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Looking for rear bumper installation tips (Question #2)
See this gap? It's about 3/16" of a gap. These bumpers are made with heavy gauge steel and are not flexible... If I tighten the bolt in the picture above I will FORCE the fiberglass of the quarter panel to deform to the geometry of the bumper. Right now, the bumper has its own geometry and the fiberglass has its own geometry. I know the fiberglass is somewhat flexible but I don't want to FORCE the fiberglass to conform to the geometry of the bumper this much because I'm worried it might cause some unnecessary stress on the fiberglass that could potentially lead to stress cracks??? Did GM ever use shims between the inside of the bumper and the fiberglass to accommodate for the geometry difference(s)? What am I missing? Please help me get this right. Thanks in advance.
No shims were used at the factory in this location that I'm aware of. so something is off, either the bumper is out of spec or, the fender or bumper are replacements.
You probably know this but I was surprised to see shims at the inner-upper bracket to the frame. I forget now how the puzzle of the bumper goes together but maybe that would affect your line up?
This imho is not unusual I would make a rubber spacer the shape of the bumper mounting surface / I have used black rubber squegies to make them in the past they are not noticeable once in place. Hope this helps Jeff
There are shims listed for the rear mounts on the rear bumpers. Maybe (maybe!) you would be better off rotating the left rear bumper clockwise as viewed from above using the specified shims. It looks like there is room for this without making the install look odd. The way you show it, the amount of rubber shims would be quite large. Alternatively you may be able to move the whole bumper toward the center of the car by means which should be somewhat obvious. Our car required some shims in various spots to repair some inelegant shim work during a previous restoration. I preferred purpose made shims like the factory ones, which I made. The bumper kit from one of the "usuals" (not bumpers, just the bolts) actually came with one (!) shim. That side bolt is not listed as having shims, so I would use a thin rubber protector, but wouldn't shim it. Latitude is available in other areas.
I have reproduction bumpers and had a similar gap - possibly bumper variance or possibly due to some body repair in the rear. I shimmed the brackets out from the frame to where they were solidly in contact with the body - maybe even forcing it out very slightly but I mean very slightly. Then I used another shim and an inner tube cushion piece to fill the gap so I could snug up the bolts without putting undue pressure on the fiberglass or forcing the bumpers into misalignment. I used the same OE type shims as used between the frame and the bracket. Looking from above my bumpers are perfectly symmetrical side to side and with the fear body contour. The shims can be seen if looking closely but are not obvious. No cracks in 30 years either. I think it looks better than a lot of the C2 fears I see with one bumper sticking 1/4 to 1/2” out past the other when looking down on the rear of the car.
Original bumpers or reproductions? It does make a difference
I'm not sure because I don't know how to tell. I bought my car 2.5 years ago and these were the bumpers that were on the car when I purchased it. They could be the original factory installed bumpers that have been rechromed or they could be replacement GM bumpers that replaced the factory installed originals when the car was restored in 2008 or they could be repros. I was careful to install the same number of shims (in the same thickness). See item 4 in the AIM drawing below.
There are shims listed for the rear mounts on the rear bumpers. Maybe (maybe!) you would be better off rotating the left rear bumper clockwise as viewed from above using the specified shims. It looks like there is room for this without making the install look odd.
Bob, I was thinking about the same thing - putting a couple more shims (item 4 in the image below) at the same location as item #4 in the image below to "rotate" the bumper clockwise (as you put it). I'll certainly add this to the list of possible fixes. Thanks.
I have reproduction bumpers and had a similar gap - possibly bumper variance or possibly due to some body repair in the rear. I shimmed the brackets out from the frame to where they were solidly in contact with the body - maybe even forcing it out very slightly but I mean very slightly. Then I used another shim and an inner tube cushion piece to fill the gap so I could snug up the bolts without putting undue pressure on the fiberglass or forcing the bumpers into misalignment. I used the same OE type shims as used between the frame and the bracket. Looking from above my bumpers are perfectly symmetrical side to side and with the fear body contour. The shims can be seen if looking closely but are not obvious. No cracks in 30 years either. I think it looks better than a lot of the C2 fears I see with one bumper sticking 1/4 to 1/2” out past the other when looking down on the rear of the car.
Last night after creating this post the following thought crossed my mind...
"What if the person/people that installed this bumper just installed it without much care. Caring little if it was done properly (or not) and with consideration to the separate geometries of the fiberglass and bumper???"
I may have to be open to this possibility, and therefore I may have to loosen all the braces (5, 7, & 8 in the AIM diagram below) and simply start over from scratch with regards to their ideal positions and the number of shims used.
I’d recommend you loosen all attachments, especially the frame to body bracket near the center-rear. It allows lateral adjustment. Snug the outboard hand tight first. If that fails, you may need to verify your body is sitting equally on the frame. You should at least have the same distance at this bolt on each side when the body is centered.
I would suggest going to Walmart and buying a bicycle inner tube. The cut it apart to make paint protection gaskets and put them between the paint and the bumper
If I understand the bumper and the body have not changed so it fit before . . so I would loosen all the bumper bolts and tighten this one first then see if you can shim or adjust the other attachment points to compensate . . with C2s there is usually compromise with bumpers.
I did as suggested above... Loosened all related fasteners and started over from scratch while trying to maintain the specs in the AIM. I played with the sequence when tightening the fasteners for the bumpers and brackets. In the end it looks great. I just needed to hear you guys tell me to loosen all related fasteners and started over from scratch. Thanks guys.
If you haven’t replaced a C2 body on a frame before, there is more slop in the body mounts than you would probably suspect. You can move it around quite a bit on the frame with the body bolts loose. I never considered how it might affect the bumper mounting though. There’s a lot of adjustment in the bumper brackets but anything is possible.
This morning I had a frame guy come over because I wanted another confirmation that the frame was square. He just left 5 minutes ago. Good news. The frame is "perfect" as he put it. He talked about "slop" and tolerances in the body mounts. If I ever have the car painted I'll lift the body off and at that time I'll look at this further but for now I'm just happy to know that the frame is square.
06-19-2023, 05:34 PM
Looking for rear bumper installation tips (Question #2)
