM #3917232 gasket looks to be a part that was released about late 1967 or early 1968, it was apparently not used in PRODUCTION for Corvettes until 1972 or 1973. While it was available in SERVICE until June, 1973 as I mentioned, it was never catalogued in SERVICE for Corvettes or any other Chevrolet application I can find. That means that while it was a Chevrolet Division part number, it was used in PRODUCTION and SERVICE for some other non-Chevrolet GM vehicles from the time of its release. It's very likely the gasket seen in the last row of Leonard's gasket photo.

Every original gasket I have ever seen, be they PRODUCTION or SERVICE examples, have been a light brown, thin, celulosic (paper-like) material as can be seen in all of Leonard's examples.

As far as diverter valve elbows go there were THREE. However, I'm only going to cover TWO here. For 1969-74 small blocks (big blocks did not use elbows for 1969 or 1971 LS-6), the diverter valve elbow was GM #3947728. This is the elbow often referred to as the "round" elbow. 1972-1974 big blocks as well as 1975-79 small blocks used the GM #3951346 elbow. This is the elbow often described as the "square" elbow. The strange thing is that the 3951346 part number appears to be a 1969 released part but I can find NO application for it before 1972. The GM #3951346 was discontinued in April, 1985 and the GM #3947748 was discontinued in March, 2000.

Keep in mind that while the above is the best information I can come up with (and it required a LOT of time to do even this), there could have been variations in PRODUCTION. As I mentioned, both of the gaskets could have been used interchangeably. It's also possible that the elbows might have been able to be used interchangeably for some applications.

[Paul D.]

Re: '72 smog pump elbow gaskets?

Joe, I have no dog in this hunt, but would like to thank you for the extensive research you always provide for the rest of us. Your replies are always educational whatever the subject matter. Again, a big THANK YOU.

[Leonard M.]

Re: '72 smog pump elbow gaskets?

I'm with Paul. I appreciate the obvious time and effort you show in your posts and find all of them interesting, as Paul said, whatever the subject matter is.
One interesting point you made in your second post was regarding the elbows.
I know that my BB 1972 is original and has the "square" elbow. Also, pictures in the J.G. and the Dobbins show '72 BB with square and '72 SB with round. Curious if any Original '72-'74 SB pumps have the square elbow.

[James G.]

Re: '72 smog pump elbow gaskets?

The square (BB) style is closer to flat on the offset, meaning basically on a flat plane, where as the round (SB) style offsets by approx .25" is the only difference I can see in design.
Whether this would interfere with anything if interchanged with one another, well I would have to study the setups.