Paint & Body Special Section
An item on the '74-and-up Corvette that is considered so significant that it is a defining feature is the urethane bumper arrangements. Designed to meet the new federal crash standards phased in during the '70s, the bumper arrangement represented a major change in the Corvette. Beginning in 1973, on the front only, and then appearing at both ends for 1974, these urethane bumper covers were innovative for their time, hiding a massive impact-absorbing system behind them. Gone were the elegant chrome bumpers of earlier Corvettes. Undeniably, the change had a major impact on styling, though arguably the form created by GM was very attractive in its own right. Still, the change is significant enough for a distinction to be made, separating C3 Corvette types between "chrome bumper" and "rubber bumper" cars, with the former considered more coveted.
The new bumper technology was not without its problems. Though the concept is virtually universal in modern vehicles, the technology of a flexible urethane cover was in its infancy at the time of its introduction on the Corvette. Without a doubt, the major detraction was the material's inability to hold its original shape over time. Perhaps the "flexible" nature of the cover was an aspect taken a little too far. For the rubber bumper Corvette owner, distortion and waves in the covers' sleekly styled lines is just a fact of life.
There are a number of solutions available to rubber-bumper Corvette owners. Of course, the covers can be replaced with new OEM covers, but the distortion problem will eventually return. The aftermarket has semirigid fiberglass replacements, which are not nearly as flexible as the originals, but have enough flexibility to conform to the rigid body panels, allowing bolt-in installation as upgraded replacements in most cases. The third alternative is a rigid fiberglass replacement for the original covers. Being rigid, these covers do not have the conformability to mate directly to the hard body panels. For a bolt-in installation, typically the panels need to be fitted, filled, and ground until the cover mates to the body with an identical seam. This involves adding filler as needed to either the body or cover, and/or finish grinding and sanding both until the profiles come together with a perfect seam.
A final alternative is to simply bond and blend the hard fiberglass covers to the body for a custom seamless joint. This approach can give a stunningly clean look, and it is only possible with a hard, fiberglass cover. A drawback of the rigid or even the semi-rigid caps is that any impact will result in body damage, and this is exasperated if a rigid cap is rigidly bonded and blended with the body panels. Considering the hazard, our decision was to assume the risk, and resolve not to slam into anything.
For our project '76 C3, seamless is exactly the look we were after. We are not looking to build our Corvette to anything like stock specs, and intend to carry out many custom modifications. With performance being the overriding goal, using hard fiberglass bumper caps has another significant advantage-the massive impact absorbing system can be deleted for significant weight savings. This modification, of course, carries a safety consideration, since removing the bumper structure negates the impact protection afforded by the manufacturer, but our primary objective here is improving track performance. Gutted of the bumper structure and fitted with just the rigid fiberglass covers, the net effect is at the front and rear; a rubber-bumper car can be substantially lighter than its earlier chrome-bumper counterpart.
For the rear of the car, we passed on a stock, sloped-back, 1976-style cover, and discussed possible optional rear treatments for the car. We wanted a spoiler, but were much more enamored with the clean, one-piece look of the later Corvettes than the add-on look of the Pace Car spoiler arrangement. Tom Keen, of Keen Corvette Parts, had just the cover we wanted-an '80-style cap designed to mate with our '76 Corvette. For the front, a custom '80-style cover is also available, but our goals were bent more toward a custom approach than swapping styling cues over model years. We preferred the looks of the stock front of the '76, though we had some custom mods in mind. If we have one gripe about the OEM front-end styling of our car, it is the massive bumper guards built into the cover, with the front license plate mount pushed up front and center. Since the replacement bumper cap we were going to use is hard fiberglass, it's a piece that can be readily modified by using established 'glassing techniques. The plan for the front was a bonding on a stock hard fiberglass '76 cover, but only after shaving the bumper guards and deeply recessing the location of the front plate.
There are numerous products for bonding the fiberglass bumper caps, including many newer high-tech panel adhesives, but we went with the proven technique of simply using Mar-Glass fiberglass-reinforced, polyester-resin filler. After all, the major body panels of the car are polyester resin, and with the large flanged mounting surfaces, there is a tremendous "purchase" area for a high-strength bond. Unlike many of the new breed of panel adhesives, the Mar-Glass is rigid and will produce a rock-solid bond that will not flex or crack over time if done properly. Since the panels were being bonded, none of the OEM mounting hardware was retained, and, in fact, no mechanical fasteners whatsoever were used to mount the panels. The bond of the Mar-Glass is all that is needed, making the finicky stock fasteners redundant. By far the hardest part of this installation was modifying the stock-style '76 front cover to our own custom nose piece. For this, we would caution that a very high degree of fiberglass working skill is required. Installing the pieces was no more than a weekend's work, and we really like the end result.