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Fuel tank inspection holes

I’ve been thinking about how to cut the large circular inspection holes in the fuel tanks. I don’t have a fly cutter and wasn’t able to source one locally. A few of the other builders have trimmed these by hand and filed them to size but I wasn’t keen on this approach as it sounded like it would take a long time. I ended up buying a nibbler attachment for my drill which worked a treat. I drilled a large hole in the rib then inserted the nibbler bit and started cutting the hole.

Starting fuel tank hole

I was able to trim almost up to the line using the nibbler which left small areas to remove using a half-round file. After filing I rounded the edges with some sand paper and finished off with some Scotbrite pad.


I then repeated this for the other tank. Next, I picked out the inspection hole covers and use a small metal ruler to centralise the plate’s position over the hole. For me, the plate sat about 5/8″ from the rib bead to be centred.


I clamped each plate into position and drilled the first hole into the rib using the pre-punched holes in the plate as a guide. The plans didn’t indicate what hole size these should be. I verified that the nutplates and bolts used here were the same size as that used for the tank skin spar union, so I drilled these to #19. As these holes were too big for clecoes, I used the plate bolts to stop things sliding around.


After double-checking which side to dimple, I used my C-frame to dimple the nutplate rivet holes on both ribs.

Fuel tank rib dimples

I then had to countersink the rivet holes on the reinforcing rings. As these had to be quite deep in to accept the dimple and allow everything to be flush, I noticed the countersinking bit wobble around a bit on the first hole, causing a slightly elongated hole. To stop this from happening, I drilled a hole in some scrap wood to use as a guide hole for the countersinking bit, and to hold it steady. I was able to countersink all the other holes without any issue using this method.

Fuel tank ring countersinks

After deburring and tidying up all pieces, I clecoed both reinforcing rings into position, ready for riveting.

Fuel tank rings ready to rivet

Build time 4 hours.


About gstrack

Husband and father of 2. Control & Instrumentation Engineer. Flying nut. Gadget geek.


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