Vet65te

I picked up a pair of 2.5 inch exhaust manifolds yesterday. Both were #901 Left Hand versions but they are in very good shape and $50 for the pair seemed reasonable. The vague story line behind them is that they were bought from a Chevy Dealer about 8 years ago. I thought GM stopped selling these rams horn exhaust manifolds a long time ago so curious as to who actually manufactured them since I thought I read that the Chinese castings available through places like AutoZone don't have any raised casting numbers on them. The casting number on each of these LH manifolds is #3797901 and they do have a large GM casting mark and both have date codes but only one is really prominent enough to decipher. That one seems to show 4-4-00.
Also, one has an additional raised casting mark that resembles a capital 'S' not too different from the Saginaw casting marks I've seen in the past. The pics didn't come out very good but hopefully the marks can be seen. Comments?



Mike T.

62Jeff

Hi Mike,
What is your question?

Jeff

Vet65te

Hi Jeff, yeah, I know, I have a tendency to throw in so many of the details of the story that the question kinda get's buried. It's sorta in the middle of that long explanation...'curious who casts these exhaust manifolds' since I doubt they were made by GM?
Mike T.

62Jeff

Sorry, I see it now.

DZAUTO

GM continued to sell the 3797901 (62-63 left, side facing holes), 3797902 (62-65 right, siding facing holes), 3846563 (64-65 left, FRONT facing holes) until about 3-4yrs ago. It's my understanding that these later manifolds were cast in Mexico under GM contract/license (the S is the logo for the company that cast them). I bought some of each just before they were discontinued about 3-4yrs ago for about $105ea.
For the most part, they are very good castings with the GM and casting numbers on them, EXCEPT, some of them have some moderate to severe grinding marks on the top surface of the manifold, which is a dead givaway that they are later service replacements, not originals.


My only regret about these manifolds is that I didn't buy a bunch of the 563 versions because they are excellent for making a 942 version (shown below). The 942 manifolds were the "plain" manifolds which were ONLY production installed (NEVER available as a service part) and they had NO PROVISION for alternator mounting.






A replacement (NOT a repo) of the 901-902 manifolds are now made in China. They are VERY GOOD mainfolds, but NO casting numbes or GM logo is on them.
Below is a comparrison of the Chinese(top) and GM 2 1/2in manifolds.




Tom Parsons

Vet65te

Thanks for the pics/info. Looking at the #901, if I wind up using both of these LH units and need to remove that bracket extension on one of them, it looks like they might not lend themselves to being 'trimmed' down as cleanly as the #563 version due to that inboard threaded side-facing hole but since that would be the end that's closest to the firewall, it's not going to show that much anyway.
Mike T.

DZAUTO

Mike,
Screw a bolt with enough threads on it so that the bolt will go all the way down to the bottom of the threads, then tighten it as much as possible so that it is thoroughly locked in the hole. After that, cut off the bolt and the boss as well as the bracket, grind them down and HEAVILY blast the manifold with a VERY coarse media to kind of restore the case appearance.
Additionally, after you have it ground down to the desired contour, and if you know someone who can put a weld over the end of the cut off bolt, then you can grind down the weld, blast it and it will be nearly undetectable.

Tom Parsons

Vet65te

Hey Tom, when the time comes to trim off that extension on one of the LH #901 exhaust manifolds, I think I may chicken out and just remove the main part of that extension and leave that one inboard threaded bolt hole alone. While I've been in this area for over 30 years, I personally dont' know of any welders capable of welding on cast iron stuff like this. There may be some around but I don't know of any offhand so I would rather not have to experiment in finding someone that knows how..

Also, I just looked (or rather 'felt') the inside of the manifold in the area of that bolt hole and noticed a 'bump' of sorts that I figured must be for the external bolt hole but oddly at the other end where there is no exterior threaded bolt hole, there is an identical inside bump.
Go figure.
Mike T.

DZAUTO

Mike,
That is EXACTLY correct. ONE basic mold is used for the INTERIOR of an exhaust manifold. That interior mold is "universal", that is, it is used for any manifold regardless of whether it is to have a gen/alt bracket for the left side, right side, front side, or, NO bracket (such as the 942 version). Thus, ALLLLLLLLLL ex manifolds get the "bumps" (or boss if you prefer) on each end inside the manifold. For a manifold where the bump exists but there has not been a hole drilled and tapped, it is possible to use a long shaft carbide rotary file and cut down that bump.
Even the replacement Chinese manifolds got the bumps on each end inside.

Tom Parsons