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If the bend is like a twist you may be able to straighten it out OK.
If it is the heavy 1973 bumper it may be pretty hard to do because it
is really stiff. The earlier ones were made of fairly thin stuff and
could be straightened fairly easily. Susan backed up too quick and too
far once and wrapped part of the rear bumper around a piece of 3" pipe.
We straightened it out OK using a piece of railroad track inside the
bumper and wooden blocks with a 20# hammer. About 5 years later I noticed a
crack running along the top edge of the bumper, and on closer examination it
also had one on the bottom edge, and under the rubber, one going across the
face. By that time I had scrapped out a 68 142 and had the rear bumper
from it which I installed. I conclude that the residual stress from the
bending and straightening along with the humid tropic atmosphere and
components from the exhaust probably wound up as stress-corrosion cracking.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma