Rear Spring Bolt Length

[Elm Zobens]

I'm in the process of re-installing the rear spring on my 69' Coupe. I'm concerned regarding the length of the four 9/16" bolts that holds the spring to the rear cover- The original bolts I removed were 3 5/16" long and the replacements I purchased thru Muskegon Brake are 3 1/2". When I measure the center of the spring where it's compressed with the center bolt, I measure 2 3/8" (9 leaf spring). When I screw the 3 1/2" bolts and seat them in the front 2 holes with the spring plate in place (this is just a trial fitting, the spring is not installed), I get around 2 3/8" total before the lock washers are even compressed.
While the rear bolt holes go thru the cover and aren't an issue, I'm concerned that the front 2 will bottom out before it really makes full contact with the spring. With a the stories I've heard of broken covers and bolts puncturing the housing, I want to be REAL sure I do this correctly the first time.

I've already chased all the threads with a tap to make sure the bolts seat fully-
Has anyone run into this situation before? Do I grind down the 2 front bolts till I have 3 5/16"? Do I try to find 3 1/4" or 3 5/6" bolts for the front instead?

Any advice would be greatly appreciated.
Thanks!
Elm.

[Charles P.]

Re: Rear Spring Bolt Length

If you use the longer bolts be prepared for some differential fluid running down your arm. The temptation is to torque like crazy to seat those lock washers. In reality all that pressure is being transferred to the cast iron diff cover which eventually fails. You know you have gone too far when the bolt suddenly seems to get easier to turn. When you back the bolt out you realize your error. Too short a bolt will leave you with only a few threads being secure. Alot of the companies that supply the springs and spring plates supply a standard 3 and 1/2 inch bolt or even 3 9/16 which may or may not apply. If you have rebuilt your spring with new liners or are using a aftermarket you have probably changed the thickness of the spring somewhat over the original setup. Your trial fitting method is the best. I would attempt to find some 3 5/16 inch bolts with the WB headmarks rather than cutting them down. I got mine from Paragon which they have listed for use with the 7 leaf spring. They list 3 and 9/16 for use with the 9 leaf which from the sound of it would be way too long for your setup. Cutting down the longer bolts is risky as the sharp edges may act like a tap if you left them too long.

[Joe L.]

Re: Rear Spring Bolt Length

Elm-----

The original bolts were of GM #3833486. The "official" length of these bolts was specified by GM as 3-1/8". However, the actual length of GM bolts often is a bit longer (or, shorter) than the "official" length. I believe that these bolts were about the 3-5/16" length you mentioned.

GM discontinued these bolts years ago and replaced them with GM #9428598. These bolts have an "official" length of 3-3/4". So, apparently, GM thinks that bolts even longer than the 3-1/2" you have been supplied will work ok.

You will likely have a hard time finding bolts in the 9/16" X 3-1/8" to 3-5/16" range. GM currently offers NONE. You MIGHT find a 3-1/4" bolt in a real good hardware store.

Also, keep in mind that the original bolts used for this application as described above, were a SPECIAL bolt and had a non-standard shank length for the overall length. This was designed to have the area exposed to the spring leaves in contact with shank and not threads. General purpose bolts, including the GM #9428598, will usually not have this configuration. It's really ok to use them, though. However, if your original bolts are in good condition, I'd reuse them.

Otherwise, I'd shorten the new bolts to match the length of the original bolts. If you do this in a professional manner, the shortening should cause no problem.

[Randy R.]

Re: Rear Spring Bolt Length

I am installing the rear spring in a 67 and have run into a similar situation. I removed a seven leaf spring and am replacing it with a used original nine leaf spring. I ordered the bolts for a 9 leaf spring from LI Corvette. I carefully threaded the new bolts in slowly. They felt like they bottomed out before they compressed the spring and lock washers. I removed them, measured the depth of the hole and comfirmed that the hole was tapped to the bottom. I then measured the stacking height of the spring and the anchor plate and have concluded that the new bolts are too long. I plan to use two of the old shorter bolts that secured the anchor plate with the seven leaf spring for the front holes and two new ones for the rear.
The bolts that were removed with the seven leaf spring have the correct head markings and appear original. I had the original nine leaf spring replaced with a seven leaf spring over twenty five years ago. I doubt the alignment shop that did the work back then would have had acccess to a "correct" bolt when they changed springs. Based on this assumption and the fact that the new nine leaf bolts are too long, maybe the shorter bolts came with the car.

[Joe L.]

Re: Rear Spring Bolt Length

Randy----

For info, both the 7 leaf and 9 leaf originally used the same spring retainer plate bolts.

[Elm Zobens]

Re: Rear Spring Bolt Length

Thank you all for the input-

I went back last night and cleaned the original bolts but decided they were just a bit too corroded for my taste (these were still the original 'WB' headed bolts).
The funny thing is, that after I cleaned them up, I 'trial' fitted the spring to the differeantial using the original bolts. To my surprise, even the original 3 5/16" bolts were long enough to bottom out before the head of the bolt made contact with the retainer (minus the lock washer)!
Since I am using the original spring, retaining plate and new GM liners, I can only guess that maybe the original bolts may have stretched slighty over the past 35 years?
Anyway, since I wasn't risking damage to the differential, I ground two of the new 3 1/2" bolts down to 3 1/4" so the heads would contact the retaining plate without the lock washers and installed them without incident.

Since the GM parts catalog specifies 3 5/16" long bolts for either 7 or 9 spring configurations, I'm not sure where these aftermarket distributors came up with 3 1/2" or even 3 9/16" long bolts?!
Seems like if someone wasn't paying attention, there would be a lot of busted differential housings out there.
For my piece of mind, anything over 3 1/4" long bolts for this application would be asking for trouble.

Thanks again!
Elm.