I wasn't happy with the way the alternate air door could be opened with the cable, but not closed again the same way. After a couple days, though, I had an idea for a simple way to make it better. First I bought a package of spring washers from McMaster. These are just little pieces of round spring steel with a bend in the middle. You can get them in many different sizes and spring rates; these are the ones I used:
I made a little 1/4" OD brass bushing on my lathe, and enlarged the pivot hole in the door up to 9/32" so it would be a loose fit over the bushing. A quick trip to the hardware store for some brass washers and I was all set…
Here is the hardware stackup. Around a #8 AN525 screw is the brass bushing, surrounded by the two spring washers which are sandwiched between a pair of brass washers. A thin AN washer goes between the lower brass washer and the screw head.
Some blue loctite on the screw threads for extra security:
Here's what it looks like when it's torqued down. The screw captures the bushing between the outer washer and the steel door aperture; the bushing remains stationary while the door rotates around it. The outer washer compresses the springs, which in turn bear on the door to keep it from flopping outward when it's open. The brass washers are just there to provide a bearing surface for the steel spring washers as they rotate.
Here's what it looks like all assembled. In theory the moving washers will tend to try to loosen the screw, so this could still be improved by using a proper bolted joint with a castle nut and cotter pin. I already had the #8 nutplate installed on the inside of the air duct, though, so I'll keep an eye on this area and make sure it's lubed periodically. Between that and the loctite, I expect it should be fine.
The little thingy on the push-pull cable in the previous photo is one of these – a machined collar more typically found on model airplane landing gear:
I decided to put the collar on the wire to give it a positive stop. In retrospect I cut the cable sheath an inch too short, but the collar helps to make sure the cable doesn't pull too far out.
I thought about making a little video of the door being opened and closed with the cable, but it's too cold in the garage. Trust me, it now works in both directions! I'm happy with the way this turned out.