1971 LT-1 Carburetor Mounting Bolts Length??????

[Paul C.]

I had my Holley carb rebuilt and when reattaching to the intake manifold the mechanic said he thought the bolts where too long. He wants to cut them off and shorten them. I told him don't do anything until I check with the NCRS experts. Exactly how much should the carb mounting bolts (studs) extend out from the intake manifold? The carb and manifold all are correct numbers. Am getting it judged this year--I hope--don't want to screw this up.

Thanks Paul

[Terry M.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

I had my Holley carb rebuilt and when reattaching to the intake manifold the mechanic said he thought the bolts where too long. He wants to cut them off and shorten them. I told him don't do anything until I check with the NCRS experts. Exactly how much should the carb mounting bolts (studs) extend out from the intake manifold? The carb and manifold all are correct numbers. Am getting it judged this year--I hope--don't want to screw this up.

Thanks Paul

Paul

About this long.

9B1 acc pump(1).jpg9B1 fig 6 better.jpg

Please note the unthreaded portion at the top of the stud and the configuration of the nuts. Good luck to you and your mechanic.

[Paul C.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

Terry:

Thank you sooo much for the help and pictures. I relayed the message--do not cut the bolts.

Paul

[David L.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

I was doing some research today on the carb to intake studs for a 1970 Z28 and according to my 1970 Chev. Parts Catalog (Oct. 1970) and my 1972 Corvette Parts Catalog the 1970-1972 Corvettes w/SHP 350 & 454 use the same studs, GM # 3967475 in Gr. 3.288 (5/16"-18-24 X 1 7/8"). I have 3 studs that measure 1.82" O.L. in the photo below which I believe are GM # 3967475 even though they are slightly shorter (0.055").

0.08"+/- tapered end
0.50"+/- exposed threads (12 threads, estimated from Terry's photo)
0.28"+/- nut (washer head type)
0.44" Holley carb thickness @ hole
0.02"+/- gasket thickness
0.50"+/- depth into intake (5/16"-18 threads)
----------------------------------------------
1.82"+/- total overall length, very close to theoretical length of 1 7/8" (1.875")

Dave

[Terry M.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

I was doing some research today on the carb to intake studs for a 1970 Z28 and according to my 1970 Chev. Parts Catalog (Oct. 1970) and my 1972 Corvette Parts Catalog the 1970-1972 Corvettes w/SHP 350 & 454 use the same studs, GM # 3967475 in Gr. 3.288 (5/16"-18-24 X 1 7/8"). I have 3 studs that measure 1.82" O.L. in the photo below which I believe are GM # 3967475 even though they are slightly shorter (0.055").

0.08"+/- tapered end
0.50"+/- exposed threads (12 threads, estimated from Terry's photo)
0.28"+/- nut (washer head type)
0.44" Holley carb thickness @ hole
0.02"+/- gasket thickness
0.50"+/- depth into intake (5/16"-18 threads)
----------------------------------------------
1.82"+/- total overall length, very close to theoretical length of 1 7/8" (1.875")

Dave

Just a guess on my part, but I think my base gasket is more than 0.020 thick. I am not sure of an easy way to measure it without taking the carburetor off. I might do that some day, but I am not prepared to do that soon. Otherwise your numbers seem pretty good to me.

DSCN5962.jpgDSCN5961.jpg

[David L.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

Terry,

The gasket thickness was just a wild guess and I guess I was off quite a bit. I actually measured the base thickness on my 3367 Holley and a 5/16"-24 washer head nut. The 5/16"-18 threads measure 0.5" on the 3 carb studs in my photo.

Why do you think the 1 7/8" studs were used. It seems like the 1 1/2" studs would work.

Dave

[Roy V.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

David, my 72 has a thick heavy type of dense cardboard type of material used as a gasket in addition to the thickness of the carburetor base (see attached photo)
Did the '71 LT-1 have them as well?


Regards
Roy

[Joe L.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

David, my 72 has a thick heavy type of dense cardboard type of material used as a gasket in addition to the thickness of the carburetor base (see attached photo)
Did the '71 LT-1 have them as well?


Regards
Roy

Roy-----


Yes, they did but the insulating gaskets were not the same for 1971 and 1972. 1971 used GM #3989390 whereas 1972 used GM #3999250 or 3999259. How were the gaskets used for 1971 and 1972 different? I've forgotten but I don't think it was in any way that could be discerned with the carburetor mounted on the engine.

[Jim T.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

Paul

About this long.

[ATTACH=CONFIG]40706[/ATTACH][ATTACH=CONFIG]40707[/ATTACH]

Please note the unthreaded portion at the top of the stud and the configuration of the nuts. Good luck to you and your mechanic.

Terry do you know how many Holley equipped Corvettes used the same nuts in your picture?

[Terry M.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

Terry,

The gasket thickness was just a wild guess and I guess I was off quite a bit. I actually measured the base thickness on my 3367 Holley and a 5/16"-24 washer head nut. The 5/16"-18 threads measure 0.5" on the 3 carb studs in my photo.

Why do you think the 1 7/8" studs were used. It seems like the 1 1/2" studs would work.

Dave

I have no clue why the length used was chosen. As you can see from the earlier photos and the OPs question, there are a lot of exposed threads and one might easily think the studs too long. I don't have a 1970-72 AIM handy. Were the studs installed at the vehicle assembly plant? I would guess at Winters foundry, but I am not sure without looking. Perhaps the equipment used to install them dictated a certain length. All that is guesswork on my part, however.

[Terry M.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

Terry do you know how many Holley equipped Corvettes used the same nuts in your picture?

For 1970 1312, for 1971 1957, and for 1972 1761. What you didn't want a wise a$$ answer? OK then for 1970-72 ALL of them.

All the LT1s would be my serious answer.

[Joe L.]

Re: 1971 LT-1 Carburetor Mounting Bolts Length??????

I have no clue why the length used was chosen. As you can see from the earlier photos and the OPs question, there are a lot of exposed threads and one might easily think the studs too long. I don't have a 1970-72 AIM handy. Were the studs installed at the vehicle assembly plant? I would guess at Winters foundry, but I am not sure without looking. Perhaps the equipment used to install them dictated a certain length. All that is guesswork on my part, however.

Terry-----


Carburetor studs were installed at the engine plants.