One of the challenges with a traditional build—where everything isn’t defined by plans or a kit—is figuring out where to drill “arbitrary” holes. You might know you need a hole in the firewall to route wiring, but where exactly should it go? There are usually many viable options—but some locations that look perfect on one side might conflict with a weldment or structure on the other. The plans often won’t help—you’re on your own.
Careful measurements can help, but they’re often hard to make: nothing’s square, your ruler is too long or too short, and the precision you need isn’t easy to achieve.
Here’s a trick—especially handy when working with stainless steel firewalls, aluminum, or even composites. Buy some small magnets—you can get a hundred online for just a few bucks. Stick two of them together so you know which side is positive and which is negative. Mark the “outside” face of each magnet with a marker to make them easier to place.
Now, reach inside (a helper can come in handy) and place one magnet against the inside of the firewall. From the outside, “fish” for it using the second magnet. Once they snap together through the firewall, move them as a unit to your desired location. Then, simply circle the magnets on both sides. You now have matching drill locations—inside and out!
Excellent tip, Paul! I’ve done almost exactly the same thing to mark acoustic guitar tops when, gluing, installing braces, etc. Those tiny “rare earth” magnets are very strong and do a great job staying in place. Often I will hold a magnet in place manually on the blind side of the top (reaching through the soundhole), while other times, I will hold it temporarily with tape. The companion magnet then snaps into place very reliably on the outside of the top.