So the battery mount is set aside for a while. That left two more firewall tasks, one simple and one less so. The first was just drilling the two mount holes for the oil/fuel pressure manifold – that was easy enough. The second was finishing up the holes for the three control cable penetrations.
For these cables, I decided to go with steel eyeball-type mounts. These provide a nice sturdy seal, but also allow for some finessing of the angle at which the cable goes through firewall. The fun, though, is that each of these requires six screw holes around the perimeter, in addition to the actual passthrough hole. The eyeballs come with nice drill templates, but they assume that you’re starting from scratch – that is, you’d use the template to make the six screw holes as well as the center of the pass through, then drill them all. Well, I had two 1/2” prepunched holes, and one 1/8” one…so I couldn’t use the templates as provided.
After exercising a decent amount of brain power figuring out an approach for this, I hit upon a solution, which revolved around using the actual backer plate as a template. The first step was to open up the prepunched hole to just under 1 1/8” using a step drill, and then I could use that step drill bit to hole the backer in place, using the passthrough hole as a reference. Then I used a 5/64” drill bit to start a dimple through one screw hole, then finished that hole to 1/8”. That allowed me to put one screw in, plus the step drill for centering, and follow the same procedure for the other five screw holes.
This procedure worked out fairly well, except for one thing – using step drill on the thin stainless steel doesn’t make for a really great hole. In fact, it tends to wrap the steel just around the hole, while also creating an enormous burr on the backside of the hole. I spent a pretty annoying amount of time reducing that burr to a manageable state, but in the end the hole still isn’t super clean.
I didn’t really like the idea of using this same approach on the other two holes, so I paused and did some research fro better ways to approach this. At some point, I realized that I had a much better tool for this – a knockout punch set I bought a while back. The only problem was figuring out where I’d put the thing…and I spent probably a solid hour hunting around the hangar (this time is not included in tonight’s logged build hours).
All that searching time paid off. One of the punches was the exact size I needed for the passthroughs, which was a nice bonus. I got the second and third holes punched, but stopped short of drilling the screw holes – I’ll leave that for another night. This photo shows how much nicer the punched holes (right and center) are compared to the drilled one (left):
There’s also one more bit of complexity, which my be visible above – the rightmost hole is really close to the firewall support angle, to the point that the punch took a small bite out of the angle. Because of this, the backer plate isn’t going to be able to sit flush against the firewall without some additional creativity. I have a plan, though – the angle piece is .063” thick, and I have plenty of .063” scrap, so I should be able to just fabricate a shim to go between the backer and the firewall; this will effectively make a full flat surface for the backer plate to rest on.