L71 Exhaust Manifolds

[Kerry A.]

The RH exhaust manifold on my 67 L71 3880828 has the "GM" marking. I have noticed that some manifolds have a "GM1" or "GM2". What is the difference? I'm looking to replace my manifold and want to understand the differences before doing so. Thank you.

[Joe L.]

Re: L71 Exhaust Manifolds

The RH exhaust manifold on my 67 L71 3880828 has the "GM" marking. I have noticed that some manifolds have a "GM1" or "GM2". What is the difference? I'm looking to replace my manifold and want to understand the differences before doing so. Thank you.

Kerry------


"GM1" and "GM2" and similar markings are PATTERN NUMBERS. Patterns are what the sand molds are made from. They have no significance as far as originality is concerned. In general, GM had more than one pattern for any cast part. There probably weren't a lot of patterns for the Corvette big block exhaust manifolds since they had very limited application and comparatively few were needed for PRODUCTION requirements.

[Kerry A.]

Re: L71 Exhaust Manifolds

If I understand correctly, as long as the manifold part number and casting date is correct either the GM GM1 or GM2 is acceptable. Thank you Joe.

[Keith B.]

Re: L71 Exhaust Manifolds

What's the reasoning behind changing the manifold

[Kerry A.]

Re: L71 Exhaust Manifolds

I'm in the process of a complete engine rebuild and when the manifolds were removed it was discovered that at some time the RH had been poorly repaired. How it stayed together is probably a testament to not have driven the car very much. After removing the old repair we found the manifold was damaged far worse than originally thought. A LOT of weld build will be necessary including the forming of a new mounting ear where the exhaust connects to the manifold. I'm able to locate original manifolds but either they have the incorrect date or have AIR holes

[Keith B.]

Re: L71 Exhaust Manifolds

As you may know once installed you can not see the date

[Kerry A.]

Re: L71 Exhaust Manifolds

Yep no date judging. I guess I'm too much of a purist and want everything to be right. However, I'm finding out I may want to bite the bullet on this one

[Joe L.]

Re: L71 Exhaust Manifolds

If I understand correctly, as long as the manifold part number and casting date is correct either the GM GM1 or GM2 is acceptable. Thank you Joe.

Kerry-------


But NOT ONLY "GM1" and "GM2". It's possible there were other pattern numbers used during the period.

[Kerry A.]

Re: L71 Exhaust Manifolds

Joe,

If I recall correctly I read an article that referenced GM8. As my original post pointed out I had no idea what the numerical character after the GM represented. So I have learned a lot. I just love this forum.

[Kenneth B.]

Re: L71 Exhaust Manifolds

JOE is correct. There were 2 patterns on a mold plate. Thus pattern 1&2. for each hand. If extra tooling was ordered the next set would be 3&4. This was done so if a casting problem cane up in machining the problem pattern could be fixed or new ones used.