The service manual says to torque the bolt to 45ft-lbs after the shims are securely in place. The wall thickness at the point where the bolt passes through the frame is fairly heavy with reinforcement. Supposedly the bolt squeezes the frame walls and holds the shims in place. I say this is a bunch of BS - there is no way that bolt is going to squeeze the frame enough to exert significant force on the shims. I'm about to pull the trailing arms from a '74 so I will use this opportunity to conduct a test. With the trailing arm and shims removed, I will measure the distance between the bolt holes using an inside micrometer. I will then insert the bolt, torque it to specs and measure again. Anybody out there want to speculate how much the difference between the two readings will be??? I'll post the results next week.
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Re: 63-82 Trailing Arm Bolts
Don't know where you read that the bolt should squeeze the frame walls to hold the shims in. Not true. You should be filling the entire gap with shims, with force if required to insert the last one. -
Re: 63-82 Trailing Arm Bolts
If I did the math correctly, your going to need some way to spread the frame walls back out next week. A 7/16-20 nut that is driven to a torque of 45 ft lbs, exerts a clamping force of 5654.86 pounds. I don't remember the exact thickness of the frame wall but I'd say that much force is more than enough to cause some serious tweaking. The formula goes like this...
6.2832R
Q = F x _______
P
F=force at end of wrench: (45)
R=length of wrench in ft. (1)
p=lead of thread (.050)
Q=loadComment
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Re: 63-82 Trailing Arm Bolts
Mike, you are exactly right...the conclusion that "Supposedly the bolt squeezes the frame walls and holds the shims in place" is BS. You have read something between the lines that isn't in the service manual.
When you disassemble the trailing arms from the frame you will discover that there are, as Mike Ward says above, the same total thickness of shims (6 @ 1/8"?) in each trailing arm frame pocket. The way the trailing arms are aligned is to move shims from one side of the trailing arm to the other, moving the trailing arm inboard or outboard.
The total thickness of shims always stays the same and is divided to fill the pocket on both sides of the trailing arm. When torqued, the bolt load is transferred to the frame pocket material, through the tightly packed shims, through the cylinder that forms the pivot hole in the trailing arm, through the shims on the other side to the frame pocket...there is no space for the frame to "squeeze" together if ALL the shims have been properly replaced. If properly installed, the design will have no problem with 45 ft-lbs and there will next-to-zero frame deformation.
The best strategy when removing the trailing arms is to wire the shims removed from each side of each arm together, and tag them for re-installation in the same position. If the car was driveable before you started work, then the current shim setup should work for driving the car for realignment.Comment
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Re: 63-82 Trailing Arm Bolts
Mike,
I noticed when I was installing the shims in my '67 trailing arms that they were tighter, although just slightly so, after the bolt was inserted and tightened. It appears to put pressure on the frame, which relaxes again after loosening the bolt. Although tightly packed, the shimmed could be moved slightly before the bolt was installed, but not after.Comment
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Re: 63-82 Trailing Arm Bolts
Rob,
That is, by design, exactly how it's supposed to work. Once the shim pack total is established so there is zero clearance, the pack will be snug but it still requires torquing the pivot bolt to specs to secure the shim pack. The frame rails DO flex and tighten the shims when the pivot bolt is torqued and also prevent the bushings from rotating as the suspension goes through it's travel. If this bolt isn't torqued to specs and the shim pack secured, the bushing outer sleeve will be allowed to rotate instead of rotating within the rubber as it's designed to do.
Also, the pivot bolt should only be torqued to the correct specification when the suspension is in the correct "ride height" position, not when the arm is allowed to hang free at it's lowest position. In other words, if you torque that bolt when the car is completed and back on it's wheels, it will be in the correct position after you move the car back and forth several feet to allow the rear camber to reset to it's normal ride height.Comment
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Re: 63-82 Trailing Arm Bolts
The main reason I doubt that the bolt exerts much force (if any) is my experience with '63's. The '63 didn't have a cotter pin to hold the shims in place. It was quite common for the shims to shake loose and fall out, which is why GM added the cotter pin. Another item of contention I have is with the bushing/sleeve design. The twin bushings are secured in place by friction to the trailing arm, and the inner sleeve is flared so it can't move around the bushing. The ONLY WAY the trailing arm can rotate is for the inner sleeve to move freely around the bolt. The bushings act as dampers only, they do not affect rotation. The design of the trailing arm bushings is very different from control arm bushings.Comment
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Cotter pin question
Mike,
Does the mid-year shim pack get a cotter pin from the factory?
On the C3s the cotter pin doesn't show up until the last week of 1970 production (mid-July 1970), even though the hole for the cotter pin is in the frame through much, and maybe all, of 1969 production.TerryComment
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Re: Cotter pin question
Terry-----
Mid-years did not use a cotter pin for supplemental shim retention.In Appreciation of John HinckleyComment
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Re: 63-82 Trailing Arm Bolts
Mike, trailing arm shim packs until late 70 did not have the cotter pins because the original shims were not slotted. Early IRS shims had a full hole on the bolt end (PIA for alignment). I believe the slotted shims and cotter pins started in late 70. Any 63s you saw with slotted shims had been realigned using service replacement shims. The cotter pins are there to insure safety (loss of shims) if the bolts are not torqued properly.
In my previous post, I incorrectly referred to the bushing inner sleeve as a pivot hole...in fact, when the bolt is torqued properly, it clamps the bushing inner sleeve to be stationary between the shim packs on each side of the trailing arm. All movement of the trailing arm should be occurring in the rubber bushing; there should be no relative movement between the bushing sleeve and the shims. If the 63 you observed had lost shims, it probably had not been properly torqued.Comment
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Re: Cotter pin question
Terry my original owner 70 with a G08 on the trim tag was bought with no cotter pins. The original shims have holes and the bolt has to be removed to change the shims for rear toe alignment.Comment
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Re: Cotter pin question
I would expect that Jim, but thanks for the info. The last week of 1970 production, when the cotter pins began to be used, was the last week of July -- G 27 to G31. I recall seeing one car in that range with a cotter pin on one side and no cotter pin on the other.TerryComment
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