Back Drilling the Firewall

Last week, I posted about Starting on the Firewall for my airplane. With the corners cut out, it is time to install the sheet of stainless steel. Installing the firewall on a Bede BD-4C airplane is easy:

1. Take a big sheet of stainless steel
2. Put a zillion holes in it that exactly line up with the holes that are already in the fuselage
3. Remove the bolts from the fuselage and reinstall them through both the sheet of stainless steel and the fuselage.

Sounds hard, yes? Actually, it really is easy. In an hour, having never done this before, I was able to put the first five holes in my firewall. The rest will go much faster, now that I know what I am doing. This is what it looks like now (you can click any photo to see it larger):

I got a blind hole spotter which is a little cylinder of steel that fits into an existing bolt hole and has a sharp point on it. I removed a few bolts from the top corners of the fuselage (leaving the rest of the bolts in place so that nothing would fall apart). Then I stuck the blind hole spotter into a bolt hole, like this.

The holes, in case you are wondering about scale, are for AN3 bolts, which means that they are 3/16 inch in diameter.

Next, I laid the sheet of stainless steel over the blind hole spotter, like this:

Once the firewall is in place, some firm pressure with a thumb is all that is necessary to make a dent.

I got a couple of carbide drill bits; they will last longer against the stainless steel than regular drill bits. I drill a small pilot hole first, and the dent from the blind hole spotter is enough to keep the bit from wandering.

Then I follow up with a 3/16 inch drill bit, enlarging the hole to its final size.

Lather. Rinse. Repeat. About 50 times.