Diverter Valve Muffler?

[Henry J.]

I have come across part number 3938790 which is held out to be a 68 Corvette diverter valve muffler. I cannot find this number in any of the resources I have in hand, but I can find 3918966 [68 AIM] and its replacement 1492575 [Oct. 70, Corvette P&A, Gr. 3.672].

I would be very pleased to learn whether 3938790 was a valid part number and, if so, what the part was and its application.

Thank you, in advance, for your consideration.

[Edward B.]

Re: Diverter Valve Muffler?

Henry, 3938790 comes up as being used on the 68 Camaro, 68 Chevelle, and 69 Nova and 69 full size cars.

Ed

[Joe L.]

Re: Diverter Valve Muffler?

Henry, 3938790 comes up as being used on the 68 Camaro, 68 Chevelle, and 69 Nova and 69 full size cars.

Ed

Ed and Henry------


I can't find that the 3938790 was ever used or cataloged for the 1968 applications but it's possible it was used. It was definitely used for the 1969 Nova and full size Chevrolet applications.

In any event, there were 3 different part numbered muffler assemblies. First was the GM #3918966 used for all 1968 Corvette applications as well as most other 1968 application. It was also used for 1969 L-88/ZL-1. It was replaced for SERVICE by the GM #1492575.

The GM #3938790 was used for some 1969 applications and may also have been used for some 1968 applications.

All three of the above-referenced mufflers were virtually identical. I never could understand why there were different part numbers. However, the 3918966, a Chevrolet part number, and the 1492575, another GM divisional part number, could actually have been different part numbers for the same part. Presumably, the 3938790 differs in some way but I'll be damned if I can figure out what it is.

The 1968 model year was a year for which virtually all GM applications using AIR and diverter valves, used diverter valve mufflers which were attached to the valve with a spring clip. After 1968, most GM applications used diverter valve mufflers that were "crimped on" or, no muffler, at all. As far as Corvettes go, all 1969 Corvette applications used diverter valves that were "crimped on" EXCEPT L-88/ZL-1. 1969 L-88/ZL-1 used the same valve as 1968 with the spring clip attached muffler.

[Edward B.]

Re: Diverter Valve Muffler?

Joe, I was going by the P&A so it may have been used in 1968 as a service replacement. The 1968 Camaro and 1968 Nova AIM's don't show the part number for the muffler, and all the optional engines state "Same as production", so there's no info on what part number was supplied at the factory.

Ed

[Henry J.]

Re: Diverter Valve Muffler?

Joe:

The 3918966 muffler, shown below, is the one-piece "early" muffler for 68's. This would be the muffler that was "staked" to the diverter valve, rendering it non-serviceable. I have no idea what's inside this muffler.

Asm1.jpg

Chevrolet Service News (December 1967) makes clear that the 3918966 was replaced by the "later" 1492575 muffler, shown below. The 1492575, when used in conjunction with the 7035403 adapter tube, made for a field-serviceable muffler arrangement for the diverter valve. Chevrolet Service News (February 1968) confirmed that this change was made in production after approximately January 15, 1968.

Pic2.jpg
Pic3.jpg

The spare muffler, of unknown number, that I own (not pictured) does not have a square-mesh screen inside, just like the 1492575 pictured above. Interestingly, the muffler shown below, also of unknown number, clearly does.

Pic5.jpg

Perhaps this is the difference between the 3938790 and the 1492575.

[Joe L.]

Re: Diverter Valve Muffler?

Joe:

The 3918966 muffler, shown below, is the one-piece "early" muffler for 68's. This would be the muffler that was "staked" to the diverter valve, rendering it non-serviceable. I have no idea what's inside this muffler.

[ATTACH=CONFIG]70042[/ATTACH]

Chevrolet Service News (December 1967) makes clear that the 3918966 was replaced by the "later" 1492575 muffler, shown below. The 1492575, when used in conjunction with the 7035403 adapter tube, made for a field-serviceable muffler arrangement for the diverter valve. Chevrolet Service News (February 1968) confirmed that this change was made in production after approximately January 15, 1968.

[ATTACH=CONFIG]70035[/ATTACH]
[ATTACH=CONFIG]70039[/ATTACH]

The spare muffler, of unknown number, that I own (not pictured) does not have a square-mesh screen inside, just like the 1492575 pictured above. Interestingly, the muffler shown below, also of unknown number, clearly does.

[ATTACH=CONFIG]70036[/ATTACH]

Perhaps this is the difference between the 3938790 and the 1492575.

Henry------


Very interesting. I did not realize that any 1968 was originally equipped with a crimped-on diverter valve muffler. I've never actually seen one. Thus, I just assumed that the 3918966 was a spring clip-retained muffler. I have NOS examples of the 1492575 and the 3938790. I know those are both spring clip-retained mufflers. One of these days I'll have to dig them out and see if I can discern the differences between them. I cannot find that either ever replaced the other for SERVICE.

The thing that's really strange about this is that Chevrolet apparently decided to ditch the crimped-on muffler in early 68 in favor of the spring clip-retained muffler, even to the extent of the added complexity of the needs-to-be-added adapter tube. Then, for 1969 most applications went to back to a crimped on muffler (albeit of a different crimped-on design) which was never even available separately from the valve in SERVICE.

By the way, where did you get the photo of the 3918966? I've never seen this view of one.

[Henry J.]

Re: Diverter Valve Muffler?

Joe:

I got the photo of the 3918966 from an e-bay ad for an "early" 68 muffler. [Yes, I know that's dangerous.] I had never seen one before, so I kept it for my on-going research on A.I.R. hardware/systems.

I would like to see a picture of a known-correct 7035403 adapter tube. I've searched, but never found one so I can't compare with the tube on the diverter valve of my February-built 68 BB.

Here's what I THINK happened at St. Louis during 68 production: All diverter valves, for both SB and BB applications, came to assembly simply with an open aperture for the installation of the muffler. In early production, the one-piece 3918966 was attached by "staking" the spigot in place, just as the AIM depicts. Circa mid-January, the 1492575 appears. This muffler requires the 7035403 adapter tube as it doesn't have a spigot. The adapter tube is "staked" into position in the diverter valve and then the muffler is attached using the horseshoe clip. This was a running change, arising from serviceability considerations, but the 68 AIM was never revised to reflect it.

Can I prove any of this? No. I've inferred all of it based on my reading of Chevrolet literature.

[Edward B.]

Re: Diverter Valve Muffler?

I agree that the 3918966 muffler was "early" for 1968 because I purchased an early to mid 1968 smog pump (dated the 353rd day of 1967) a few months ago locally, and although the muffler was missing, the "hole" for it is in the divertver valve (see picture). Note that this is a 28296 "DC" diverter valve (see attached).

Also, from my Camaro research, the 3918966 "press-on" muffler was used from approx August, 1967 through December, 1967. It was then superseded by the "clip-on" 1492575 muffler. I don't have an exact date when the "clip-on" was discontinued, but it was superseded by the "crimp-on" muffler in late 1968. Like Joe indicated, more than likely it never saw 1968 production and was only used for the 1969 model year.

Ed

[Henry J.]

Re: Diverter Valve Muffler?

Ed:

Thank you for the additional information and the excellent pictures of your diverter valve. Nice catch on that pump/valve!

[Joe L.]

Re: Diverter Valve Muffler?

I agree that the 3918966 muffler was "early" for 1968 because I purchased an early to mid 1968 smog pump (dated the 353rd day of 1967) a few months ago locally, and although the muffler was missing, the "hole" for it is in the divertver valve (see picture). Note that this is a 28296 "DC" diverter valve (see attached).

Also, from my Camaro research, the 3918966 "press-on" muffler was used from approx August, 1967 through December, 1967. It was then superseded by the "clip-on" 1492575 muffler. I don't have an exact date when the "clip-on" was discontinued, but it was superseded by the "crimp-on" muffler in late 1968. Like Joe indicated, more than likely it never saw 1968 production and was only used for the 1969 model year.

Ed

Ed------

The early 1968 valve apparently had the 3918966 muffler STAKED to the valve body from what I see in the AIM. So, the valve you have pictured should have stake marks seen on the flat surfaces on either side of the orifice. Presumably, the same staking would need to have been performed to retain the 7035403 tube.

The muffler shown in your third photo is the style used for most 1969 and later valves until the time that the mufflers were eliminated. I suppose it might also have been used for late 1968 but I have nothing that would indicate that. In any event, this style muffler was never available separately from GM in SERVICE. It was supplied with the valve and, apparently, if one decided they needed a new muffler, one would have to buy a new valve.

[Edward B.]

Re: Diverter Valve Muffler?

Boy Joe. My eyes are hardly what they used to be, but I'm not sure I see any stake marks on the valve. There's "something" at the 12 o'clock position (see attached), but I'm not sure it's a stake mark.

Ed

[Joe L.]

Re: Diverter Valve Muffler?

Boy Joe. My eyes are hardly what they used to be, but I'm not sure I see any stake marks on the valve. There's "something" at the 12 o'clock position (see attached), but I'm not sure it's a stake mark.

Ed

Ed------


I think the stake marks, if present, should be at the 3 o'clock and 9 o'clock positions. This valve looks like there is a scuff mark on the lower perimeter of the orifice indicating that a muffler or tube was once installed. There is no way I can see that the muffler or tube could have been an interference fit in the valve since there would have been no way to press them in.

If present, I think the stake marks should be quite prominent and on the sides of the boss that the orifice is machined into. I cannot see how any staking on the top could have been accomplished with the muffler in place.

[Edward B.]

Re: Diverter Valve Muffler?

Joe, I don't see anything at all at the 3 or 9 o'clock positions even though the AIM note shows "To be staked securely (2) places at asm" (see attached).

The only thing I see is what appears to be a casting "nub" at the 12 o'clock position (which we've already discussed), and the moon shaped "scuff mark" (actually, a bit of a "gouge") at the 5 to 9 o'clock position that you noted.

Ed

[Henry J.]

Re: Diverter Valve Muffler?

Ed and Joe:

I did notice the same apparent lack of stake marks that Joe observed. This caused me to examine the diverter valve on my car.

On my valve, there are no stake marks either, nor is there the casting nub as on Ed's. My 7035403 adapter tube is simply a very poor friction fit to the valve. Also, the font for the number 28296 is smaller, I think, than that on Ed's.

I note that my Corvette P&A Catalogs clearly state that the service replacement valve for this application (7036447) is stamped "28296". Perhaps the valve on my car is a service replacement. Perhaps the intended staking operation was never carried out.

[Edward B.]

Re: Diverter Valve Muffler?

Henry, the production diverter valve for your car (if it's a 427), was 7028296 (stamped 28296 on the valve itself). The SERVICE replacement was 7036447 (also stamped 28296 on the valve itself).

If it's a service replacement, more than likely it would have the "clip-on" muffler (HERE'S ONE ON EBAY).

If yours has the "staked-on" muffler, it's more than likely the original and not a service replacement.

Ed