I have overhauled my 72' Differential, axle shafts, bushings and rear wheel bearings. In the course of the work I have tourqued all fasteners to their specified values, from the 72' GM service manual. However, I broke the ears on the differential cover. I did this when I tourqued the rear spring backing plate to the specified tourque of 70 ft-lbs. Well, that was quite a surprise. Upon further research of additional reference material, I have found that another value for these bolts - 25 ft-lbs. Which is correct? 70 ft-lbs seems like alot and 25 ft-lbs seems on the light side.
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Re: 72' Rear Spring Backing Plate
the trick is to load the springs first, then do the finish torque. Essentially, the spring has to be flat when the plate is being torqued to prevent breakage. 70 is correct torque, (lubricated threads etc..) -
Re: 72' Rear Spring Backing Plate
Bill-----
Bill Clupper said it! But, I'll add this: what you have experienced is the reason that the VAST MAJORITY of rear covers "lost an ear". These covers rarely break in service.In Appreciation of John HinckleyComment
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Re: 72' Rear Spring Backing Plate
Bill and Joe,
I appreciate your responses. Unforntunately, I followed the procedure that you outlined. Additionally, I touqued the bolts evenly in a crosisng pattern. I have checked the spring for defects. It does not seem to be bent. It sits flat on its side on the garage floor. The 7 leaves and liners are in good shape. I looked carefully at the cross member that the differential mounts to, it does not seem to have a front or back orientation and it is not bent. I installed new bushings all around and U joints. If the differential is bolted to the cross member and the front mounting bracket fits up perfectly it is hard to believe any thing is wrong with the angle of the differential relative to the rest of the suspension. I am ordering a HD replacement cover. Any other thoughts or advice? I am stumped and do not want to break off any more ears!Comment
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Re: 72' Rear Spring Backing Plate
Bill,
It may be possible that the improper installation procedure was used to install the spring a previous time (perhaps by the previous owner) which caused weakening of the ears. Although you may have followed the best procedure, damage may have already been done the last time the spring was installed and the break resulted when you added the final stress required to separate the ear from the rest of the backing plate.TonyComment
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Re: 72' Rear Spring Backing Plate
Tony and Bill-----
Yes, that's exactly what I would suspect. The ear could have been cracked from previous improper spring installation/removal. Check the broken off ear. If there is ANY corrosion evident on ANY part of the fracture surface, this will almost prove that this is what happened. If there is no corrosion evidenced, this "test" will be inconclusive inasmuch as the crack may not have been of a nature to allow "internal" corrosion to occur.In Appreciation of John HinckleyComment
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Re: 72' Rear Spring Backing Plate
Joe, Tony and Bill,
I believe that I have solved this riddle. In looking at this situation, I noticed that the original spring bolts are much longer than the replacements that I purchased. I also noticed that the down angle of the axle shafts with the spring on and car lowered seemed greater than normal. The car also appreared to be at a much higher ride hieght. While the spring is a seven leaf model, I believe that the spring rate of this spring is higher than that of a std 7 leaf. The longer bolts were used to adjust the ride height to the appropriate level.
The failure mechanism was too much spring pressure applied to the differential cover. The cracks are clean, no corrosion. The stock bolts are 2" shorter that the long bolts the previous owner used. I simply overloaded the ears with the combined wieght of the car, compressed spring and at some foward angle due to the shorter bolts. Thoughts?Comment
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Re: 72' Rear Spring Backing Plate
Joe, Tony and Bill,
Forget the previous message. I simply overtourqued the 4 bolts with the car on the ground. When I inspected the backing plate it was bowed. Moral of the story: Automatic Tourque wrenches (Clicker type) are not good on a sprung plate. I will use an old style, rod type next time.Comment
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