Fabricating, or building, goes hand in hand when building a hot rod. But for many of us, we don't have welding skills, nor do we have a shop full of fabrication equipment to actually construct complex pieces. If that fits your skill and equipment set, how do you fabricate the simple little bits? What we're talking about are flat-mounting brackets, block-off plates, backing plates or "doublers," and so on. These are pieces typically fabbed from aluminum, and they're things the average guy can build himself in his own garage with a minimum of specialized tooling or shop equipment.

A couple of things we've recently built include a firewall (hole) block-off plate that serves double duty as a roll control mount (for old timers, "Line Lock"), along with a large doubler plate for a heavy aftermarket electric fuel pump. What's up with the "doubler"? Simple. The inline aftermarket electric fuel pump for our car is designed with a fuel return system and the total pump and return assembly makes for a heavy piece. The only place we could mount it was on the sheetmetal kickup over the back axle. The original mounting bracket for the fuel pump has a relatively small pattern. Over time the weight of the pump, coupled with the small pump mounting pattern will erode the kickup sheetmetal. In simple terms it will tear. The solution? Build a healthy doubler to distribute the load. Simultaneously, the doubler covered up the sins of a past owner (who drilled a couple of unwanted holes in the kickup).

We have a similar situation with a big fuel filter. It too will mount on the kickup, and although not as heavy as the fuel pump, there's likely sufficient heft in place to eventually pull through the lightweight sheetmetal in the trunk. Another piece we're building is a plate to hold two battery charger (or booster) jacks hidden underneath the car. To complicate things, the mounting bracket we plan on using for the fuel filters is on back order. But we do have a factory drawing of an identical clamp. We'll use that to lay out and build the doubler plate. On the other hand, we already have the charger jacks in place. That means we can do the work with all of the parts in hand. And again, we'll do it all with a few common tools.

Alloys
Before we even think about cutting, drilling, or grinding, we should look at the material choices. Most doublers and brackets we build for ourselves are constructed from aluminum. Most common is 3003, and it's easy to obtain, but we use 5052 aluminum alloy for most brackets and backing plates. There are a number of aluminum alloys you can use for building stuff, but 5052 works well for the backyarder. Here's why: 5052 is one of the (if not the) highest strength alloy within the range of non-heat treatable aluminum materials. It has very good fatigue strength. It has a very good resistance to corrosion (that's why it is often used in marine, commercial, and specific aviation applications, and that's one very big reason we use it), but the best attribute as far as the homebuilder is concerned is the workability. It can be formed into rather intricate shapes when annealed. It doesn't tear as easily as 3003 or 1100 alloys. It can be welded with a TIG. And when finishing, it cleans up very nicely. You can purchase 5052 in a number of thicknesses. We've found that a material thickness of 0.100-inch (plus or minus) works well for most doublers, mount plates, and brackets. FYI, 5052 is also commonly available in 0.025-, 0.032-, 0.040-, 0.050-, 0.063-, 0.080-, 0.090-, 0.125-, and 0.1875-inch thicknesses.

Edge Margins-Close Counts
The next thing you have to get a handle on is the edge margin. What in the world is that? Basically, it boils down to how close you can drill a hole to the edge of a piece of metal. If a hole is drilled too close to the edge, there's a chance the material might tear away while under periods of stress. That pretty much defeats the purpose of something like a doubler. The distance between the drilled hole and the edge of the metal is called an edge margin. A specified edge margin is the distance from the center of a hole to the nearest edge.

In aircraft construction, the standard edge margin works out to twice the diameter of the hole. For example, if you have a 3/16-inch hole, the center point of that drilled hole should not be closer than 3/8 inch from the edge of the material (3/16-inch drilled hole X 2 = 3/8 inch). What if you have to drill a series of holes in a single piece (pretty common, particularly when it comes to doublers)? The aircraft industry has an answer for that too: The minimum separation between hole centers works out to three times the diameter. If you have a piece that mandates multiple 3/16-inch holes drilled for fastening purposes, each hole center must be at least 9/16-inch apart (3/16-inch drilled hole X 3 = 9/16 inch). If you have something like an aluminum doubler and it's mounted to the car by way of a series of holes, then each hole must maintain the edge margin and the series of holes must be a specific distance apart.

What follows is a standard aircraft maintenance chart laying out acceptable 2X numbers for edge margins as well as 3X numbers for hole separation, using the most common drill sizes:

Edge Margin and
Hole Separation
(All figures in inches)
Drilled 2X 3X
Hole (Edge (Separation)
Diameter Margin)  
3/32 3/16 9/32
1/8 1/4 3/8
3/16 3/8 9/16
1/4 1/2 3/4
5/16 5/8 15/16
3/8 3/4 1 1/8
7/16 7/8 1 5/16
1/2 1 1 1/2
9/16 1 1/8 1 11/16
5/8 1 1/4 1 7/8
3/4 1 1/2 2 1/4