Here's a variation on the Skybolt (or other quarter-turn) cowl fastener installation process that automatically results in your studs and receptacles lining up almost perfectly.
Start with your cowl drilled and clecoed to the mounting flanges. Leave all the holes sized to #30 for now. If you have to adjust the fit slightly, do it by elongating the holes in the fiberglass cowl - leave the #30 pilot holes in the flanges where they are.
I did not use Skybolt's store-bought flanges, which come with the large holes pre-drilled... if you have these, I think this method should still work fine as long as you use the SK-RVC inserts. If you're not using the pre-made flanges, drill all the holes for your rivets and locking pins at this point, but don't enlarge the center hole past #30.
The tool that makes this possible is a mini hole saw with a 1/8" pilot on it. I have a Hougen hole cutting kit that doesn't strictly have a pilot, but it does have a spring-loaded pin on the end that works well enough for this application.
Here's most of the process in one image: Remove one cleco, cut through the fiberglass using the piloted hole cutter, then reinstall the cleco with a 1/8" fender washer underneath it. Repeat the same steps for every fastener location. The end result should be a row of holes in the cowl that are all perfectly centered on the underlying flange holes.
The Skybolt grommets require a 15/32" hole, and the closest hole cutter I have is 7/16", so I cut all these holes slightly undersized in-place and then enlarged them the last 1/32" on the workbench with a unibit.
If you are making your own flanges, at this point you can remove the cowl from the airplane and then use the same tool to drill the flanges up to 5/8". Since you're using the same pilot hole for everything, the holes should all be concentric.
What I like about this method is that you don't have to worry about having a unibit wander off-center, and it still works fine even if the holes in your fiberglass are wonky or buggered up. In my case, I started with a few fairly ratty holes in my upper cowl, but the end result was as perfect as I could hope for.
Start with your cowl drilled and clecoed to the mounting flanges. Leave all the holes sized to #30 for now. If you have to adjust the fit slightly, do it by elongating the holes in the fiberglass cowl - leave the #30 pilot holes in the flanges where they are.
I did not use Skybolt's store-bought flanges, which come with the large holes pre-drilled... if you have these, I think this method should still work fine as long as you use the SK-RVC inserts. If you're not using the pre-made flanges, drill all the holes for your rivets and locking pins at this point, but don't enlarge the center hole past #30.
The tool that makes this possible is a mini hole saw with a 1/8" pilot on it. I have a Hougen hole cutting kit that doesn't strictly have a pilot, but it does have a spring-loaded pin on the end that works well enough for this application.
Here's most of the process in one image: Remove one cleco, cut through the fiberglass using the piloted hole cutter, then reinstall the cleco with a 1/8" fender washer underneath it. Repeat the same steps for every fastener location. The end result should be a row of holes in the cowl that are all perfectly centered on the underlying flange holes.
The Skybolt grommets require a 15/32" hole, and the closest hole cutter I have is 7/16", so I cut all these holes slightly undersized in-place and then enlarged them the last 1/32" on the workbench with a unibit.
If you are making your own flanges, at this point you can remove the cowl from the airplane and then use the same tool to drill the flanges up to 5/8". Since you're using the same pilot hole for everything, the holes should all be concentric.
What I like about this method is that you don't have to worry about having a unibit wander off-center, and it still works fine even if the holes in your fiberglass are wonky or buggered up. In my case, I started with a few fairly ratty holes in my upper cowl, but the end result was as perfect as I could hope for.
