I'm looking at replacing the screws that hold my Aileron on. They are starting to show some signs of rust red discoloration around the paint. I found the screws on Aircraft Spruce, and they offer both the Cad and the SS. The parts manual calls for the Cad, but if it won't hurt, I think I want to go with the SS.

I am looking at getting MS27039-1-10 or MS27039C-1-10

I should also say that I am considering doing a paint job on her this fall. Is this something I should wait until then to replace, or do it now.

Any opinions from the great BT minds?

Also any thoughts on the painting process? I'm looking at doing it myself with my A&P and a local guy who works for a local airplane paint shop. I'm a CSOB and need to get her painted this fall as the previous paint job was not done properly and there is some micro bubbling under the top coat.

I bet the cad ones are stronger.....

Piper Aileron screws are supposed to be structural.

I concur. SS hardware is usually not as strong as cad. If the application is structural, "going pretty" might end badly. This has happened with Navions before, where the wing skin is structural (no spars!), and the inspection plates are held on with structural, not SS, screws. Pulling rusty cad hardware and going SS shiny has caused airplanes to come apart in the air.
Robin

So far, I've learned that SS is not as strong as Cad and that SS is not structural, just pretty.

This is why I love BT. I'm glad I checked with you all before ordering.

So is this something that I am ok waiting on until paint? I assume the heads need to be painted to prevent corrosion of the CAD structural screws correct?

Hi Robin. Rip here. There ARE structural stainless steel screws, but you have to be careful!
I've even seen a few Navions with PK (sheet metal) screws in the area you mention.

Fly safe.

Hello
As stated above the MS27039 is a structural screw in both the cad plated and the corrosion resistance steel (-C). The cad plated alloy steel and crescent steel are very close in tensil and shear loads. I would use the cad plated one since you are ultimately going to paint over them. Look at the attached images and you can look at the mil-spec on these screws

As others said... As long as they are MS27039, they are structural regardless of material. There are some alloys of stainless that are almost as strong as carbon steel, but in general always weaker. The design of a MS27039 screw is to be much stronger in shear with a small unthreaded shank to take the bearing loads.

The stainless version of the MS27039 screw is a dull gray color, not the bright stainless color. This is due to the metalurgic additives to give it it's strength. After a while, they will still rust, I'd go with the cad plated ones.

Thanks for all the advice!

I'm looking for the nuts and am having trouble finding them. MS20365-1032C

It doesn't look like Aircraft Spruce has the exact product although did find it over at Wicks.

Is AN365C-1032 a suitable alternative or should I just go with Wicks?

The stainless used in these fasteners isn't your typical stainless. Whilst the chrome content is similar to your typical 304 / 18-8 / kitchen sink stainless steel - it has a significant amount of Nickel which gives it good high temperature properties (and also is a dull grey colour). Furthermore it has various heat treatments to improve strength.

It is worth noting that the strength figures in the reference spec are the ultimate strength. IMHO, a more practical measure is the yield strength - as that is when the material starts to stretch.

The cad plated screws in the spec can be any one of the materials listed, the one I looked up with 4130 which has a room temperature yield of 161000 psi. The stainless screws to A-286 / AMS 5731 have a yield strength at room temperature of 85000 psi. (AMS 5732 screws will be a bit stronger, but how many fabricators will put the material through another heat treatment process where they don't have to?)

So you can see that whilst the ultimate strength of the cad plated and the stainless is about the same, when the load is increased on them, the stainless screws will start deforming at a little over half the load on the carbon steel screws.

Also the AN365C-1032 is interchangable with the MS20365-1032C according to this: https://www.airpartsinc.com/shopexd.asp?id=556