Top cowl to firewall hinge attachment

Just finished trimming my epoxy/glass cowl to fit the -6A firewall and am starting to fit the cowl to firewall hinges. After searching my (ancient and mostly not helpful ) builders manual, scanning the -6 and -7 prints in my collection, I still can't figure this one out.

On the curved hinge section across the top of the firewall with the undersized pin installed (0.090" pin in 0.125" hole), what keeps the aft edge of the cowl in line with the forward edge of the top skin? I'm thinking in flight differential pressure loads and/or engine vibration and/or curvature mismatch between the cowl and firewall as possibly causes of movement. (My cowl is about 1/16" high in the center, a couple inches either side of center line, fits well everywhere else.)

With the cowl and firewall hinge halves aligned on center, there is +/- 0.0175" (0.035" total) vertical movement possible between the cowl and skin. OK, in reality the cowl can not really move below the skin by much because it will be stopped by the hinge eyelets on the firewall side. But......it can move up by some amount, depending on the initial alignment of the hinge halves. Has anyone offset the nominal position of the hinge halves using shims to prevent the cowl from moving up?

I have searched everything I could find about cowl fitting on the Van's Air Force site, the Matronics RV list, and my favorite "wonder how they did it?" builder websites. Lots of information, lots of great photos of the process......but nothing that answers my questions.

5753 flying.......somebody has to have figured this out....other than using camlocks

cowling

I sanded my cowling down to fit the skin on the the boot cowling. I just kept sanding the cowl down until it dropped down onto the hinge. After this is done, I installed the hinge pin. The plans tell you to use the .090 but I took a .125 pin and stuck it in a drill and tapered it down with a file and sand paper on the bends around the top of the cowl. This way it will go down into the hinge on the curves but it is full thickness on the top part of the cowling. The top cowl will bulge with air pressure but there is little you can do about this. I drilled the hinges in place and then I found out what thickness of shims I needed to make the top of the cowling match the top of the boot cowling skin. Some areas on the cowling are thicker than others and you might have to sand these down a bit. After this is done, I took it all apart, sanded the hinges with scotchbrite, sanded the cowling were the hinge goes, sanded the shims and riveted it all back together with rivets and JB weld epoxy. Make sure you clean all the areas prior to applying JB weld with MEK. Put a pin in the hinge while you squeeze the rivets. This helps the hinge to stay straight and it also keep the epoxy out. Clean after all is done with some MEK making sure there is no epoxy in the hinge eyes. I also put a piece of .063 strip where the 4" gap is for the hinge pins to go. This will bulge way up if you don't put a stiffner in there. I also put some tabs on the side of the fuselage where the pins can't go just below the ending of the hinges on the curves. This helps keep the sides from bulging as well. The cowling is very hard to fit and no two are alike. It took me about 3 months of on/off work to get the cowling to fit, hinges riveted on, all fiberglass work done, prepped and painted to completion. If you take your time, it will turn out great.

Thanks for the tip on using a tapered 1/8" steel pin.

My cowl does not need to be sanded to fit. It sits down on the hinge without any restraint (no tape, no clamps, etc), except for a small high spot in the very center. The high spot is a small enough length it can be pulled down to the hinge......as long as there is no excessive clearance in the hinge from using an undersized pin.

I understand the cowl may bulge in flight with the engine cooling air pressure. Oh well, that comes with the design. A tighter hinge pin fit along the top will minimize bulging.