'66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

[Gary B.]

[Thomas N.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Gary:
Enclosed is pictures of my 67 rear end with the carrier bracket with the round reinforcements. The bolts are original (I replated). You will see the 3846867 with the castellated nut on the front, and the 193490 bolt without the castellated nut (normal nut) and cotter pin hole. If you need anything else, let me know.

[Gary B.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Tom,

Thanks. Maybe I can just cut off the dog nose on the 2nd bolt I have.

Another odd thing about the front bolt that uses the castellated nut is that the '66 AIM shows it using a lock washer both adjacent to the head and adjacent to the nut. I don't recall ever seeing that done before. I know the lock washer can go against the head or against the nut, but I didn't think two lock washers on the bolt was ever done.

Gary

[Larry M.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Gary:

This was recently discussed on either NCRS or Corvette Forum tech board. This bolt is a bit long for the trailing arm application and cotter pin does not engage the castled nut unless a few thick flat washers or additional lock washers are used as spacers.

https://www.forums.ncrs.org/showthre...-Frame-Spacing

Larry

[Joe L.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Tom,

Thanks. Maybe I can just cut off the dog nose on the 2nd bolt I have.

Another odd thing about the front bolt that uses the castellated nut is that the '66 AIM shows it using a lock washer both adjacent to the head and adjacent to the nut. I don't recall ever seeing that done before. I know the lock washer can go against the head or against the nut, but I didn't think two lock washers on the bolt was ever done.

Gary

Gary------


GM #193490 was never available from GM in SERVICE. For this bolt GM advised "procure locally".

[Gary B.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Thanks all.

Gary

[Gary B.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Gary:

This was recently discussed on either NCRS or Corvette Forum tech board. This bolt is a bit long for the trailing arm application and cotter pin does not engage the castled nut unless a few thick flat washers or additional lock washers are used as spacers.

https://www.forums.ncrs.org/showthre...-Frame-Spacing

Larry

Larry,

Very interesting. That's a problem that I would never have expected that I might have. Time will tell.

Gary

[David B.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

GM #193490 is 7/16-20 x 4 1/4 long zinc

[Joe L.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

All,

I’m working on the rear differential and rear suspension on my ’66 and I have two questions about bolts. The ’66 AIM lists the same GM part number for the trailing arm pivot bolt and the forward bolt for the front mount of the rear differential. That bolt has a bull nose and it takes a cotter pin thru the bull nose along with a castellated nut. Paragon sells a two-bolt kit (1798K-2) that is the identical for the trailing arm pivot location and the front different mount location. But the ’66 AIM shows the two bolts for the front differential mount have different part numbers. The front bolt is GM 3846867, which is indeed the same as the GM part number for the T-arm pivot bolt. However, the aft bolt for the front, rear differential mount does not show a cotter pin in the AIM and the GM part number is 193490. I assume 193490 does not have a bull nose and does not take a cotter pin or use a castellated nut, but I’m just guessing since I’ve never seen an example of the 193490 bolt. Does anyone know how GM 193490 differs from GM 3846867?


Gary

Gary-------


Yes, the forward bolt, GM #3846867, was the same bolt as used for the trailing arm pivot bolt but only for 1963-68 (except 1968 with M-40). For 1968 with M-40 and all 1969-79 the differential forward bracket was revised to include thick reinforcing plates on both sides. So, longer bolts were required. These bolts were the same configuration as the 3846867 but were about 1/2" longer. They were GM #3921658 and, later, GM #458987.

By the way, the 1968 w/M-40 and all 1969-79 bracket, GM #3921657, was far superior to the brackets used earlier.

[Gary B.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Joe,

Do you know the actual lengths of GM 3846867 and GM #3921658/GM #458987?

The length of the Paragon reproduction of GM # 3846867 is 4 7/16", from under the head to the tip of the dog nose. I'm wondering if the Paragon bolt is shorter than GM #3846867.

Gary

[Joe L.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Joe,

Do you know the actual lengths of GM 3846867 and GM #3921658/GM #458987?

The length of the Paragon reproduction of GM # 3846867 is 4 7/16", from under the head to the tip of the dog nose. I'm wondering if the Paragon bolt is shorter than GM #3846867.

Gary

Gary-------


I do not have the actual, MEASURED length of the 3846867 but GM says it is 4-9/16" in length. I will be obtaining one soon, though. I do have NOS GM #458984 which replaced the 3846867 for 63-76 SERVICE and was used in PRODUCTION for 1977-82. GM says that this bolt is also 4-9/16". The NOS measures 4.52"

Except as installed on my car, I do not have a GM #3921658. However, I do have NOS examples of the GM #458987. That bolt measures 4.81".

[Gary B.]

[Alan D.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Gary,
64 uses the same bolt, 3846867, and washers (one under bolt head & one under nut) And yes the pin is outside the nut on both sides.
Unfortunately have not found a method to measure the bolt yet.

[Gary B.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

Alan,

It's interesting that the problem with length seems to have persisted for at least '64, '65, '66 and part of '67.

Gary

[Alan D.]

Re: '66 T-arm pivot bolts vs aft, front diff mount bolt; Different GM part numbers

And the 63AIM also shows same bolt and washer configuration.