64 rear spring

[Doug L.]

Hi Guys,
I'm back to working on my rear spring. A few months ago I concluded that it was not an original spring, based on it having 10 leafs. Since I had nothing else with which to replace it and since it seemed to be in good condition, I restored and reassembled it. Today I attached it to the differential.

I had a very difficult time getting the 4 attachment bolts started. Eventually I had to put a large "C" clamp on the spring, one side at a time, near the end of the upper-most leaf. I compressed the spring enough to allow me to get one bolt started without its lock washer, then worked my way around the spring plate until I had all started. I then removed them one-at-a-time to install the washers.

My concern now is that the bolts don't thread very far into the differential. I can't tell about the 2 front bolts since they thread into blind holes, but I can measure the depth that the 2 rear bolts thread into their holes. Those rear holes are ~3/4" deep. The bolt, although not yet torqued, extends only 5/16" into the hole. I assume the front bolts are approximately the same.

What I am wondering is whether the spring is original but had 1 leaf added. The next-to-bottom leaf is not upturned on its ends, is cut square and is ~1/4" thick. The height of the stack of springs (with the 10 leafs) is 2-3/8" and the leaf in question is ~1/4" thick. Does anyone now what the factory height of the spring stack is supposed to be or how far the bolts should enter their holes? I know the front bolts had little clearance to the end of the holes in the diff making protrusion of the diff case a potential problem if the washers were not installed. If the front bolts are only 5/16" into their holes like the rear bolts are, I'm not certain the spring will be stable.

Thanks in advance for any insight on this situation.
Doug

[Joe L.]

Re: 64 rear spring

Hi Guys,
I'm back to working on my rear spring. A few months ago I concluded that it was not an original spring, based on it having 10 leafs. Since I had nothing else with which to replace it and since it seemed to be in good condition, I restored and reassembled it. Today I attached it to the differential.

I had a very difficult time getting the 4 attachment bolts started. Eventually I had to put a large "C" clamp on the spring, one side at a time, near the end of the upper-most leaf. I compressed the spring enough to allow me to get one bolt started without its lock washer, then worked my way around the spring plate until I had all started. I then removed them one-at-a-time to install the washers.

My concern now is that the bolts don't thread very far into the differential. I can't tell about the 2 front bolts since they thread into blind holes, but I can measure the depth that the 2 rear bolts thread into their holes. Those rear holes are ~3/4" deep. The bolt, although not yet torqued, extends only 5/16" into the hole. I assume the front bolts are approximately the same.

What I am wondering is whether the spring is original but had 1 leaf added. The next-to-bottom leaf is not upturned on its ends, is cut square and is ~1/4" thick. The height of the stack of springs (with the 10 leafs) is 2-3/8" and the leaf in question is ~1/4" thick. Does anyone now what the factory height of the spring stack is supposed to be or how far the bolts should enter their holes? I know the front bolts had little clearance to the end of the holes in the diff making protrusion of the diff case a potential problem if the washers were not installed. If the front bolts are only 5/16" into their holes like the rear bolts are, I'm not certain the spring will be stable.

Thanks in advance for any insight on this situation.
Doug

Doug------



I think you have a later GM replacement spring which has a spring height somewhat greater than the original spring.

The original spring had a compressed height of right at 2.1". This may vary slightly, though, due to varying thickness of replacement spring liners.

I don't think your current bolts are engaging enough threads for the spring you have. The length of these bolts is critical----you don't want them too short and you don't want them too long (particularly for the two forward bolts that are blind-tapped).

You need bolts that are about 1/4" longer than the ones you have so that you will have about 9/16" of thread engagement. Assuming that your current bolts are 3-5/16" long, Paragon has a bolt available that is 3-9/16" long which should be just about right. These are their number 6647K and sells for about 10 bucks a set.

[Doug L.]

Re: 64 rear spring

Hi Joe, Gary,
Thanks for your input. If the original spring was 2.5" and the replacement spring was thicker, my situation doesn't make sense given my spring stack measures 2-3/8" Gary's data makes more sense. If the original spring thickness was 2.12" and 1 leaf was added, the total would be close to what I have. Since the original spring had 9 leafs at 2.121" and mine has 10 leafs at 2.375 with the 2nd leaf .25 thick, I'm wondering if I can simply remove the second leaf resulting in a 9-leaf spring with assembled height of 2.375 minus 0.25" = 2.125"? Any thoughts on doing this?

Thanks-Doug

[Gary B.]

Re: 64 rear spring

Doug,

I just noticed you're talking about a '64 spring, not '63. The specs fort he '64 (thru '74) 9-leaf spring are:

GM # 3850839
Two stage design (as is the one in your photo)
Assembled thickness: 2.121
Spring rate: Stage 1: 140 @ 1360 lb
Stage 2: 196 @ 1965 lb
Leaves 1 thru 3 were 0.214" thick
leaves 4 thru 9 were 0.194" thick

Sorry for the confusion.

Gary

[Joe L.]

Re: 64 rear spring

Hi Joe, Gary,
Thanks for your input. If the original spring was 2.5" and the replacement spring was thicker, my situation doesn't make sense given my spring stack measures 2-3/8" Gary's data makes more sense. If the original spring thickness was 2.12" and 1 leaf was added, the total would be close to what I have. Since the original spring had 9 leafs at 2.121" and mine has 10 leafs at 2.375 with the 2nd leaf .25 thick, I'm wondering if I can simply remove the second leaf resulting in a 9-leaf spring with assembled height of 2.375 minus 0.25" = 2.125"? Any thoughts on doing this?

Thanks-Doug

Doug------


I would not recommend removing a leaf from the spring. The spring must be maintained as-designed---this includes the entire leaf count. I would simply use bolts that are about 1/4" longer than the ones you have now.

If you wish to use the bolts you have, then I would obtain a new reproduction 9 leaf spring.

[Gary B.]

64 rear spring

Doug,

After taking a closer look at the spring in your photo, there are several odd aspects. The shortest leaf in the curved group has ends that are not typical for a 64-74 Corvette leaf spring. Plus it looks like there is no liner between leaves 2 and 3, and one or both of those leaves is incorrect. I'd suggest looking for a good, used 9-leaf GM spring and starting over, since there's no other way to correct leaf 6 (7 in your spring).

Gary

[Joe L.]

Re: 64 rear spring

Doug,

I just noticed you're talking about a '64 spring, not '63. The specs for '64 (thru '74) spring are:

GM # 3850839
Two stage design (as is the one in your photo)
Assembled thickness: 2.121
Spring rate: Stage 1: 140 @ 1360 lb
Stage 2: 196 @ 1965 lb
Leaves 1 thru 3 were 0.214" thick
leaves 4 thru 9 were 0.194" thick

Sorry for the confusion.

Gary

Gary and Doug------


I had the spring height dimension incorrect in my previous post and I've corrected it. I had measured the compressed height of my original, rebuilt GM #3850839 spring but I must have originally read the caliper incorrectly. Anyway, I re-measured it and came up with 2.085". The difference from original spec is likely due to the fact that the replacement spring liners are slightly thinner than originals.

In any event, to use the spring which Doug has which is 2-3/8" thick, he still needs bolts which are about 1/4" longer. If the current spring provides the correct ride height based upon his previous experience with it and is otherwise in good condition, I would simply use 1/4" longer bolts and this spring. If he were to obtain a new 9 leaf reproduction spring, it still might not be the same compressed height as the original specs and it might not provide the correct ride height, either.

[Gary B.]

64 rear spring; Your goal?

Doug,

What is your goal with the spring? If having your car judged isn't an issue and all you need is a functional spring, and if you're happy with the ride height that your current springs gives you, then I agree with Joe about using slightly longer bolts.

On the other hand, if you plan on having your car judged, then you'll have to find either a good used spring, which is not that hard, or you'll have to buy a repro spring, such as the one from Eaton, which judges well. With the Eaton spring there is a difference of opinion as to the correctness of the ride height as you read in the archives; some claim the ride height with the Eaton spring is correct; some claim the ride height is too high. It's hard to reconcile these observed differences.

Gary

[Michael F.]

Re: 64 rear spring

Doug------


I would not recommend removing a leaf from the spring. The spring must be maintained as-designed---this includes the entire leaf count. I would simply use bolts that are about 1/4" longer than the ones you have now.

If you wish to use the bolts you have, then I would obtain a new reproduction 9 leaf spring.

I agree, go get an Eaton springs replacement. After trial and error. The only springs that gave me the factory ride and height.

[Frederick W.]

Re: 64 rear spring

Eaton's website describes 3 springs:

"For the years 1963 through 1979 Chevrolet Corvette's 3 different 2 1/4 Inch wide rear leaf springs were used.

There was the standard duty 9 leaf spring, P/N ML357, in which the 6 longest leaves were curved and the 3 short one were straight.

In the spring business we call this a 6 and 3 or a 6/3 spring.

The Heavy Duty spring was a 7 leaf, P/N ML715.

There was also another 9 leaf spring, P/N ML813, which all the leaves were curved.



The difference between all 3 springs is the spring rate. (The amount of weight that is needed to deflect a spring one Inch is the down and dirty description of spring rate).

The spring rate for ML357 (the 6/3 spring) is 140 Lbs. for the 1st 6 leaves and 196 Lbs. for when all 9 leaves are touching.

The spring rate for ML715 (the 7 leaf spring) is 305 Lbs.

The spring rate for ML813 (the 9 leaf all curved leaf spring) is 196 Lbs.

All 3 springs are designed to have a slightly negitive arch, about 3/8 of an Inch, when they are at their designed load. The designed load for the ML357 and ML813 is 1,360 Lbs. and for the ML715 it is 1,325 Lbs.

In order for all 3 springs to be at the same height when under load, each spring has a different free arch.

The free arch of the ML357 is 8 11/16 Inches. The ML715 is 5 7/8 Inches. And the ML813 free arch is 7 1/2 Inches.

The correct way to measure the free arch is for the spring to be off the vehicle. Place a straight edge across the ends of the spring and measure down to the main plate next to the center bolt.

While we use the correct type of spring steel, SAE5160 High Alloy Spring Steel, the thickness's of the leaves have been adjusted to sizes the steel mills now produce. Way back when, the mills would produce quantities as low as 5 tons of a size. Today over 20 tons of a size is the bare minimum of a size they will produce. And that is a lot of Corvette springs.

While the overall thickness of the spring have slightly increased, less than 3/8 of an Inch, the fit, feel, function and performance of the springs are unchanged. The free arches have been lowered a bit from the factory specs in order to maintain stock ride height.

We do not paint our Corvette Leaf Springs so you can apply the finish of your choice.

Each spring includes full length liners between the leaves.

The end of the leaves are as close to OE as possible.

And each spring ships with clips banding the leaves together so they do not fan out during shipment. The clips are easily removed."

Note that there were apparently 2 different 9 leaf springs with different rates. Also note the comment that "THE ENDS OF THE LEAVES ARE AS CLOSE TO OE AS POSSIBLE".

[Gary B.]

Eaton's claims re: end treatment; Unbelievable

Frederick,

"The end of the leaves are as close to OE as possible.".

If Eaton makes this claim, then they must think we are dumb enough to believe it. The tapered rolled ends on original mid-year springs were made by some manufacturing process back in the 1960's. To claim that the current end treatment on Eaton springs is as close to OE as possible is simply ludicrous. The Eaton end treatment is obviously not that close to the original. The end treatment on the 7-leaf spring now sold by America's Finest Corvettes (AFC) indicates that it is indeed possible to better approximate the original end treatment. If AFC follows thru with their promise to make a cosmetically acceptable 9-leaf spring with the correct ride height, then Eaton's sales to NCRS people will decrease substantially OR Eaton will simply have to find a way to improve the end treatment of the Eaton spring.

Competition for sales among vendors does result in improved products.

Gary Beaupre
Northern California Chapter
NCRS # 28818

[Alan D.]

Re: 64 rear spring

Just remember to full torque the 4 bolts when the full weight of the car is on the spring.