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Upper cowl hinge drilling alignment

amaris

Well Known Member
Hello all, I could use some advice. I'm fitting the upper cowl and when looking at the firewall tabs where the hinge will attach, without the top skin clecoed into place, the firewall moves around. How do I drill my hinge with either the clecos in the way or without the top skin but making sure my firewall tabs are all in alignment so when I do go to final rivet, everything lines up?

I thought about marking a reference line on my hinge and making sure each rivet hole of the firewall lines up with that. But wondering if that is actually the correct alignment once the top skin is in place.
 
Proseal

I put to top skin on before the firewall forward stuff.
I had glued the firewall and top skin together with the proseal firewall sealant using the clecos. There are better materials than proseal for this seal (see other posts here on VAF). I also riveted the areas of the firewall to skin that did not have a hinge, like the gap near the oil door, the area near the upper engine mounts, etc. This kept the firewall in place for the top hinge installation. YMMV.
 
Thanks, John. I'm not ready to put the top skin on for good, I have too many other bits and pieces to finalize in the subpanel.
 
Holes in the top skin are the guide, not the firewall tabs. Use the top skin to drill the first two holes in the hinge, as the sets up the alignment. Do this while the skin is held firmly in place minus a few clecos near the drill spot, as the alignment can change with the curvature. You can remove 2 or 3 clecos near the hole to be drilled and the hinge will flex around the others. With a piece of wood to protect fingers, sandwich the three layer together with your fingers. Using skin hole as a guide, the drill bit will force the SS tab to center and put the hole in hinge. It is SS and difficult to drill so no worries about wallowing out the hole like aluminum. You can line up the SS tab with the drill bit before squeezing it together to get them close enough for the bit to center the second hole. Cleco the hole you just drilled and remove the next cleco beyond your gap and proceed. Be sure the hinge aligment doesn't change as you do the first few holes.

Larry
 
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Hi Larry,
Thanks, appreciate the detailed info! I'll give that a go and see if I'm getting things lined up properly.
 
Hinge alignment

Holes in the top skin are the guide, not the firewall tabs. Use the top skin to drill the first two holes in the hinge, as the sets up the alignment. Do this while the skin is held firmly in place minus a few clecos near the drill spot, as the alignment can change with the curvature. You can remove 2 or 3 clecos near the hole to be drilled and the hinge will flex around the others. With a piece of wood to protect fingers, sandwich the three layer together with your fingers. Using skin hole as a guide, the drill bit will force the SS tab to center and put the hole in hinge. It is SS and difficult to drill so no worries about wallowing out the hole like aluminum. You can line up the SS tab with the drill bit before squeezing it together to get them close enough for the bit to center the second hole. Cleco the hole you just drilled and remove the next cleco beyond your gap and proceed. Be sure the hinge aligment doesn't change as you do the first few holes.

Larry

Always try to leave both halves of the hinge together with the pin in place.
A few small C-clamps and some small 1/2 strips of 1/4" plywood work well to hold the hinge nice and tight till clekos are installed.
I recommend a little math to figure out if a shim is needed and the thickness required before final riveting of the hinge. I needed a .032" shim for the cowl to sit flush but I adeed a strip of carbon fiber and thin layer of fiberglass to all the cowl edges.
 
If you're needing to put the cowl on now, the advice that the top skin sets the position is correct.

You can always install the top skin temporarily with pull rivets and proceed and drill them out later. Did that, no big deal.

Cheers
 
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