C3 Bumper Cover Install - Rubber Bumper Blues
A Step-By-Step Replacement Of A Urethane Bumper Cover
From the May, 2010 issue of Corvette Fever
By John Pfanstiehl
Photography by John Pfanstiehl
If your Corvette was made in the last 38 years or so, it has a rubber bumper on both ends. OK, it's actually a urethane flexible bumper cover, but rubber bumper makes a better sounding title for the article. And even if your Corvette is pre-'73, your other cars almost certainly have rubber bumpers. So follow along to see what's involved with rubber bumper replacement, refinishing, and fitting.
It amazed me that auto manufacturers almost universally changed to painted bumpers on cars. If I were a conspiracy enthusiast, I'd think the paint companies had a hand in this. Sure, those molded bumpers look pretty when new. They're sculpted and clean looking, and they blend into the bodylines. Plus, they were made to take a low-speed hit with no damage, right? Well, if it's just a small tap squarely with something very smooth, they might come away unblemished.
Cracks in the paint are never...
Cracks in the paint are never a welcome sight, and they are all too common on rubber bumpers. These occurred after this car rolled into another at about zero miles an hour while waiting at a stoplight. Notice there are not even any scuffs or chips at the point of contact. Before considering repainting bumper damage like this, inspect the rubber under the paint. If it's cracked, a new bumper cover is the best long-term solution.
But most encounters are not so kind, and neither is time. If kept out of the elements, a steel bumper will be the same 10,000 years from now. By comparison, a rubber bumper will have aged within 10 years, and it may be severely arthritic well before the car becomes a classic.
Replacement of the stock urethane bumper cover can be accomplished with basic hand tools. In fact, power tools won't even fit into most of the tight spaces you'll visit during this project. A yoga class would be of more help here. Corvette owners can choose both the parts and procedures to suit them. The pros and cons of stock urethane bumper covers and aftermarket fiberglass bumper covers are, well, covered. And the new bumper cover can be taken to a paint shop off the car as shown, or it can be installed first and the car driven to the shop. In any case, a handful of wrenches and a little patience can renew the look of a beat-up bumper.
Bumper Cover Options and Fitting
After baking in the Florida...
After baking in the Florida sun, this bumper cover cracked, split, and popped apart. Hard to believe but true, this was not the result of a collision. It happened spontaneously while sitting out in the sun for a few hours after being washed.
The original Corvette bumper covers were urethane and were very flexible. The flexibility was a necessity for bumper systems that could absorb minor impact without costly damage. The disadvantage was the wavy appearance of the cover, and in early years the dull finish that came from flex additives in the paint. To overcome those problems, aftermarket suppliers soon made fiberglass replacement bumper covers. Of course, the fiberglass covers are more rigid-and therefore are more likely to be damaged by a small impact-but they have the potential of being fitted to achieve a great appearance. Plus, the cost of fiberglass bumper covers is less than half the cost of urethane replacement covers.
The advantages today of a replacement urethane bumper are originality, and possibly a better fit. The better fit is possible when only the bumper cover is being replaced, and no additional bodywork or repainting is being done on the adjoining body panels. The flexibility of the urethane cover provides a bit of wiggle room to push and pull the cover to fit the dimensions and angles of the top panels and fenders.
The cover still felt flexible...
The cover still felt flexible to the touch, even though it was at the end of its useful life. This shows why it's wise to make sure the bumper is not brittle before spending money on repairs or refinishing.
By comparison, a fiberglass cover has little give. If it doesn't closely match the body panels on your car, bodywork is needed. Grinding the bumper where it's too high or adding filler where it's too low. However, if bodywork and repainting is also being done on the top panel and fenders, then a fiberglass bumper cover has the potential of achieving a fit and appearance superior to a urethane bumper.
In short, a replacement bumper cover will never fit perfectly without bodywork. However it may fit acceptably, or even fit better than the original bumper cover. The fit problem is due to variances in manufacture of both the body and the bumper cover. The bumper has to align with three different body surfaces: the top panel (upper surround) and both fenders. Not only are there differences in manufacture of each of these three parts, in the shark years the three panels are glued together, so there is significant variation in spacing between the panels, in their alignment and in the angles between them. Therefore, the same bumper cover may fit well on one car, but not fit well on other cars.
In addition to dimensional variations in the body, there will be dimensional variations in bumper covers as they are manufactured-even when they come from the same mold. Injection-molded parts are not perfectly identical. Small changes in temperature of the mold, temperature of the plastic, material variations, injection pressure, and cycle time all can make dimensional changes in the finished part.
|'73-'74 Front Bumper Cover Urethane (GM)
|'73-'74 Front Bumper Cover Upper Retainer
|'73-'79 Front Bumper Cover Outer Retainer
|'73-'79 Stock Front Spoiler
|Difficulty Index - 2 Wrenches|
|Anyone’s Project: no tools required||1 Wrench|
|Beginner: basic tools||2 Wrenches|
|Experienced: special tools||3 Wrenches|
|Accomplished: special tools and outside help||4 Wrenches|
|Professionals Only: send this work out||5 Wrenches|
The new urethane bumper cover...
The new urethane bumper cover from Zip came well protected. The new cover was made from the factory molds and uses a more advanced urethane compound that is less likely to sag and wrinkle as it ages. Fiberglass covers are another option and are described in the accompanying sidebar.
The exterior surface finish...
The exterior surface finish on the urethane cover was excellent. There were no pits, sand scratches, or blemishes. The mold flashing was well trimmed, and the exterior surfaces were primed.
Order the upper retainer,...
Order the upper retainer, and both left and right side retainers for the bumper cover. Most Corvette owners think their car is rare, but it would be really rare if the old retainers were still in good shape after decades exposed to the elements.
The studs can become loose...
The studs can become loose in the upper retainer strip, just like the originals. JB Weld cured the problem. Trim any cement after it dries because the retainer fits tightly against the cover and excess cement may cause dimples in the cover that will be visible after painting.
This project is ripe for "while...
This project is ripe for "while you're there..." side jobs. The lower front valence was ready for replacement after losing a bout with a 5-gallon bucket of spackle that dropped off a truck. It's a good time to consider replacement or refurbishing of the signal lights and grilles.
Begin by jacking up the front...
Begin by jacking up the front and removing the grilles. This opens up access for the bolts and screws of the lower valence and the spoiler. Spray penetrating oil or WD-40 on the exposed threads of all the bolts and screws. Life will be easier if you can save and reuse the original fasteners.
Be alert for different screws...
Be alert for different screws in different locations. The factory used a number of different fasteners, and if any subsequent repair work was done, that greatly increases the likelihood of a mixture of screws. A hex-head sheetmetal screw is on the outboard end of the spoiler, while a machine screw occupies the hole right next to it.
The different screws you encounter...
The different screws you encounter can look pretty similar. With its tapered thread, the machine screw on the left looks a lot like the sheetmetal screw on the right. On older Corvettes, the fasteners you encounter are like a box of chocolates.
Take note of the different...
Take note of the different types of trapped washers on the bolts, too. The bolts with larger washers are used for the large holes in the lower valence, while the bolts with the smaller holes fit in the indented or recessed locations.
Removal of the lower valence...
Removal of the lower valence and spoiler should be easy if the fasteners aren't rusty. You'll appreciate how much room you have to work here when it's time to reach inside to remove the bumper cover nuts.
If replacing or refinishing...
If replacing or refinishing the lower front valence, pry the clips off now and bag them. It's easy to forget and throw the old panel away with the clips still attached.
Closely compare each replacement...
Closely compare each replacement part with the original. Check the overall dimensions and shape and the size and location of every hole. It's much better to learn of any problems now instead of after repainting or during assembly.
Unbolting the side retainers...
Unbolting the side retainers takes a little patience. On the '73 convertible we are using for this article, the location of the upper nuts does not allow even a ratchet. It's wrench-only and one sixth of a turn at a time.
Place the upper retainer on...
Place the upper retainer on the bumper cover to show where the studs are located. Manually raise and lock the headlamps for better access to the nuts holding the upper retainer.
Fortunately, the majority...
Fortunately, the majority of the studs and nuts are visible from underneath if you look hard enough. The exception is the outer nut. A very short extension on a 1/4-inch ratchet or a 3/8-inch-long socket will get the job done.
Let's hope that you don't...
Let's hope that you don't face a motley assortment of fasteners like these when removing the bumper cover. These definitely are not original. Where did the repairer even find four-sided nuts?
Many brackets, reinforcements,...
Many brackets, reinforcements, and braces become visible when the front bumper cover and bumper bar are removed. A steel reinforcement is pop-riveted to the fender on the lower inside edge, and another brace is pop riveted to the fender on its front edge.
Always Bag 'em Danno. Take...
Always Bag 'em Danno. Take the time to label and bag the fasteners. You'll be glad you did when it's time for reassembly.
It's not a pretty sight with...
It's not a pretty sight with the bumper cover removed. This is one job you'll want to finish as soon as possible. The large steel bumper bar is normally hidden behind the cover.
Drill the four 3/16-inch rivets...
Drill the four 3/16-inch rivets that attach the lower retainer to the bumper cover. The lower retainer is sturdier than the upper retainer and often can be reused.
Test-fit the upper retainer...
Test-fit the upper retainer to the upper panel. Clear or elongate any of the holes if necessary.
The upper retainer is not...
The upper retainer is not symmetrical; one corner is trimmed. Place the trimmed corner up when the retainer is installed into the bumper cover.
It takes some effort to work...
It takes some effort to work the upper retainer into the bumper cover. Start by inserting the center two studs and then work out to the ends on each side.
Test-fit the bumper cover...
Test-fit the bumper cover after installing the retainers. Center the cover to the top panel and then make sure the bumper can be moved up or down as needed to make it flush with the top panel.
Check the fit on both sides...
Check the fit on both sides at the fenders, too. There's only so much you can do at this stage, but at least be sure the holes are sufficiently elongated to achieve the best fit possible between the bumper cover and the body panels.
Elongate the holes if needed...
Elongate the holes if needed to enable the cover to be moved up, down, or to one side. A small rat-tail file or a Dremel tool are the best tools here. If the holes are significantly enlarged, small washers should be used under the nuts for the retainer.
A coat of POR 15 will keep...
A coat of POR 15 will keep the retainers from rusting, and provide a little help keeping the studs tight in the upper retainer. Two cautions: (1) make sure the POR 15 doesn't get on the threads-it's like cement, and (2) wear gloves or you'll be sporting POR 15 spots for days.
Install all the retainers...
Install all the retainers to make sure they fit properly and to hold the urethane cover in its proper shape. Without the retainers, the bumper cover is too flexible for painting off the car. Mask all the exposed threads on the retainer studs.
If you're in Florida, California,...
If you're in Florida, California, or the Atlanta area, Carsmetics is an option for painting the bumper. They specialize in small repairs and apply DuPont paint that comes with a lifetime warranty (good at any DuPont AOQ shop). The cost was $350 for the bumper cover and lower valence (both painted off the car).
This shows how brittle the...
This shows how brittle the old cover had become-it broke in two when dropped in thick grass. The cover has one last service to perform. It will be used as a spray test panel by the painter to make sure the color match is acceptable.
While the bumper is being...
While the bumper is being painted, it's a good time for the "while I'm there..." projects, like refinishing the screws and grilles in semi-gloss black.
With the bumper cover and...
With the bumper cover and bar removed, there was easy access for replacing the top panel support bracket and the energy-absorbing bolts. The large square die on the bolt is slightly smaller than the bolt shaft diameter. When the bumper bar is forced rearward during an impact, the die is pushed down the shaft stretching and extruding it to safely absorb some of the impact.
Rivet the bumper cover to...
Rivet the bumper cover to the lower retainer with 3/16 aluminum rivets with large heads. This can be done before painting, or after to keep the rivet head natural. The rivet heads will be covered by the lower valance panel.
After the bumper cover is...
After the bumper cover is installed, the job is nearly done. It's a perfect time to replace a ragged spoiler. When trying to order a spoiler, it helps to know that it's also called an air dam or air deflector.
The last step is installation...
The last step is installation of the grille and lamp assemblies. The holes in the mounting brackets allow a little adjustment. If more is needed, elongate the holes. Bending the brackets risks cracking paint or fiberglass because they are riveted to the fenders.
New bumper cover install completed-sleek,...
New bumper cover install completed-sleek, shiny, looking fine and ready for at least another twenty years. Nip/Tuck couldn't do a better looking nose job.