70 LT-1 carb base gasket

[Joe L.]

Re: 70 LT-1 carb base gasket

Gm #3969835


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[Joe L.]

Re: 70 LT-1 carb base gasket

Since the choke rod is adjusted by bending, it would simply need to be bent to accommodate whatever thickness gasket/insulator is installed. No need for a different choke rod. The fuel line is long enough to be bent easily. I believe everything else (vacuum lines and accelerator cable) is flexible enough to self-adjust for the different thickness.

Terry------


Yes, most choke rods can be adjusted by bending. However, in PRODUCTION I believe that a specific choke rod was used for each application. I do not think that assembly line workers were charged with the task of adjusting them.

[Dennis D.]

Re: 70 LT-1 carb base gasket

The information and observations are amazing.

With that said ,I wonder about the practical application. Had the thick insulating gasket been used for heat issues from the EEC system? As Joe mentions, why then the thick gasket on 70 non EEC?

I have heard of many heat percolating and such issues on 70 LT1's. Mine in fact has the fuel block and lines foil wrapped as well as the metal insulator.(no thick gasket though). Lot of heat prevention. Not a california car.

Needless to say, I've undone several issues contributing to that problem.

I'm just wondering if the thick insulating gasket is installed, the problem goes away.

[Joe L.]

Re: 70 LT-1 carb base gasket

The information and observations are amazing.

With that said ,I wonder about the practical application. Had the thick insulating gasket been used for heat issues from the EEC system? As Joe mentions, why then the thick gasket on 70 non EEC?

I have heard of many heat percolating and such issues on 70 LT1's. Mine in fact has the fuel block and lines foil wrapped as well as the metal insulator.(no thick gasket though). Lot of heat prevention. Not a california car.

Needless to say, I've undone several issues contributing to that problem.

I'm just wondering if the thick insulating gasket is installed, the problem goes away.

Dennis------


The primary purpose of the thick insulating gasket was not to help ameliorate heating caused by the EEC system as this system causes no heat to be generated. The purpose was to help prevent heat from causing gasoline vapors to emanate from the carburetor.

I believe the thick gasket is a benefit on any carbureted car, EEC or not. However, it could cause problems when operating in very cold climate conditions.

[Ron G.]

[Patrick H.]

Re: 70 LT-1 carb base gasket

Didn't keep a record of that VIN, wish I had....

Gary Bosselman

194370S412824

https://www.mecum.com/lots/CA0815-219701/1970-chevrolet-corvette-lt1-coupe/

[Terry M.]

Re: 70 LT-1 carb base gasket

Terry------


Yes, most choke rods can be adjusted by bending. However, in PRODUCTION I believe that a specific choke rod was used for each application. I do not think that assembly line workers were charged with the task of adjusting them.

That makes some kind of sense, but I have never seen different part numbers for different choke rods. Perhaps there were PRODUCTION only examples.

[Joe L.]

Re: 70 LT-1 carb base gasket

That makes some kind of sense, but I have never seen different part numbers for different choke rods. Perhaps there were PRODUCTION only examples.

Terry-------


Oh, there were LOTS of them. From 1966, the beginning of the divorced choke era, through 1974, the end of the divorced choke era, there were no less than NINETEEN different auto choke rods used on Corvettes. ALL were once available in SERVICE. None are anymore.

By the way, The Doc has excellent reproductions of many of them.

[Patrick H.]

Re: 70 LT-1 carb base gasket

By the way, The Doc has excellent reproductions of many of them.

Unfortunately he's closed at present, with no scheduled or predicted time of reopening.

[Terry M.]

Re: 70 LT-1 carb base gasket

Terry-------


Oh, there were LOTS of them. From 1966, the beginning of the divorced choke era, through 1974, the end of the divorced choke era, there were no less than NINETEEN different auto choke rods used on Corvettes. ALL were once available in SERVICE. None are anymore.

By the way, The Doc has excellent reproductions of many of them.

I should have spent more time on my response to Patrick. I knew there were many (I didn't know as many as 19), but I didn't know there were many for 1970 LT1. How many were there for 1970LT1?

[Joe L.]

Re: 70 LT-1 carb base gasket

I should have spent more time on my response to Patrick. I knew there were many (I didn't know as many as 19), but I didn't know there were many for 1970 LT1. How many were there for 1970LT1?

Terry------

There were 3 for 1970 LT-1:

GM #3917616-------1970 LT-1 with EEC [this one could not have been used for with EEC application as it was the same one used for 1967-69 Z-28; this one must have been for "except EEC". And, it must have been used for the "thin" carb gasket as there's no way that 67-69 Z-28 used the thick gasket as it did not even exist then]. Discontinued without supersession March, 1997.

GM #3970094-------1970 LT-1 with EEC. Discontinued June, 1970 and replaced by GM #3917616 [see above]

GM #3972124-------1970 LT-1 except EEC. Discontinued without supersession in 1980.

By the way, while there were 19 rods used for Corvettes, some of these were also used for other GM applications. However, there were also many other rods used for other GM applications that were not used on Corvettes. So, in total, there were a lot more than 19.

[Timothy B.]

Re: 70 LT-1 carb base gasket

Dennis------


Well. empirical information is always the best providing that it's validated with a sufficient number of observations. That seems to be the case in the matter of this gasket. But, there is kind of a complicated history here.

In the VAST majority of cases, cars are built the way that GM says they should be built. I would say this is true about 99.8% of the time. This situation might qualify as falling into that 0.2%. The PRODUCTION information is a bit "sketchy" for 1970 and the SERVICE information (which is, of course, derived from PRODUCTION specifications) is a bit "sketchy", too.

For 1970 without EEC (NA-9) the AIM does not specify a carb gasket for the LT-1. It does specify a gasket for the still born LS-7 which, presumably, should have used the same gasket as LT-1. For the LS-7 without EEC it specifies the gasket as GM #3881847.

For 1970 with EEC the AIM specifies the carb gasket for LT-1 as GM #3972156. I do not have an example of this gasket. However, it was replaced in December, 1971 by GM #3999198. The latter is a thick, insulating gasket very similar to those pictured in the aforementioned photos and in my attached photos. I believe the 3972156 was identical or nearly identical. The 3999198 did not have plastic compression control spacers at the bolt holes and I highly doubt the 3972156 did, either. For the still-born LS-7 with EEC the gasket specified was GM #3969835. The latter is a square bore "sandwich" type gasket with fiber gaskets on either side between an expansive aluminum plate. While this is NOT the EEC gasket specified in the AIM for LT-1, it could have been used for LT-1 and, I understand, has been the gasket observed on 1970 LT-1's with EEC.

As I mentioned, SERVICE parts information is also "sketchy" for 1970. The 1970 edition specifies the following (with my added comments in[]):

1970 Corvette (E.E.C. w/Sp.H/Per) (350)------------3972156

1970 Corvette (H.D. w/E.E.C.) (454)----------------3969835

1970 Corvette (E.E.C. w/Sp H/Per 350, H.D. 454)---3881847 [I believe this is an error and should have said "except E.E.C." as this gasket could not have been used for EEC applications].

The last edition of the P&A catalog specifies the following (presumably after errors have been corrected and supersessions applied. My added comments in[]:

70 Y (350) W/SP.H/PER (except E.E.C.)-----------682914 [this is the square bore gasket which superseded the 3881847 in December, 1973]

70-71 Y (350) W/E.E.C., W SP. H/PER)-----------3999198 [previous 3972156]

Of course, we know that all 1971 LT-1 were equipped with EEC so we know that the thick, insulating gasket was used for 1971. But, from what's been observed, 1970 LT-1 with E.E.C were equipped with the 3969835, even though that's not specified in either the AIM or the P&A catalog.

The thing I do not understand is this: why would 1970 LT-1 without E.E.C. have been equipped with the thick, insulating gasket? This gasket type was never used for any other non-EEC applications that I am aware of. So, "logically", the gasket used for non EEC 1970 applications would have been the 3881847, just as used for most other 1966-69 square bore, Holley applications and just as GM specified in the P&A catalogs. But, that does not mean it was. This could be one of those 0.2% situations. They don't arise very often, but they do arise.

By the way, 1971 LT-1's were specified in both AIM and P&A catalogs as using the GM #3989390 insulating gasket. This gasket was discontinued in July, 1972 and replaced by the GM #3999198 which was discontinued without supersession in May, 1991.

1972 LT-1's were specified as using the GM #3999259 insulating gasket. This is the gasket that has the compression controlling spacers at the bolt holes. It never replaced the 3972156/3989390/3999198 for SERVICE nor did the latter gaskets ever replace the 3999259 for SERVICE. The 3999259 was discontinued without supersession in December, 1984.

The GM #3881847 is 0.0625" thick and the GM #3999198 is 0.260" thick. The GM #3869835 has a gasket of 0.020" thickness on the carb side, 0.158" thickness on the manifold side, and a total thickness with plate of 0.270". Attached are photos of NOS examples of the GM #3881847, GM #3999198, and GM #3969835.


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I am trying to follow this post and FWIW, I wanted to add that I have a GM major carburetor kit # 3979474 that has a base gasket exactly like the 3881847. It measures with my calipers approx. .075-.080 depending on how hard you try and compress, I would post a picture but no need except for the GM and what looks like a S (or some design) embossed in the gasket.

I don't know about the late sixties Holley carburetors but the earlier carbs had vapor vents in the venturi to perk vapor to atmosphere. Maybe the thicker gasket like this even without EEC helped isolate the heat from the carburetor bottom when the vents were eliminated.