9 leaf rear spring install?
Thread Starter
Le Mans Master
Joined: Jun 2019
Posts: 5,328
Likes: 2,095
From: Loveland
9 leaf rear spring install?
I just went from the 7 leaf to the 9 leaf rear spring in my 68. I have been doing a lot of research on the DIY. I came across this torque spec page for the 68 corvette. I keep seeing on this forum the spec for the 4 main anchor bolts, 70 lbs. This spec page confuses me now. Look at, "rear suspension leaf spring anchor plate", 30 ft/lbs. Under, "stabilizer shaft", rear spring to carrier" 50 ft/lbs. I couldn't find 70 ft/lbs any place. WHAT?
http://vetteworks.tripod.com/Torque.htm
http://vetteworks.tripod.com/Torque.htm
Thread Starter
Le Mans Master
Joined: Jun 2019
Posts: 5,328
Likes: 2,095
From: Loveland
Don't get me wrong. I wasn't asking for an opinion. Or a "this is what I do"
Burning Brakes
Joined: Dec 2005
Posts: 1,164
Likes: 223
From: Pawleys Island, SC
I have noted the discrepancy in various quoted specs. On my '77 I've always gone with the least number, 30ft-lbs. Spring on and off a number of times, no issues noted. The '77 has 7/16 bolts vs. your 9/19" bolts but I don't believe that enters into the spec requirement at all. FYI using anti-seize (as I think should) will result in up to 15% increase in the resulting torque. Just make sure to only snug them up good with weight off. Complete the torque with weight on wheels/spring. Use new lock washers.
Last edited by revitup; Aug 8, 2019 at 09:17 PM.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
The smaller bolt will have a lower torque requirement. Bolt torque is intended to cause the bolt to stretch in an elastic manner (not plastic deformation) so that clamping load will be enough to hold that assembly together. Larger clamping loads with larger bolts. So, for bolts of the same grade, [maximum] torque spec for a larger bolt will always be higher.
Burning Brakes
Joined: Dec 2005
Posts: 1,164
Likes: 223
From: Pawleys Island, SC
The smaller bolt will have a lower torque requirement. Bolt torque is intended to cause the bolt to stretch in an elastic manner (not plastic deformation) so that clamping load will be enough to hold that assembly together. Larger clamping loads with larger bolts. So, for bolts of the same grade, [maximum] torque spec for a larger bolt will always be higher.
Thread Starter
Le Mans Master
Joined: Jun 2019
Posts: 5,328
Likes: 2,095
From: Loveland
Thanks guys for good info.. I guess my diff cover has been changed. My 68 has 7/16 inch bolts. I had to go with longer bolts because the 9 leaf spring is 1/2 inch thicker than the 7 leaf. I knew when I had the torque wrench set to even 50 lbs that 70 was way to much. I am still curious where members got the 70 lb spec.
When you install a new spring, either original steel or a new composite spring, don't be surprised if your car sets higher or lower. I see on this forum that people think there is something wrong with the spring when the car sets wrong, even to the point of removing 1 or 2 leafs. The height of the car is 100% dependent ONLY on the length of the bolt on the end of the spring. If the spring has more bend and stiffness, the bolt has to be longer for the correct ride height and visa versa. Get the new bolt that's a bit longer and adjustable. This makes it easier to install and get the right ride height.
The 9 leaf spring does make the ride smoother. I do have what I think is an original 7 leaf spring for sale.
When you install a new spring, either original steel or a new composite spring, don't be surprised if your car sets higher or lower. I see on this forum that people think there is something wrong with the spring when the car sets wrong, even to the point of removing 1 or 2 leafs. The height of the car is 100% dependent ONLY on the length of the bolt on the end of the spring. If the spring has more bend and stiffness, the bolt has to be longer for the correct ride height and visa versa. Get the new bolt that's a bit longer and adjustable. This makes it easier to install and get the right ride height.
The 9 leaf spring does make the ride smoother. I do have what I think is an original 7 leaf spring for sale.
Last edited by kodpkd; Aug 9, 2019 at 10:44 AM.
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Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,112
From: Crossville TN
I can't imagine a Grade 8 bolt being specified for that application (leaf spring retention). I would think a Grade 5 bolt, which has greater ability to absorb bending moments would be more appropriate. Maybe that's why the AIM specifies 70 ft-lbs (which is about right for a 9/16" Gradw 5 bolt).
Melting Slicks
Joined: Sep 2016
Posts: 2,162
Likes: 940
From: Wauconda IL
Thanks guys for good info.. I guess my diff cover has been changed. My 68 has 7/16 inch bolts. I had to go with longer bolts because the 9 leaf spring is 1/2 inch thicker than the 7 leaf. I knew when I had the torque wrench set to even 50 lbs that 70 was way to much. I am still curious where members got the 70 lb spec.
FYI, I'm in the process of putting my rear suspension back together after dropping the diff to replace worn side yoke. I just tightened my bolts to 45 ft. lbs., based on the info in this thread. That's with a composite spring.
Last edited by sunflower 1972; Aug 11, 2019 at 02:08 PM.
