1963 rear end differential cover Bolt attachment for the leaf spring

[Gerald C.]

One of the holes in the rear end cover where the spring attaches is oval. It looks like they tried a helicoil and it wasn't done properly so they just used a smaller diameter bolt with a nut at the end. I need to repair that hole and the helicoil seems to be the right way to go. Before I buy the correct one, am I seeing in the parts book that I have that these bolts are 9/16-12 thread??? It seems like an odd size, but that's what I'm interpreting.

Thoughts?

[Larry M.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

That sounds right..........it is also what Paragon shows in their catalog.

Larry

[Joe L.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

One of the holes in the rear end cover where the spring attaches is oval. It looks like they tried a helicoil and it wasn't done properly so they just used a smaller diameter bolt with a nut at the end. I need to repair that hole and the helicoil seems to be the right way to go. Before I buy the correct one, am I seeing in the parts book that I have that these bolts are 9/16-12 thread??? It seems like an odd size, but that's what I'm interpreting.

Thoughts?

Gerald-------


9/16-12 was the size for all 1963-1977 Corvettes. 1978-79 went to 7/16-14.

[Gerald C.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Thanks. I found a kit for the 9/16-12 helicoil.

[Edward J.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Jerry be sure to use the correct size bolts I believe the were 4 1/8 inches long and the lock washers were thick.

[Larry M.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Paragon shows 9-leaf spring bolts at 3-9/16 inches. It probably does not matter for the two rearward bolts that go thru open holes, but for the two forward bolts that thread into the blind holes, you need to measure the hole depth and spring height etc. and "cut to fit". Failure to do this can result in cracking the rear cover.

Also, do not tighten any of the four bolts (or any of the rear suspension bolts) beyond snug/finger tight until the weight of the car is back on it's wheels and the car ride height is at the correct level as specified in the AIM.

If you need guidance on measuring and cutting the (blind hole) rear bolts to the correct size, just let me know. I have written about this on a few of our forums, and should be able to provide details for you.

Larry

[Joe L.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Jerry be sure to use the correct size bolts I believe the were 4 1/8 inches long and the lock washers were thick.

Edward------


The GM spec for the original GM #3833486 bolt is 3-7/32" in length.

[Larry M.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Edward------


The GM spec for the original GM #3833486 bolt is 3-7/32" in length.

Joe and Gerald:

I used the Paragon bolts for my 67 car with 9-leaf suspension. As I recall, I cut about 1/4 inch off the two forward bolts. So this would make them 3-5/16 inches long (3-10/32 inch.........versus the Joe/GM spec of 3-7/32 inch).

But takes your measurements nevertheless. Spring liners thickness can and will change this bolt length dimension some. Current replacement/reproduction liners are a different thickness than originals. Lock washer thickness will also affect.

Larry

[Joe L.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Joe and Gerald:

I used the Paragon bolts for my 67 car with 9-leaf suspension. As I recall, I cut about 1/4 inch off the two forward bolts. So this would make them 3-5/16 inches long (3-10/32 inch.........versus the Joe/GM spec of 3-7/32 inch).

But takes your measurements nevertheless. Spring liners thickness can and will change this bolt length dimension some. Current replacement/reproduction liners are a different thickness than originals. Lock washer thickness will also affect.

Larry

Larry-------


The GM #3833486, the original bolt used in PRODUCTION for all 1963-74 Corvettes, was discontinued from SERVICE in March, 1988 and replaced by GM #9428598. The latter differed from the 3833486 in two ways: it was a trimmed head (non-"indented") and it was 3-3/4" in length. Apparently, the reproduction bolt mimics at least the length of the GM replacement, NOT the original. Use these bolts as-is for any 1963-74 and problems will ensue.

[David B.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

GM part# 9428598 9/16-12 x 3 1/4" phosphate coated GM 4435-M, code A spec.

[Tom E.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Heed Larry’s advice, this is what happens if you don’t measure and install the correct size bolt. Discovered During initial disassembly of my 67 roadster. I knew something wasn’t right when I removed one of the forward bolts and all the oil drained out of the rear.

D6D3C5B2-6887-4B40-9A6A-832843234AA0.jpg

[Larry M.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

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Tom:

Same thing on my friend's 1969 Corvette (small block) that we are restoring. Removed the rear spring and the gear oil drained onto the floor.

Larry

[Joe L.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

GM part# 9428598 9/16-12 x 3 1/4" phosphate coated GM 4435-M, code A spec.

David------


I got my length information from a GM published spec, too. So, I guess we need to find one and measure it.

[Gerald C.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

I cut the two from bolts to just over 3 1/4" as to avoid differential penetration yet long enough to take the correct torqueing.

[Joe L.]

Re: 1963 rear end differential cover Bolt attachment for the leaf spring

Well, I obtained a few NOS GM #9428598. It turns out that the GM specs in the P&A catalogs were incorrect showing that bolt is 3-3/4" in length. The bolt is actually 3-5/16" in length and virtually identical in length to the GM #3833486.

GMboltcatalogpage.pdf

DSCN3832.jpgDSCN3833.jpgDSCN3837.jpg

The GM #3833486 was a special bolt that was released in late 1962 for use on 1963 Corvettes and, thereafter through 1974. It is special in the sense that it has a non-standard length of thread vis-a-vis its total length. I believe the reason for this is to keep the spring leaves in contact with the un-threaded shank. When this bolt is installed on a 1963-74 Corvette with 9 leaf spring there should be no more than about 1/4" of thread showing beneath the ear of the differential cover. The GM #9428598 is a "standard" bolt with standard thread to total length. If it is used as a replacement for the 3833486 there will be about 3/4" of thread showing. The bolts are otherwise the same except that the 3833486 is a GM 280-M material grade (SAE 5) and the 9428598 is GM 300-M (SAE 8).

The typical manufacturer's headmark seen on the 3833486 is "WB". However, other headmarks may have been used. The headmark on my NOS examples is "P" while the 9428598 is "LE". The 3833486, used in PRODUCTION from 1963 to 1974, was available in SERVICE until March, 1988 when it was replaced by the 9428598.

I also have a set of reproduction bolts. These bolts have the "WB" headmarking but are 3-9/16" in length. However, if these bolts are shortened by about 1/4", they will be virtually identical to the original 3833486. I do not know if these reproductions are still available as I purchased these many years ago.

DSCN3835.jpgDSCN3836.jpg

So, how did the reproduction source get the length wrong? Well, maybe they were not reproducing the 3833486 but were actually reproducing the the GM #348389. So, what is the GM #348389? It's the spring plate bolt used for 1975-77 Corvettes which used the same spring plate as 1963-74. I know nothing about it. It was also a special bolt but it was never available in SERVICE and I have no specs on it.