67 rear spring bolt

[Randy R.]

How long is the center bolt for the rear spring? I have a standard nine leaf spring and would like to know how far the bolt should extend past the bottom of the spring.

Thank you.

[Joe R.]

Re: 67 rear spring bolt

Randy,

Not getting the message here. The center bolts (4) do not extend past the bottom of the spring. There is a plate and the bolts threads go up into the rear end housing. The bolt heads are down. You need to be careful as the front two will bottom out in the housing and snap off or punch a hole into the the housing and create a leak. The bolts are grade 3 and very soft. These bolts are 3 and 1/4" long on the shank of the bolt. That does not include the lock washer which is present upon installation.

Hope this helps,

JR

[Joe L.]

Re: 67 rear spring bolt

Randy-----

The CENTER bolt was originally about 3-5/8" long. However, it doesn't remain that length very long. It needs to be that long when the spring is assembled (at the spring factory or in SERVICE). However, after assembly of the spring and installation of the nut, the spring bolt is cut-off so that only a very small amount of threads remain outside the nut. Whatever tooling was used in PRODUCTION to do this usually resulted in a very crude, "chopped off" looking end. Using a pair of bolt cutters will simulate this if you wish. I use a hack-saw, though.

One tip: most of the spring center bolts available today, including those available from GM under GM #3723010, have a head which is too long. The original head was about 1/4" long and the replacements are about 3/8" long. You MUST grind the head of the replacement bolt so that it is about 1/4" (or slightly less) long. The longer head will not fit into the drilled recess in the differential with the spring seated flush on the cover. Attempting to torque the 4 spring attachment bolts down with a too-long center bolt head will often cause cracking off an ear of the differential cover. Believe me, FAR more covers have had the ear cracked off due to this "phenomenon" (or, improper spring de-arching) than were EVER caused by "spontaneous failure" of "weak" original design covers.

[Randy R.]

Re: 67 rear spring bolt

Thank you for the information. Especially, the note regarding the head.

[Rory Riedy]

Re: 67 rear spring bolt

Joe,

In your response to Randy you mention "improper spring de-arching". Exactly what do you mean by this. Is there a proper sequence in re-installing the rear leaf springs. Before or after the struts? Before or after the half shafts? What works best?

Thanks

[Joe L.]

Re: 67 rear spring bolt

Rory-----

The best sequence to install the spring is before installing the struts and half shafts but after installing the trailing arms.

The spring MUST be de-arched prior to torquing down the 4 spring retaining bolts. The procedure for de-arching is well described in the Chevrolet or Corvette Factory Service Manual. This procedure MUST be followed or one risks cracking off one of the differential cover ears.

Basically, the spring needs to be FLAT (i.e. de-arched) before the bolts are torqued down.

[Chuck R.]

Re: 67 rear spring bolt

Hi Rory,

I believe what Joe is speaking about is the sequence of pulling the spring up against the diff. cover with as little adverse strain to the two rearward cover ears.

If you attempt to pull the bolts up to specified torque with the end links not connected, the curve (arch) of the spring will hit the center of the diff. cover putting a tremendous amount of avoidable stress on the bolts and their mounting locations.

To avoid the risk of snapping off an ear, the spring is first mounted to the cover with the bolts just snug enough to keep it in place.

Then the spring is mounted to the end links on each side which will "de-arch" the spring to the point where you could then pull up the four mounting bolts to spec. without fear of hearing the dreaded SNAP!

Make sure that you put some "loctite" on the clean bolt threads prior to pulling it all up tight for the last time.

If I had my druthers on what sequence I should have followed?

Well, I think I would definately have the strut rods and half shafts connected to the trailing arms before I connected the spring to the trailing arm assembly.

Chuck

[John H.]

Re: 67 rear spring bolt

Oddly enough, neither the '67 nor '69 Chassis Service Manuals (don't have any later ones) say anything about deflecting the spring to ride height before torquing the four mounting plate bolts; they just say to position the spring to the cover and fully torque the four bolts, then the procedure goes on to finish the rest of the assembly of the rear suspension, with the car still on a lift. Wonder how many covers had ears broken off at dealerships before mechanics figured out you should load the spring before torquing the mounting plate bolts?

[Joe L.]

Re: 67 rear spring bolt

John-----

I think that it's the number one reason, by far, for broken covers. The second biggest reason is the too-long head on some replacement spring center bolts. All other reasons for broken covers are so miniscule in number that they're not worth talking about.

The above 2 things, although totally un-related to any deficiency in the original design covers, spawned an industry! That industry being the production and sale of "HD" differential covers. Some of these covers are of a design which even interferes with spare tire carrier installation, but folks use them to "prevent the cover from cracking and leaving them stranded".

The "HD" covers do have a definite advantage, though: with one of them you can screw up during installation of the spring and stand a better chance of not "suffering the consequences" of a broken cover ear. Of course, if you install the spring correctly, you won't have anything to worry about as far as a broken cover is concerned. I've never heard of anyone that has broken a cover ear on the road. And, those few for which this has happened I'm very confident that the ear was first cracked, but unnoticed, during improper spring installation.

[Chuck R.]

Re: You know those GM mechanics

They weren't engineers, so I'll bet they had it figured out pretty durned quick Mr. Hinckley

Bu Dum Dum!

Chuck

[Timothy B.]

Re: 67 rear spring bolt

This topic has bothered me for some time, and maybe now I can address it. I think I understand the rationale described above for not torquing the center attach bolts with the rear spring free, but something just doesn't seem right. It sounds like the rationale is based on the belief that if you torque down the bolts with the spring free, that a single bolt will attempt to take on too much of the load unevenly of straightening out the spring under the bracket and therefore crack the housing. If this is true, then would it also work simply to tighten all bolts only say a half turn or so at a time until all bolts hit their target torque together? Wouldn't that also keep a single bolt from taking too much load?

Before having heard in the various forums that the bolts should be torqued with the spring flat, I thought it should be done the opposite. The CSM didn't say one way or the other as was pointed out. The argument for doing it the other way (and I am not saying this is correct) is that if the bolts are tightened with the spring flat to spec values and then the spring is later released and allowed to return to full arch, that forces created within the bracket will be tremendous. It seems to me that it would take a huge amount of force to hold the spring flat under the bracket without the aid of the large lever arm created by pushing up on the ends of the spring. Intuitively, I would have thought that if the bolts were torqued with the spring flat and then later released, then THAT is when the ears would break on the housing. This is when the forces stored in the flattened spring section under the bracket attempts to release itself and ADDS to the forces already in the bolts at full torque.

If, on the other hand, you carefully torque the bolts up to spec value evenly with the spring free, and later mount the endlinks to the trailing arms, some of the load on the center bolts is actually released with the spring flat.

Some thoughts on this alternate view, for the sake of discussion?

[Joe L.]

Re: 67 rear spring bolt

Tim-----

The problem is that when you attempt to torque down the spring bolts without de-arching the spring, you are, effectively, trying to de-arch the spring with these bolts. As the spring is installed on the differential cover in the free state, there is an arch between the spring retainer bolts on each side. For the spring to be torqued down, this arch has to be removed. However, to remove that arch, the spring would have to be completely flattened or de-arched.

This can't be accomplished by using the spring retaining bolts to do it. However, in the process of "trying to achieve the unachievable", tremendous torque has to be applied to the bolts. This causes 2 things to occur: first, the spring retainer plate bends and distorts. Second, the ears break off of the differential cover.

[Timothy B.]

Re: 67 rear spring bolt

Joe, the part that doesn't make sense to me is the statement "...tremendous torque has to be applied to the bolts..." by trying to use the bolts to flatten the spring. The CSM for my 67 shows a torque value of 65 ft-lbs for the spring bolts. Assuming that friction is the same in either case, 65 ft-lbs torque in effect places the bolts in the same axial tension load whether they are torqued with the spring relaxed or not. Once the 65 ft-lbs is reached, then the bolts are under the same load. Clearly, 65 ft-lbs torque and the the resulting load placed on the ears of the diff. housing isn't enough to break them. The loads necessary to break the carrier would have to come from other effects, such as those I described above. If you torque the bolts with the spring flat, then the spring will try to straighten itself when the spring is disconnected from the trailing arms and allowed to re-arch. Isn't this what would place extreme loads on the bolts and therefore the differential carrier because it is adding to the torque already placed on the bolts?

This topic reminds me of some strange final exam question my statics or dynamics professor would have come up with in college! Shall we sketch out a force diagram? :-)

[Terry M.]

Re: 67 rear spring bolt

I think part of the problem is that as the bolts are tightened the arch remains in the center portion of the spring because to flatten that portion, the spring between the ears would have to get longer. The bolts hold the outer ends of the arc from sliding, so the center portion is captive. I suspect what breaks the ears is the outward force of the spring trying to grow longer as it is flattened.
Just my guess though -- I'm no engineer.

[John H.]

Re: 67 rear spring bolt

Tim -

When the rear spring was assembled to the diff cover at St. Louis, it was done in a compression fixture that "flattened" the rear spring and held it that way until the plate and bolts were installed; the 65 ft-lbs. torque only had to secure the plate, not flatten the spring while being torqued.