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Replacement cowl screws - structural or non?

We're replacing our cowl screws as many of them are stripped and I'm completely unsure of what screws to purchase. It looks like there are both structural and non-structural options, as well as cad plated vs stainless ones. We currently have tinnerman washers for all of them and would continue using those going forward. Any advice on what to purchase? We've looked at torx screws but I'm not sure if there are aviation grade ones available.
 
Screws

If you want stainless MS24693C is what you want. 100 degree countersunk to match the countersunk washers. You need to measure your existing screws.#8 will measure approximately .160 diameter, #10 .187
If you search using the MS number you will find a chart with the various lengths.
 
Screws

You'll want MS24693C5 (AN507-8R8, Stainless) -or- MS24694C5 (AN509-8R8, Stainless) -- that's what I used, IIRC ;)

The AN509 are "structural", so they can be used with shear loads across the unthreaded shank portion.
 
You'll want MS24693C5 (AN507-8R8, Stainless) -or- MS24694C5 (AN509-8R8, Stainless) -- that's what I used, IIRC ;)

The AN509 are "structural", so they can be used with shear loads across the unthreaded shank portion.

It fills me with so much joy that the original builder of my plane is still active and helping me out 15 years after he built it.

Do you recall if you originally used AN509 or 507 for the screws going around the circumference of the cowl? The ones that are in there right now appear to be the non structural type as the threads go all the way up to the head, but I’d like to replace them with the correct ones if those are incorrect. Also, do we want stainless or cadmium coated? It appears both are options from Spruce. Thanks so much for your help!
 
Careful

I would be careful with stainless installed in nut plates. The stainless may gall. It happened to me with regular locking nuts when I started my firewall forward and choose stainless from the hardware store for adel clamps screws. YMMV
 
Structural on fiberglass?

If I am understanding this correctly, the screws are going through fiberglass to hold the cowl on. It is hard for me to imagine a screw failing before the fiberglass it is holding on does. I have skybolts and cant imagine they are structural. Even with large tinnerman washers, would the screws be structural?
 
Stick with the 100 degree screws for the reasons mentioned.

And I would steer away from Torx fittings. If you find yourself away from home and have to pull the cowling, a Phillips screwdriver is much easier to find and borrow at an airport.
 
It fills me with so much joy that the original builder of my plane is still active and helping me out 15 years after he built it.

Do you recall if you originally used AN509 or 507 for the screws going around the circumference of the cowl? The ones that are in there right now appear to be the non structural type as the threads go all the way up to the head, but I’d like to replace them with the correct ones if those are incorrect. Also, do we want stainless or cadmium coated? It appears both are options from Spruce. Thanks so much for your help!

Brian,

Glad you are flying/enjoying the airplane. It's really your choice of which screw to use -- here are the pros/cons as I see it:

The AN509/MS24694 is going to provide a little more capability in shear loading over the AN507/MS24693. However, the application of the screws in the cowl edge is under tension and not shear.

Since structural capability isn't a requirement, I would defer to cost. AN507's are $0.09, vs AN509's which are $0.17 (1/2" - 17/32")

IMHO Stainless looks better than cadmium plated. However, stainless material tends to "gall" in K1000-xx plate nuts, so use a little lubricant/ anti-seize on the threads before installing (e.g. Boelube "wax" in the push tube works well.)

At one time, I used stainless Tinnerman washers. I've since changed to undersized (#6 for #8 screws) 39006F flush finish washers. They have almost no flange, the countersink size is only slightly larger than the screw head, and they provide a clean finished look and wear reduction on the hole.

Cheers,

Brian
 
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