1975 Rear Spring Bolts

[Daniel H.]

What is correct length of the four bolts that holds the rear spring to the differential? I bought a nine-leaf aftermarket spring with the liners installed. The spring stack measures 2.66 inches. The vendor said to use 4-inch bolts but to me that sounds a little bit long. I don't have the original spring to measure the stack height, nor do I have the old bolts. So, if someone could tell me what the original spring stack height is or what bolts to use with the aftermarket spring, I would appreciate it.


Thanks,
Danny

[Gary B.]

Re: 1975 Rear Spring Bolts

Danny,

Based on information from an article I wrote for the Corvette Restorer many years ago, 2.66” is thicker than any standard 9 or 10 leaf standard rear spring. Here’s a table from that article.

Also can’t you measure the depth of the blind holes in the rear differential cover, and the thickness of the bottom plate under the spring, and your stack height, and figure out from that if your bolts are too long or not?

Gary

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[Daniel H.]

Re: 1975 Rear Spring Bolts

Thank you Gary. So, the spring I bought has only 9 leaves instead of 10 which makes it not NCRS correct I'm guessing. If I look at the 64-74 in your chart it makes sense why the vendor is saying to use the 4-inch bolts instead of the 3 and 9/16 ones. I will check how deep the bolts go into the cover and then do the math. Thank you for your quick reply!

[Gary B.]

[Joe L.]

Re: 1975 Rear Spring Bolts

What is correct length of the four bolts that holds the rear spring to the differential? I bought a nine-leaf aftermarket spring with the liners installed. The spring stack measures 2.66 inches. The vendor said to use 4-inch bolts but to me that sounds a little bit long. I don't have the original spring to measure the stack height, nor do I have the old bolts. So, if someone could tell me what the original spring stack height is or what bolts to use with the aftermarket spring, I would appreciate it.


Thanks,
Danny

Danny------

The 1975-77 rear spring plate retainer bolts were GM #348389 and, unfortunately, were never available in SERVICE. Thus, I have no specs on them.

As has been suggested I would measure the total thickness of the spring you use, add the thickness of the plate and washer, measure the thread depth of the differential cover bolt holes and use a bolt that's about 3/16-1/4" shorter than the total of the aforementioned.

[Daniel H.]

Re: 1975 Rear Spring Bolts

Thank you Joe