What's the deal with exhaust manifolds having the same part, with or w/o A.I.R. smog?

[71CHEVYSHORTBED402]

I have a 1971 BBC, but the same issue may apply to SBC. At any rate:

According to GM's parts list, printed 1979, 3994045 LH is not for A.I.R (i.e., smog). But if you look online, you'll find the 3994045 part number on manifolds with the four port holes while others are without the four port holes. What the heck, same part number for both? My manifold is without.

Same issue with the 353030 RH, only vise versa. Per the book these are supposed to be for A.I.R. Some have the four port holes while others don't. My manifold is without. That said, what's the small hole for located on the 353030?..... For all I know it's just on some of them. It's located right above the "RH" mark. See the picture below for a reference. Maybe it's to mount a heat shield? The hole is plugged up on mine.

Note: I don't have A.I.R. I believe they were put to use in 1972. I imagine it varied by state.

Incidentally, there's little information on the web for 333846 RH ("not for A.I.R.") and 343787 LH ("for A.I.R."). The 3994045 LH ("not for A.I.R.) and 353030 ("for A.I.R.) are far more common, which of course are the one's I have.




Another reference. GM's parts book, 1979: http://www.gmpartswiki.com/getpage?pageid=21508

[davischevy]

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[71CHEVYSHORTBED402]

Here's a 3994045. You know, the one "not for A.I.R."? Don't know why the 30s have a ridge that runs along the side, but the 45s don't. I'm unable to locate the other two parts (333846 RH / 343787 LH) for comparisons. No pictures to be found.

http://www.ebay.ca/itm/1973-75-Chevy...item51d4ea5493


Maybe the guy responsible for sporting part numbers on exhaust manifolds got creative. I could be way off base.

[burb71]

It seems to me that they cast the manifolds, then just modified "drilled"some for A.I.R.
As to say they weren't cast with the A.I.R. ports in them.

[71CHEVYSHORTBED402]

Originally Posted by burb71

It seems to me that they cast the manifolds, then just modified "drilled"some for A.I.R.
As to say they weren't cast with the A.I.R. ports in them.

Could be. Only issue with that is the parts manual. Granted they could be replacement parts, but that doesn't explain why "3994045 = not A.I.R." etc.

[71CHEVYSHORTBED402]

???

[MagmaJct]

I can't answer your specific question, but I can give a general answer.

Anything a part manufacturer can do to reduce their inventory, they will do it. Why maintain two or more part numbers that are actually very universal?

I replaced a radiator on my wife's friend's Kia. Her Kia has a manual transmission. But the radiator I got came with the transmission cooler in it. I still worked.

My passenger exhaust manifold developed a crack on my 350 SBC. I got a new one on eBay, and it had the AIR ports drilled and tapped. I just bought cast plugs and plugged them.

I don't believe the '71 SBC utilized the AIR system. (Referring to my '71.) It was not uncommon for auto manufacturers to install emissions control devices for a year or so until they could "tune" their engine design to accomplish the same emission reduction goal mandated by the EPA or CARB without the device. When the EPA or CARB would lower emissions standards again, those same emissions control devices got put back on, again, until they could "tune" their engine design again.

Regards,
Mike

[71CHEVYSHORTBED402]

Makes sense Mike.

Any original big blocks out there with the original manifolds? What are the part numbers? Are they set up for A.I.R? Do you have A.I.R.?

I was looking to replace my 353030 RH (Not A.I.R.) to match the casting for my 3994045 LH (not A.I.R.). I've yet to see a RH to match. Seems if I really wanted to do this, I'd have to buy repro.

[SS Tim]

Service parts in general, running changes, vendor changes, inventory depopulation are but a few reasons. You are reading a snapshot catalog from 1979. It will not show you parts histories after that point.

As a point and case, in the Parts History Catalog 87A December 1981
333846 was discontinued and advised use of 353030 as a replacement.

A 76 catalog shows 333846 superceed 3994046.

As I have said repeatedly trying to find the exact production part usage with a service parts catalog is torturous and even then a service is often not the same as a production part. If you are not looking at a 1972 published catalog consider the number a "will fit". In this case it looks like the 353030 is a 454 manifold being backfitted. It makes sense and helps depopulate the part numbers as old stock is used up.

If you are looking for a specific P/N manifold I'd suggest posting a WTB in the classifed section being as clear as possible as to what you expect to see.

[71CHEVYSHORTBED402]

Thanks Tim. Definitely understand chasing down the exact part numbers can be chasing your tail. Sometimes it's just good enough to be close.

It's just curious they have the same part with or w/o A.I.R. ports (and why the hole above the RH mark). As for the 353030 being backfitted, I had the assumption it was original, and the LH was replaced with the 3994045. Been a long while since the engine was rebuilt, I don't remember. But you guys might be right the RH was replaced with a 353030. Some came with ports some not. Same applies to the 3994045.

Be interesting to hear what's on original 71 402s though. I do have a thread for chasing down a manifold(s), hopefully without ports.