67 Corvette Trailing Arm Bolts Through the Frame Spacing

[Stephen L.]

[Gary B.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Stevve,

I can see two flat washers under the head. Is it one thick washer and a star washer under the nut?

Gary

[Stephen L.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Stevve,

I can see two flat washers under the head. Is it one thick washer and a star washer under the nut?

Gary

[Thomas N.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Steve:
Thanks for the picture.
It appears there are two washers under the head of the bolt, and on the opposite side a washer, star lockwasher and castellated nut. And the cotter pin barely engages into the slots of the castellated nut. It really clarifies and validates what we are seeing. Thanks so much!

[Gary B.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Steve:
Thanks for the picture.
It appears there are two washers under the head of the bolt, and on the opposite side a washer, star lockwasher and castellated nut. And the cotter pin barely engages into the slots of the castellated nut. It really clarifies and validates what we are seeing. Thanks so much!

It looks like adding a lock washer or another flat washer would improve things some more.

Gary

[Gary B.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Another interesting observation is that the '66 AIM differs from the '67 AIM in regard to this fastener construct. The '67 AIM has a revision dated 3-30-67 that added a lock washer to the outboard side, adjacent to the castellated nut. The lock washer is GM 103328, which is the so-called hi-collar lock washer, which is a heavy duty, thicker lock washer. Given the fact there already is a castellated nut and cotter pin used on that bolt, a lock washer should not be needed for its normal function. Given what we know about the bolt being too long, it would appear that the lock washer was added to serve as a spacer.

Gary

[Mark M.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Thomas, I checked the out to out measurement of the square pads on an original 65 frame. 3.45 and 3.52 inch

[Gary B.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Mark,

I get 3.46" on the driver's side on my '66.

Gary

[Thomas N.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Thanks everyone for the responses. I'm going to plate some thicker washers and put it together to get engagement of the cotter pin with respect to the castellated nut.

[Gary B.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Thomas,

One more question. Are your pivot bolts original? Or repro? Based on my measurements of a bolt that I believe is original, it is about 0.2" longer than the repro bolts. The shorter repro bolt might exist because customers complained that original length bolts were too long. Just guessing.

Gary

[Joe L.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

All------


Below find a photo of a known-original GM #3846867 bolt used for the trailing arm pivot bolt for 1963-76 Corvettes. This bolt was used in conjunction with a GM #3774712 slotted nut. When the nut is installed on the bolt and threaded down to the point at which the cotter pin properly installs per the instructions in UPC 0 of the AIMs, there is a dimension of 3.79" between the base of the bolt head and the clamping surface of the nut. As others have reported, the dimension between the seating plates on the frame is somewhere between 3.45" to 3.52". This dimension should be the same for all 1963-82 Corvettes and I expect that the differences involve either measuring differences or manufacturing tolerances. I'll use 3.47", nominally, for the purposes of this discussion. Subtracting the 3.47" from the 3.79" results in a dimension of 0.32". This is the length that would have to be occupied by some sort of "spacer" if the slotted nut was to be installed as per UPC 0 of the AIMs. However, according to the 1963-66 AIMs, the only "spacers" that were to be installed were 1 GM #3693853 on each side. These washers were 0.09" thick so the two would total only 0.18" of the necessary 0.32". So, during the 1966-67 period, one of two things HAD to have occurred: either the cotter pin was not installed as per UPC 0 OR additional spacing washers were also installed.



For 1967-76 an additional "spacer" was added to the AIMs. This was a GM #103328, a split lock-washer of 0.14" thickness. This would have been marginally sufficient to occupy the aforementioned space and allow the cotter pin to be installed properly on the nut. However, using a split lockwasher in this assembly is a bogus way to provide a spacer in an assembly using a slotted nut and cotter pin. But, that's apparently what was done.

After 15 years, GM finally got this right for the 1977-82 model years. For these model years the GM #3693853 washers and 103328 lockwasher were eliminated and replaced by two GM #475359 flat washers, one on each end of the assembly. These washers are similar to the 3693853 EXCEPT they are 0.14" thick. So, these washers, nominally, occupy the space required. After 1977, GM cataloged these washers to be used for the trailing arm bolt assembly for all C2 and C3 Corvettes.



Another change that took place for 1977 and later was the replacement of the GM #3846867 bolt by the GM #458987. However, as far as I can tell, except for finish and, possibly, a very slightly longer thread length for the 458987, the two bolts are otherwise identical. The GM #458987 also replaced the 3846867 for 1963-76 SERVICE.



All of the above-referenced parts are now GM discontinued.

[Gary B.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

...For 1967-76 an additional "spacer" was added to the AIMs. This was a GM #103328, a split lock-washer of 0.14" thickness. This would have been marginally sufficient to occupy the aforementioned space and allow the cotter pin to be installed properly on the nut. However, using a split lockwasher in this assembly is a bogus way to provide a spacer in an assembly using a slotted nut and cotter pin. But, that's apparently what was done.

...

I have the original pivot bolt hardware that came off my '66, and I can tell you that in addition to a flat washer next to the head and another flat washer on the castle nut side, there indeed was a 0.14 lock washer next to the nut. That completely filled the 0.32" gap. So, the practice of adding a lock washer was happening on the assembly line prior to the '67 model year, even though the lock washer is not shown in the '66 AIM. The assembly line workers must have noticed the gap problem and come up with their own solution.

Gary

[Alan D.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Just for reference, here is an original from my Mar64 car.

[Mark E.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

It's hard to see through the undercoating, but is the nut drawn down so the cotter pin isn't locking the nut?

If yes, then based on Joe's post this seems like a major design and safety defect that GM didn't correct for years.

[Gary B.]

Re: 67 Corvette Trailing Arm Bolts Through the Frame Spacing

Mark,

The cotter pin in Alan's photo is not fully in a slot of the castle nut. It's hanging in the breeze. That is the problem. And yes, I think it was a potential safety concern until resourceful workers on the assembly line started to add an additional spacer, which in the case of multiple unmolested 66s that I've seen, including mine, was a 0.140" thick lock washer. Which as Joe points out, is not the ideal way to fill that space. But it works.

Next, we should talk about that same dog nose bolt, but used at the forward of the two horizontal bolts that attach the tongue/snubber mount to the front of the rear differential. The '66 AIM calls for two 103328 lock washers, each 0.140" thick, one next to the head of the snubber front bolt and one next to the castle nut. In that case, there simply is not enough space to use those two lock washers and still be able to insert the cotter pin.

So, GM was 0 for 2 in terms of accuracy in the AIM with regard to the proper use of that pilot tip bolt with castle nut and cotter pin.

Gary