1964 Trailing Arm Bolt description

[Ed S.]

Trying to determine the description of the bolt that secures the front of trailing arm to the chassis on a 64 Corvette. AIM specifies PN 3846867, used with 2 flat washers & a castle nut (and cotter pin).

The bolt that I removed from my car, which may be original, is a grade 5, 4 1/2 inch long, is designed for a castle nut and has a "P" on the head of the bolt. It appears that the original coating on this bolt & nut is black phosphorous, they are clean, not rusted and a lot closer to black than shiny silver.

One vendor sells a zinc plated bolt, that looks correct but has a "TR" on the head, it does not give a length in the parts catalog.

Question is - is my 4.5 inch "P" bolt correct? If not, what is the correct marking for the head? Also, what is the correct finish of the nut & bolt, black ph. or zinc?

[Gene M.]

Re: 1964 Trailing Arm Bolt description

I believe the P bolt you have is correct. The multi sourced item would have other ID head supplier markings. Marks such as TR for Town Robbins. A well known supplier too. As for the finish in the later years the phosphate finish was loosing way in favor of zinc with a follow on heat treatment. Not sure when the change took place but NOS parts started to show up with the zinc finish. Most any car I took apart the trail arm bolts were to rusty to say for sure what the original finish was. Cutting them out was pretty standard practice for me.

Just as a general rule (few exceptions) was a grade 5 or 8 bolt had a phosphate finish to avoid extra process of heat treatment that is required with electroplating.

[Edward J.]

Re: 1964 Trailing Arm Bolt description

Ed I happen to have a couple of original bolts and there are P head markings, and have a couple flat washers and a regular hex nut, not the typical castle nut. and the bolt does have a hole drilled for a cotter pin. plating on both of these bolts were Zinc.

[Ed S.]

Re: 1964 Trailing Arm Bolt description

Gene and Ed,
Thanks for the reply and explanation. For judging purposes, I am getting the impression that unless the JG specifies a certain finish, it could reasonably be one of several finishes, zinc and black ph being the most common. That said, I think I will have the bolts that I have which suspect are original recoated in phosphate.

But Gene you said something else that got my attention - "to avoid the extra process of heat treatment for electroplated parts". Does this mean that if one were to send off fasteners to a commercial plating shop to be electroplated with zinc the fasteners would have to also be heat treated to ensure their integrity? Do commercial plating companies normally do this? Does it apply to DYI plating kits like you can get from Caswell and Eastwood?

[Joe L.]

Re: 1964 Trailing Arm Bolt description

Gene and Ed,
Thanks for the reply and explanation. For judging purposes, I am getting the impression that unless the JG specifies a certain finish, it could reasonably be one of several finishes, zinc and black ph being the most common. That said, I think I will have the bolts that I have which suspect are original recoated in phosphate.

But Gene you said something else that got my attention - "to avoid the extra process of heat treatment for electroplated parts". Does this mean that if one were to send off fasteners to a commercial plating shop to be electroplated with zinc the fasteners would have to also be heat treated to ensure their integrity? Do commercial plating companies normally do this? Does it apply to DYI plating kits like you can get from Caswell and Eastwood?

Ed-------


There were only 2 different part numbered bolts used for the trailing arm pivot. The first was GM #3846867 which was used from 1963 to E1977 (however, there may have been another bolt used for early 1963 that I have no record of). The second was GM #458984 which was used from late 1977 thru 1982. The 3846867 might have specified optional finishes. If so, one manufacturer could have used black phosphate and another zinc plated. The GM #458984, which was also SERVICE for 1963+, was a virtually identical bolt in every respect. I've never figured out why it needed a new part number. I've never seen a 458984 with any finish other than black phosphate, though.

[Larry M.]

Re: 1964 Trailing Arm Bolt description

Ed-------


There were only 2 different part numbered bolts used for the trailing arm pivot. The first was GM #3846867 which was used from 1963 to E1977 (however, there may have been another bolt used for early 1963 that I have no record of). The second was GM #458984 which was used from late 1977 thru 1982. The 3846867 might have specified optional finishes. If so, one manufacturer could have used black phosphate and another zinc plated. The GM #458984, which was also SERVICE for 1963+, was a virtually identical bolt in every respect. I've never figured out why it needed a new part number. I've never seen a 458984 with any finish other than black phosphate, though.

Joe:

Although a minor point, I have been told by Richard Fortier that the plated finish was cad and not zinc during this time. He provided cad plated "P" headmark bolts to me for my March 1967 car. FWIW.

Larry

[Gene M.]

Re: 1964 Trailing Arm Bolt description

Gene and Ed,
Thanks for the reply and explanation. For judging purposes, I am getting the impression that unless the JG specifies a certain finish, it could reasonably be one of several finishes, zinc and black ph being the most common. That said, I think I will have the bolts that I have which suspect are original recoated in phosphate.

But Gene you said something else that got my attention - "to avoid the extra process of heat treatment for electroplated parts". Does this mean that if one were to send off fasteners to a commercial plating shop to be electroplated with zinc the fasteners would have to also be heat treated to ensure their integrity? Do commercial plating companies normally do this? Does it apply to DYI plating kits like you can get from Caswell and Eastwood?

My experience is the baking will not occur unless you ask for it. Time is money.

Also you asked about the trail arm bolt FOR A '64 CORVETTE. The P head and phosphate finish is correct. Others talking about electroplating is not pertainate to 64 model year.

[Larry M.]

Re: 1964 Trailing Arm Bolt description

My experience is the baking will not occur unless you ask for it. Time is money.

Also you asked about the trail arm bolt FOR A '64 CORVETTE. The P head and phosphate finish is correct. Others talking about electroplating is not pertainate to 64 model year.

Gene:

You are likely correct.............but the 1963-64 and 1965 Judging Manuals I have all say electroplated finish (cad or zinc). Same for 1966 and 1967 cars.

But the likelihood of the early C2 cars (63-64) having a phosphate finish is good.

Larry

[Alan D.]

Re: 1964 Trailing Arm Bolt description

Here is an original from a Mar 64 car, does not answer the head marking question but may be of some use.

[Gene M.]

Re: 1964 Trailing Arm Bolt description

Gene:

You are likely correct.............but the 1963-64 and 1965 Judging Manuals I have all say electroplated finish (cad or zinc). Same for 1966 and 1967 cars.

But the likelihood of the early C2 cars (63-64) having a phosphate finish is good.

Larry

I would trust the car over the manual. As you well know there are many, many errors in the 63-64 manual. The 66-67 calling for plated finish is correct for 66-67 not the case with 63-64. In fact if one is lucky enough to have acquired the early issue NOS bolts as Joe points they were the non plated phosphate finish. Later NOS pieces followed the later production line plated finish.