Introduction – To Bond or Not To Bond
Why in the world would anybody want to rebond a bellypan if it isn't falling off? Sure, you've had a few rivets fall out, and it's delaminated in several areas, but other than keeping your butt off the pavement, what does a bellypan do? Well, for one thing, the bellypan is one of the best friends your chassis has to make it stiff. How? It acts like one continuous gusset. A stressed bellypan makes the bottom of the car very resistant to lozenging. But it can't do its job correctly without a good bond. The rivets do very little other than hold the pan tightly to the chassis while the adhesive sets. The epoxy is in fact what will take all the shear force. And once the bond has lost integrity in one area, the delamination is sure to propagate until the pan has become just a way of keeping you separate from the asphalt.

How Do I Know if I Need to Rebond My Pan?
Time is not really a good indicator. Depending on the roughness of the tracks you race, how many shows a year you do, the number of agricultural excursions you have during a season, and how bad you bottom the car, a bond could last less than a season, or a couple of years. It is best to use methods of actually evaluating the bond itself. And assuming you strip the car down to the chassis every off-season for a check of all components (you do, don't you?), these methods are pretty simple.There are several easy ways to tell. If you can actually see areas coming loose, this is the obvious sign. Other times you may be able to work your way around the perimeter of the chassis with a thin putty knife, hunting for separated areas. But arguably the best method is to just fill individual compartments in the pan with water. If it seeps into other areas, or out the side, the bond should be redone. And I don’t know any way to do just an area and assure a solid bond, so the best bet is to remove the whole pan and start over.

Who Should Attempt This Job?
Anybody, who can drill a hole straight, knows what a countersink is, and can mix sticky things together according to the directions on the container. You don’t have to have a degree in engineering or be able to outsmart MacGuyver. However, if you have ever watched an entire program on the Lifetime, Oxygen, or WE networks, or you think Christopher Lowell is a neat guy, do not attempt this project. I humbly suggest interior decorating or fashion design for you instead.

Getting Ready
So you want to rebond your bellypan. Sounds daunting, but with the appropriate tools, materials, and facilities it's a lot easier than it sounds. At a bare minimum, here is what you’ll need:

• A facility that can be heated to 70 deg F for a period of at least 24 hours for proper curing of the compound. I prefer 48 hrs. Of course, it should be dry. And a well-lit area is always easier to work in than a dungeon.
• 2 sawhorses
• 80 grit sandpaper
• Sandblaster (not absolutely necessary, but preferred)
• Acetone or other quick-drying solvent
• Nitrile gloves, unless you really like absorbing Acetone through your skin.
• Clean, light-colored, no-lint rags.
• Wire brush
• Stiff puttyknife
• Small Drift (to drive the rivet centers in prior to drilling)
• Hammer
• Soft mallet
• Hand drill, with recommended bit for rivet size. You may also need a countersink bit. CAUTION: Note that the countersunk rivets are a 100 deg countersink, not a standard 82 deg countersink.
• Countersunk rivets. Avdell or Cherry 5/32" with steel nails. "Pop" brand rivets won’t cut it. Buy at least 25 more than you need. Trust me. By the time this job is finished, you'll wonder where all the spares went.
• Rivet gun (air riveter is highly recommended, unless you have forearms like Popeye)
• Several lengths of 2X4
• Large C-clamps or furniture clamps
• 1 quart of adhesive (I use Hysol, but there are other compounds out there that I understand will work equally as well. In fact, there is a compound I will be experimenting with this year that is used to construct and seal aircraft wing tanks. Seems to be tough stuff and I may be recommending it in my next go-around.)
• Mixing cups (disposable) and mixing sticks
• Small trowel
• Measuring cups (disposable)
• 1 very patient helper. Wife or girlfriend not recommended as these units may boil over prematurely.
• 1 Bottle of gin to celebrate the successful bonding of the pan or wash away the pain of a job gone awry.
  
  

As a "like to have" item, you may also want to make up a rivet driver for those stubborn rivets. A piece of 1" round stock, 4" long, with a hole drilled in the center that will slip down over the rivet nail will work perfectly.

Removing the Old Pan

1. With the chassis inverted and sitting on the sawhorses, drive the rivet nails into the frame rails with the small drift and a hammer. That way the drill will not drift when you get to step #2. Of course, you’re going to have a couple hundred rivet nail stubs inside your frame rails, but unless you’re the really anal type this shouldn’t bother you. (If it does bother you I suggest that you open the bottle of gin early and unclamp your sphincter a bit.)
2. Drill out the old rivets. Take care not to enlarge the original holes, you'll want to save as many of these as possible for re-riveting.
3. The pan will likely still be adhered in some areas and loose in others. Working your way around the pan, drive the stiff puttyknife in with a mallet where the loose sections are, working your way through the adhered sections. Do not pry the pan up so it stays as flat as possible. This assumes you want to salvage the old pan. If you are not attempting to save the old bellypan, see the appendix for a special set of instructions for you, you troublemaker.
4. Now is a convenient time to check your chassis for cracks at the braze or weld joints, or anywhere the chassis is highly stressed. Don't be surprised if you find some. A formula car chassis takes a heck of a beating. You'd crack, too.
   
   

Prepping the Pan and the Chassis

1. With the chassis inverted, remove all the old bonding material with whatever method works best for you. I prefer to chisel off any big chunks to prevent excessive sand blasting or sanding. Be careful, the chassis rails have very thin walls!
2. Sand blast the bottom of the chassis (preferred) or scuff with the 80 grit sandpaper. This will give a good surface for the adhesive to grip. Do not spend lots of time around the rivet holes. They will erode quickly.
3. Wipe the surface down acetone. You can also clean with brake cleaner, but I prefer wiping because you know when you have a clean surface (the rag will not pick up any crud). We'll do this once more before bonding.
4. Inspect the rivet holes in the chassis with a sample rivet to see if they have become over-enlarged. If so, make a mark near that hole and we’ll drill a new one near it. DO NOT, under any circumstances, weld up the old holes or you may wind up with a pretzel for a chassis afterward.
5. Clean the bellypan with soap and water, removing all oil and other contaminants from the surface. Do the whole thing, not just where the frame rails rest. It is very easy to wipe oil or dirt onto the bonding surface, so best to do it all and remove any doubt.
6. Lightly blast or sand the pan. Feel around for any dings the pan may have that would prevent it from sitting flat on the chassis. Pound these out.
7. Align the pan back onto the chassis with a dozen or so rivets, and clamp in place. Drill any additional holes you deemed necessary in step 4. Countersink so a sample rivet is flush with the bottom of the pan.
8. This step is extremely important if you have an aluminum pan. Have the bellypan anodized. You can use colored or clear. This will give an excellent bonding surface to the adhesive and extend the life of the pan. Any local plating company should be able to help you out, and if you aren't in a terrible hurry they can run it with somebody else’ batch and save you some money. There is an alternate method that I’m going to play with this year, but since I haven’t done so I can’t advocate it just yet. If anyone wants to be a guinea pig along with me, try a conversion coating (you can get it from Aircraft Spruce). This is a surface treatment used to prepare aluminum for paint and adhesives that should work really well. If you're the daring type give it a try. You can do it at home and it should be cheaper than anodizing.
9. Scotchbrite the areas on the bellypan where the chassis will bond. Just take the shine down. This will give even more texture for the epoxy to bond to.
10. Finally, put your epoxy in a warm place the night before you start bonding.
    
    

There is one more thing you can do that will increase the strength of the bond significantly, but this will require some advanced fabricating skills, so I leave it as optional. You can weld sheet metal gussets into each corner of the bottom of the frame rails, thus significantly increasing the area of the bond. I have done this on my chassis and it works very well. But that is another project unto itself, and if you are interested in doing this I suggest you contact me for specific directions.

Bonding – The Meat & Potatoes
OK, now you have a chassis and bellypan prepped and ready to go. You have your mixing cups, mixing sticks, and compound handy. Don't start this late at night when you might be inclined to hurry. Start early in the day and go through a couple of rehearsals with your helper. Sounds silly, but once you mix the compound there's no going back. Not that the compound will set up before you are finished; but the time to find out your clamps won’t fit is before the frame rails are glopped up with epoxy.

1. Make one last dimensional check with the bellypan in place on the chassis. Fit two or three dozen rivets into their holes to make sure everything is going to line up properly.
2. Wipe down the chassis and pan one last time with acetone. Keep it handy through the process, you may need to clean the nose of the rivet gun at some point.
3. Mix the epoxy according to the directions on the can. Mix well, and don’t get in a hurry. You have at least an hour. About 1 pint should be plenty.
4. Trowel on a layer of epoxy to the bottom of the chassis. Don't be stingy, but don't put on so much it drips, either. A layer a little less than 1/16th inch should be about right. Smooth this layer out so there are no breaks and the seal to the pan is complete.
5. Lay the pan on the inverted chassis and start putting rivets in. I like to put in a large number in each section of the car. This will help keep the pan aligned during the riveting process.
6. Lay a 2 x 4 (2" side in contact with the pan) diagonally across the approximate geometric center of the car, and clamp in place, sandwiching the pan between the board and the chassis. You should see an amount of epoxy squish out around the perimeter. That’s a good thing. In general, the thinner the epoxy layer is, the better the performance will be.
7. Start riveting in the center of the car, putting rivets where you can access them close to the board.
8. Repeat this process, working your way in general from the center of the car outward. Make sure you are pulling the pan down tight to the chassis. If you miss a few due to the board being in the way, no worries. Just put them in once the board is removed.
9. Once all the rivets are in, spot check areas to make sure that the pan has been pulled tight to the chassis. If not, drill out that rivet (or series of rivets), pull the pan down locally with small blocks of wood and clamps, and re-rivet.
10. Flip the chassis over so the pan is sitting on the horses.
11. Wipe up the excess epoxy that squished out from under the frame rails. You can leave a small fillet. I don’t recommend leaving a large fillet, it adds nothing to the strength of the car.
12. Clean the floor and any chassis rails that may have had epoxy drip on them. Do it now. If you wait until it's hard, you'll be sorry.
13. Now let it sit there in a corner and think about what it's done for about a day. Keep the heat at least room temperature. If you have access to an oven many epoxies will respond well to an elevated-temp cure. See manufacturer's recommendations.
14. Marvel at your mechanical prowess and pop the top on that bottle of Gin.
    

Appendix for Those Putting on a New Bellypan
Sometimes it will be necessary to put a new bellypan on. Or maybe you need to add weight, so you're putting on a stainless pan. In that case, unless your manufacturer predrilled the holes in the pan, you'll have to drill new. Don't try to line up the old holes. I can almost guarantee that you'll be disappointed with the result. Even if you have the old pan as a pattern you'll still wonder why the rivets won't go in at assembly. It is ok to drill new holes in the bottom of the chassis. Just don't drill them really close to the old holes. And I like to stagger the holes alternately 1 inch to the left and right of centerline of the frame rails. Countersink carefully. It is easy to go too deep and lose grip on the rivet.




About the author...
Tom Johnson is the kind of guy who has been known to write entire articles just so he can write a blurb about himself at the end. Masquerading as an engineer for an Aerospace company by day, he can often be found shambling around the streets of Dayton, Ohio at midnight wearing little more than a dirty trenchcoat and mumbling about "motion ratios" and "velocity-dependent shock valving". He has incredibly bad dental hygiene. Moms, don't let you kids grow up to be Tom Johnson.




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