Eventually all of the original 73-82 Urethane front and rear bumpers will need to be replaced. As these flexible, stretchable bumpers age, they become brittle loosing these plastic characteristics. Whether it is from Ozone, Heat or UV light, the volatile plastic materials inside the Urethane escape leaving a harder substrate that cracks and crumbles. In severe cases, merely bumping into a Corvette with your knee can produce a crack in these bumpers.
The aftermarket solution to this problem has been a Fiberglass or Flexiglass bumper that is much more rigid and much less flexible than the Urethane originals. Unfortunately, 75-82 Corvettes do not have the dimensional exactness found in today's modern vehicles. I have often said that C3 Corvettes were not built to exacting specifications like the Space Shuttle. The original Urethane bumpers could accommodate the considerable size variation between similar models and thus be fitted satisfactorily. The replacement Fiberglass or Flexiglass bumper will not allow for this size variation and thus must be fitted to each individual model.
Reading ex-Corvette Chief Engineer Dave McLellan's new book "CORVETTE from the INSIDE" ( published 2002) confirms why the replacement bumper must often be fitted (i.e. built up or sanded down) to achieve a smooth transition from the body to the bumper. Speaking of the C3 he writes the following.
Page 74 Quote: "customers were consistently complaining about the fit and finish coming out of the St. Louis Assembly plant. But because Chevrolet was selling every Corvette it could build, the St. Louis plant did not want to hear about complaints."
Page 74 Quote: "The C3 was a tough body design to build, having been literally thrown together late and implemented with inadequate resources. The plant built the bodies, but in order to make things fit, they kept adjusting their fixtures until the body parts were as much as 1/4 inch to 1/2 inch out of position. The design called for bonding the front and rear fenders together just below their peak line and then grinding some 15 feet of seams to produce a finished surface."
Page 249 Quote: "We had come from a C3, where the entire car was essentially custom built. The St. Louis Assembly hadn't trusted anyone to give them dimensionally correct parts so everything had to be slotted or made oversized and then ground to suit. Large parts such as fenders had to be trimmed by hand, which produced gaps looked rough and irregular."
The fit and finish of parts on C4 and especially C5 would later be vastly improved when GM instituted an approach he referred to as Dimensional Management. Mr. McLellan describes this as "a strategy for ensuring all the machined parts would fit together smoothly." Nevertheless, I still love my old C3's and accept that everyone is slightly different.