70 rear spring leaf movement

[Donn M.]

My 70 LT1 had an after market rear spring, band clamps, painted liners, Etc.. My friend was doing a resto mod on a '71 and replaced his spring, which had worked well for years, with a custom setup. I checked it out and it appeared original, had the GMU stamp on the bottom leaf, so decided to use it to replace mine. Restored it, installed and made sure it had a load before tightening of all bolts. Not a fun project, but slow and easy and looks great, except higher on one side than the other. Adjusted the outer bolts slightly to get the back fender well 28-3/4" on one side and 28-1/2" on the other. Bolts have 3 threads remaining one one side and 5 on the other.
After driving it some the 2nd leaf from the bottom moved about 1/2" forward from the bottom leaf on the drivers side and of course the opposite on the right side. I released as much tension as possible without completely taking out, used blocks and clamps and pulled back together. After driving it some it has move right back out of alignment and even a little more.
I'm ready to take it all out and do over, but would appreciate any suggestions on what I may be doing wrong as I don't want to do this 3 times. I did replace the bushings and installed new shocks at the same time. I checked and the ears on the differential and they are not cracked that I can see. Thoughts appreciated .

[Ed S.]

Re: 70 rear spring leaf movement

Are you confident the center bolt is not an after market unit - they tend to have larger (taller) heads than OEM bolts - they are too tall and bottom out up against the diff case in the recessed hole - if the head is too tall, just by a fraction of an inch, it will be impossible to properly torque the spring plate to the point where it applies sufficient clamping force on the spring - to prevent movement. Using an aftermarket center bolt with an oversize head has been known to crack the diff case.

[Joe L.]

Re: 70 rear spring leaf movement

Are you confident the center bolt is not an after market unit - they tend to have larger (taller) heads than OEM bolts - they are too tall and bottom out up against the diff case in the recessed hole - if the head is too tall, just by a fraction of an inch, it will be impossible to properly torque the spring plate to the point where it applies sufficient clamping force on the spring - to prevent movement. Using an aftermarket center bolt with an oversize head has been known to crack the diff case.

Ed------


Even the GM centerbolt, GM #3723010, that was available for many years had a head that was too tall. These have to be ground down so that the head is not more than 1/4".

[Joe L.]

Re: 70 rear spring leaf movement

My 70 LT1 had an after market rear spring, band clamps, painted liners, Etc.. My friend was doing a resto mod on a '71 and replaced his spring, which had worked well for years, with a custom setup. I checked it out and it appeared original, had the GMU stamp on the bottom leaf, so decided to use it to replace mine. Restored it, installed and made sure it had a load before tightening of all bolts. Not a fun project, but slow and easy and looks great, except higher on one side than the other. Adjusted the outer bolts slightly to get the back fender well 28-3/4" on one side and 28-1/2" on the other. Bolts have 3 threads remaining one one side and 5 on the other.
After driving it some the 2nd leaf from the bottom moved about 1/2" forward from the bottom leaf on the drivers side and of course the opposite on the right side. I released as much tension as possible without completely taking out, used blocks and clamps and pulled back together. After driving it some it has move right back out of alignment and even a little more.
I'm ready to take it all out and do over, but would appreciate any suggestions on what I may be doing wrong as I don't want to do this 3 times. I did replace the bushings and installed new shocks at the same time. I checked and the ears on the differential and they are not cracked that I can see. Thoughts appreciated .

Donn------


Does your car have an original rear differential cover using 9/16" bolts to retain the spring to the cover?

[Donn M.]

Re: 70 rear spring leaf movement

Ed, Thanks for the response. I did not know there was a difference in that center bolt. The one in the leaf spring was a bit cross threaded and the rebuild kit from Corvette Central included a new bolt so I used the new after market one. Guess I should have done more research ahead of time.
Whoopee, I get to do it all over again . . . , but I now know what the problem is. Thanks again.
Donn

[Donn M.]

Re: 70 rear spring leaf movement

Joe, Thanks for the input. The bolts appear to be 9/16". They have a JH stamp on the heads. They were not replaced with the longer ones needed for the 10 leaf replacement spring and were short in the differential receiving holes. Need to take it all apart and grind the head down on the center bolt to the 1/4"max.. Thanks again.
note; when I tightened the 4 bolts, I think it was to 60#, the front was ok, but the back 2 bolts felt spongy so I didn't torque them to a full 60#. May have avoided a cracked differential. Just trying to find the good side of this problem.
Donn

[Joe L.]

Re: 70 rear spring leaf movement

Joe, Thanks for the input. The bolts appear to be 9/16". They have a JH stamp on the heads. They were not replaced with the longer ones needed for the 10 leaf replacement spring and were short in the differential receiving holes. Need to take it all apart and grind the head down on the center bolt to the 1/4"max.. Thanks again.
note; when I tightened the 4 bolts, I think it was to 60#, the front was ok, but the back 2 bolts felt spongy so I didn't torque them to a full 60#. May have avoided a cracked differential. Just trying to find the good side of this problem.
Donn

Donn-------


The reason I asked was that the 1978-79 cover, which became SERVICE for 1963-77, used 7/16" bolts (and a different spring plate). If an original 1963-77 spring is used with this cover, special spacers have to be used on either side of the spring. If this is not done, the spring leaves can displace as you described.

The spring retainer bolts should NEVER be tightened unless the spring is de-arched first (i.e. "flattened").

[Ed S.]

Re: 70 rear spring leaf movement

Yeah, apparently all after market bolts have different head thicknesses. The safest thing to do is to measure the depth of the recess in the diff case and then grind down the bolt head to a height that will ensure it doesn't bottom out. Not much force or pressure on that bolt - grinding down the head does not produce an unsafe condition.

[Donn M.]

Re: 70 rear spring leaf movement

Hey Joe, I was aware to leave the bolts slightly loose until all weight was back on the car. I just don't understand why Corvette Central wouldn't put a warning note in the "Rebuild Kit" stating 'if being used on an original spring' the bolt head will need to be ground down' as I would think most springs being rebuilt would be originals. This minor item is major work to do over and/or replacing the differential on a number matching car can be even worse. Or, is it very common knowledge that the bolt needs to be altered ?
Thanks again for all the info. Donn

[Ed S.]

Re: 70 rear spring leaf movement

Hey Joe, I was aware to leave the bolts slightly loose until all weight was back on the car. I just don't understand why Corvette Central wouldn't put a warning note in the "Rebuild Kit" stating 'if being used on an original spring' the bolt head will need to be ground down' as I would think most springs being rebuilt would be originals. This minor item is major work to do over and/or replacing the differential on a number matching car can be even worse. Or, is it very common knowledge that the bolt needs to be altered ?
Thanks again for all the info. Donn

It is just not Corvette Central. I purchased a "restoration kit" from Quanta - their kit included liners, (correct - so they say) grey spray paint and a new center bolt. The "kit" was less expensive than buying 2 of the 3 items I needed separately. The new center bolt had a huge head - way too large to function correctly. You would think if you are going to have a special bolt made you would at least have it made correctly - or..... as you suggested..... at least include a warning that the head has to be ground down.

[Joe L.]

Re: 70 rear spring leaf movement

It is just not Corvette Central. I purchased a "restoration kit" from Quanta - their kit included liners, (correct - so they say) grey spray paint and a new center bolt. The "kit" was less expensive than buying 2 of the 3 items I needed separately. The new center bolt had a huge head - way too large to function correctly. You would think if you are going to have a special bolt made you would at least have it made correctly - or..... as you suggested..... at least include a warning that the head has to be ground down.

Ed-------


The bolt sold in SERVICE for the 63+ Corvette spring center bolt, GM #3723010, had very wide application dating back to about 1957. This bolt ALWAYS had the "tall head" configuration. It could very well be and likely is that this is not the actual bolt used when the Corvette springs were manufactured but is the bolt GM supplied to SERVICE the application. So, all of the aftermarket and reproduction sources do accurately offer a correct reproduction of the 3723010 bolt. It's just that that bolt is not and never has been the same as the bolt originally used for the Corvette spring.

It's unfortunate that GM never advised in the P&A catalogs that the head of this bolt had to be re-worked when used for the 63+ Corvette application. To this day, I'm sure that has caused problems for a lot of Corvette repairers, including professional mechanics.

[Donn M.]

Re: 70 rear spring leaf movement

Joe, Appreciate your input. Makes me feel not quite so in in adequate. You and Ed very helpful. Small problem. Large redo project to correct.
Better get after it this week. Thanks again, Donn

[James W.]

Re: 70 rear spring leaf movement

Donn,

Here are a few pictures of a rear spring center bolt dimensions. This is the center bolt for my '65 with the std. 9 leaf spring. As you can see there is a big difference in the original length bolt and a replacement bolt from the Corvette vendors. Also, the replacement bolts have the shank fully threaded as compared to the original bolt. I machined a replacement bolt down to match my original for length and bolt head height. The bolt in the last picture with the "W" on it is the original bolt.

James West

[Joe L.]

Re: 70 rear spring leaf movement

Donn,

Here are a few pictures of a rear spring center bolt dimensions. This is the center bolt for my '65 with the std. 9 leaf spring. As you can see there is a big difference in the original length bolt and a replacement bolt from the Corvette vendors. Also, the replacement bolts have the shank fully threaded as compared to the original bolt. I machined a replacement bolt down to match my original for length and bolt head height. The bolt in the last picture with the "W" on it is the original bolt.

James West

James------


You can't compare the length of a bolt removed from an original spring to the length of the replacement bolt. The original bolt was once as long as the replacement bolt. This allowed the spring leaves to be compressed by simply drawing down the nut. Afterwards, the bolt was shortened. You will usually note that the end of original bolts have a "chopped off" appearance. I think they used some type of shearing tool to remove the unneeded bolt length after the spring leaves were compressed.

[Steven B.]

Re: 70 rear spring leaf movement

Ed-------


The bolt sold in SERVICE for the 63+ Corvette spring center bolt, GM #3723010, had very wide application dating back to about 1957. This bolt ALWAYS had the "tall head" configuration. It could very well be and likely is that this is not the actual bolt used when the Corvette springs were manufactured but is the bolt GM supplied to SERVICE the application. So, all of the aftermarket and reproduction sources do accurately offer a correct reproduction of the 3723010 bolt. It's just that that bolt is not and never has been the same as the bolt originally used for the Corvette spring.

It's unfortunate that GM never advised in the P&A catalogs that the head of this bolt had to be re-worked when used for the 63+ Corvette application. To this day, I'm sure that has caused problems for a lot of Corvette repairers, including professional mechanics.

Right you are Joe as usual! Years ago I purchased the bolt and other items when changing the spring and found the thicker head compared to the original. I wondered why - until I installed.