68 L79 Exhaust Manifold Numbers

[Richard D.]

Hey Folks:

In looking at my exhaust manifolds, seems the LH side is OK - 3872765, but the RH side has a number (not sure if these are read correctly)

374703X
or
314708X

I did not see the AIR (missing on this car) threaded holes in either though.

What would the RH one be from (if you can figure out the numbers provided)?

And, if AIR holes are missing from each, is the LH side the "right" one (I would assume not)?

Thanks for any input -

Regards,

Rich

[Joe L.]

Re: 68 L79 Exhaust Manifold Numbers

Rich-----

GM #3747038 is the casting number used for a 2" outlet ram's horn-style manifold used on many passenger cars and Corvettes through about 1965. This manifold was used for many years. It does not have provisions for AIR fittings and it was never originally used for any 1968 Corvette application.

GM #3872765 is the left side exhaust manifold used for 1966-1971 Corvettes with AIR. It has provisions for the AIR tube fittings. It was originally used for ALL 1968 Corvette small blocks.

[Richard D.]

Re: 68 L79 Exhaust Manifold Numbers

Joe, thanks (as usual) for the info.

Yes, I noticed the "provisions" for the AIR on the LH side (bosses?).

Ques. 1 - Are they both 2" outlet? I would assume they are, but if they are not, how does this affect the exhaust balance and performance of the motor?
Ques. 2 - Since the the LH has the provisions, but is not tapped out, then it cannot be "original" to the car? Can it be tapped out and made "right".
Ques 3. - How hard are the correct manifolds to come by, and are repros any good if I have to go that route?

Thanks again for your help,

Rich

[Joe L.]

Re: 68 L79 Exhaust Manifold Numbers

Rich-----

I've never seen a GM #3872765 which was not drilled and tapped for AIR. Apparently, at some point, this casting must have been used in SERVICE for the GM #3846559 finished manifold. Except for the AIR bosses, the manifolds are the same. However, I don't think that any of these untapped 3872765 were ever used in PRODUCTION. I would not rule it out, completely, but I doubt it. I can tell you FOR SURE, though, that NONE were used for 1968 small blocks, ALL of which were originally equipped with AIR. So, if it's untappped, it absolutely can't be original to the car, even though the casting is right.

Theoretically, the casting you have could be drilled and tapped for the AIR fittings. However, it would take a pretty good machinest. It's more than just drilling and tapping. Seats have to be machined and the hole diameters are different on the front and rear of each fitting. It would require drilling a hole for the 7/16" tapping to a certain depth and then drilling a smaller hole for the inner tube (extension which the tubing nut seats on).

Both of the manifolds that you have are 2" outlet.

The correct manifolds that you need are a drilled and tapped 3872765 for the left side and a drilled and tapped GM #3872778 for the right side. Late 1968s may have used GM #3932481 for the right side. The 3932481 were made both with and without AIR tappings, although all 68-69 Corvette versions were drilled and tapped. All of these manifolds are GM-discontinued. I don't know which ones are reproduced, if any are.