72 LT1 engine mount bolt

[Rick M.]

One of my main engine mount bolts holding it to the frame is longer than the other. My question is should both of them be the same length or did they have different sizes. Thanks in advance for any help.

[David L.]

Re: 72 LT1 engine mount bolt

Rick,

My 1970 AIM lists GM # 399803 as the bolt part number (GM # 3958007 was replaced by GM # 399803 theoretically on 6/7/69).

My 1972 Corvette Parts Catalog (Sept. 1971) lists GM # 399803 in Gr. 0.002 (2 required) for the 1972 Corvette. The size is listed as 7/16"-14 X 4 7/16", 300-M. This bolt is also listed in Gr. 6.509.

Enclosed is a photo of a pair of Corvette engine bolts (assumed to be 399803) with "RSC" head markings, 3 washers (103342) and 2 nuts (9422299).

Dave

[Michael G.]

Re: 72 LT1 engine mount bolt


Nuts are of the "locking" type". Midyear used a similar application.

[Rick M.]

Re: 72 LT1 engine mount bolt

Mine have all that. One is longer than the other. Are they suppose to be the same?

[Michael G.]

Re: 72 LT1 engine mount bolt

Mine have all that. One is longer than the other. Are they suppose to be the same?

Yes, same length. Drivers side bolt will face towards the rear of car.....passenger side bolt will face towards front.

[Joe L.]

Re: 72 LT1 engine mount bolt

Rick,

My 1970 AIM lists GM # 399803 as the bolt part number (GM # 3958007 was replaced by GM # 399803 theoretically on 6/7/69).

My 1972 Corvette Parts Catalog (Sept. 1971) lists GM # 399803 in Gr. 0.002 (2 required) for the 1972 Corvette. The size is listed as 7/16"-14 X 4 7/16", 300-M. This bolt is also listed in Gr. 6.509.

Enclosed is a photo of a pair of Corvette engine bolts (assumed to be 399803) with "RSC" head markings, 3 washers (103342) and 2 nuts (9422299).

Dave

Dave-----

I don't think the L1969-E1970 motor mount bolt, GM #3958007 was ever available in SERVICE.

The GM #399803 was used from mid 1970 through 1982. This bolt was assigned as "group of prime use" to group 6.509 as you mention. This is a group which includes steering gear retaining bolts. In fact, the bolt was cataloged by GM for 1970-82 Corvette steering gear applications. However, this must be some sort of mistake since Corvette steering gear retaining bolts were a round head with square neck type bolt. The 399803 was used for 64+ passenger car, Chevelle, and Vega steering gear mounting, though.

How was the 399803 different from the 3958007? I don't know. However, the 399803 is still available from GM to this very day.

[David L.]

Re: 72 LT1 engine mount bolt

Dave-----

I don't think the L1969-E1970 motor mount bolt, GM #3958007 was ever available in SERVICE.

The GM #399803 was used from mid 1970 through 1982. This bolt was assigned as "group of prime use" to group 6.509 as you mention. This is a group which includes steering gear retaining bolts. In fact, the bolt was cataloged by GM for 1970-82 Corvette steering gear applications. However, this must be some sort of mistake since Corvette steering gear retaining bolts were a round head with square neck type bolt. The 399803 was used for 64+ passenger car, Chevelle, and Vega steering gear mounting, though.

How was the 399803 different from the 3958007? I don't know. However, the 399803 is still available from GM to this very day.

Joe,

I agree. I could not find GM # 3958007 in any of my vintage Chev. parts catalogs or parts history manuals.

I have a pair of NORS 6258154 engine mounts (replacement for 1963-1982) in boxes dated May 1981. Each mount includes the bolt, 2 washers, and nut. The bolt appears to be GM # 399803 (7/16'-14 X 4 7/16", black phosphate coating or something similar, "RSC" marking on head) but the nut is not GM # 9422299 ("distorted" 7/16"-14) but rather a run of the mill 7/16"-14 nut.

Dave