70 LT-1 carb base gasket

[Dennis D.]

Was this the correct carb base gasket for non california 70 LT-1's?

[Joe L.]

Re: 70 LT-1 carb base gasket

Was this the correct carb base gasket for non california 70 LT-1's?

Dennis------


No, this is an insulator-type gasket and was not used for 1970 LT-1 without EEC. The original gasket was GM #3881847 which was discontinued in December, 1973 and replaced by GM #682914 which was discontinued without supersession in October, 1984. These gaskets had the same general configuration as the gasket you picture but were only about 3/32" to 1/8" thick.

Dr. Rebuild has excellent reproductions.

[Dennis D.]

Re: 70 LT-1 carb base gasket

Thanks joe. The only gasket I found on dr R was this one and the one with the shield. Will check again

[Joe L.]

Re: 70 LT-1 carb base gasket

Thanks joe. The only gasket I found on dr R was this one and the one with the shield. Will check again

Dennis------


Dr. Rebuild #3726122.

[Terry M.]

Re: 70 LT-1 carb base gasket

I am not so sure Joe. The gasket that came on my 1970 LT1 sure looks a lot like the picture Dennis posted.

[Joe L.]

Re: 70 LT-1 carb base gasket

I am not so sure Joe. The gasket that came on my 1970 LT1 sure looks a lot like the picture Dennis posted.

Terry------


If your car originally had a gasket like the one pictured, then it was not originally equipped with the gasket it was supposed to have. The gasket used for 1970 LT-1 without EEC was the same as that used for 1966 L72 and 1967-69 L-88 as well as the still-born 1970 LS-7. GM #3881847. It was a gasket as I described.

The gasket Dennis pictured is the style used for 1971-72 LT-1 all of which, of course, were equipped with EEC. Is it possible that some 1970 LT-1 were also equipped with this gasket? Yes, I suppose it's possible. But, it's not the gasket they were scheduled to have according to GM.

[Terry M.]

Re: 70 LT-1 carb base gasket

I had the opportunity to examine known original gaskets installed on low-mileage Corvettes. I thought the 1971-72 gasket had plastic inserts in the bolt holes to limit the compression of the gasket, whereas the 1970 does not. Someplace I have thickness measurements of the gasket/spacer/insulator from my 1970, but due to the aforementioned compression the area around the bolt holes is less than the center of the length and width. Now I will have to search my files for those dimensions. One of those gaskets had the part number stamped into it, but I can not now remember which, nor can I remember that number. I didn't try to remember all that because of the difficulty of judging all those details.

[Gary B.]

Re: 70 LT-1 carb base gasket

That's the gasket being made by Richard Fortier. He used Terry's gasket as a sample. Terry's car is 21xx VIN. He also used a gasket off a 5,000-mile 70 LT1, Vin 57xx, March built car. I sent Richard both to use as a sample. It's very close, he did a very good job. They measure roughly 7/32" thick. My 70, VIN 145xx, July built, uses the same gasket. It's a 39k mile car. It's also the gasket that was on Ed Foss's 2100 mile 70 LT1. Didn't keep a record of that VIN, wish I had....

Late 71's and all of 72's used a gasket with black plastic insulators in them. Fig. M 5.23 in the current 70-72 TIM&JG.

This is what I have seen on low-mile, original, unrestored cars. It probably differs than what GM service manuals calls for. And not trying to start an argument with Joe. Just what I have observed....

Gary Bosselman

[Dennis D.]

Re: 70 LT-1 carb base gasket

That's the gasket being made by Richard Fortier. He used Terry's gasket as a sample. Terry's car is 21xx VIN. He also used a gasket off a 5,000-mile 70 LT1, Vin 57xx, March built car. I sent Richard both to use as a sample. It's very close, he did a very good job. They measure roughly 7/32&quot; thick. My 70, VIN 145xx, July built, uses the same gasket. It's a 39k mile car. It's also the gasket that was on Ed Foss's 2100 mile 70 LT1. Didn't keep a record of that VIN, wish I had....<br />
<br />
Late 71's and all of 72's used a gasket with black plastic insulators in them. Fig. M 5.23 in the current 70-72 TIM&amp;JG.<br />
<br />
This is what I have seen on low-mile, original, unrestored cars. It probably differs than what GM service manuals calls for. And not trying to start an argument with Joe. Just what I have observed....<br />
<br />
Gary Bosselman

Gary. Are you saying that the insulator gasket WAS used on 70 non Calif LT-1's?

[Gary B.]

Re: 70 LT-1 carb base gasket

Yes. The only cars I have seen the thin gaskets on, are the California LT1's with the aluminum plate sandwiched with the thin gaskets.
But, I've not seen all 70 LT1's. This is Terry's gasket:

20171201_145707.jpg

[Terry M.]

Re: 70 LT-1 carb base gasket

Gary was able to find that information before I did. Thank you Gary.

[Dennis D.]

Re: 70 LT-1 carb base gasket

Yes. The only cars I have seen the thin gaskets on, are the California LT1's with the aluminum plate sandwiched with the thin gaskets.
But, I've not seen all 70 LT1's. This is Terry's gasket:

[ATTACH=CONFIG]98930[/ATTACH]

So possibly thin gaskets with heat shield EEC cars.
Thick gasket otherwise.

Would that effect other elements associated with the carb, such as the choke rod length/configuration?

[Terry M.]

Re: 70 LT-1 carb base gasket

So possibly thin gaskets with heat shield EEC cars.
Thick gasket otherwise.

Would that effect other elements associated with the carb, such as the choke rod length/configuration?

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.

[Joe L.]

Re: 70 LT-1 carb base gasket

So possibly thin gaskets with heat shield EEC cars.
Thick gasket otherwise.

Would that effect other elements associated with the carb, such as the choke rod length/configuration?

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.


DSCN3688.jpgDSCN3689.jpgDSCN3687.jpg

[Joe L.]

Re: 70 LT-1 carb base gasket

Gm #3999198:


[ATTACH=CONFIG]98941[/ATTACH][ATTACH=CONFIG]98942[/ATTACH]