reecewallace wrote: ↑Fri Jan 26, 2024 2:31 am….
Really appreciate this! I didn't know anything about MS vs SS, so appreciate the explanation. I will order the MS versions - I'd rather replace the screws more often than galvanic corrosion.
The term SS applies to the alloy used, such as Stainless Steel, …and in aviation hardware that is usually designated with a “C” in the PN.
The abbreviations of AN, MS, NAS are in reference to hardware quality standards. An Ad Hoc and unofficial explanation of AN/MS/NAS is…
The development of aircraft special hardware needs was identified by the army and the navy in the early part of last century, and a joint committee known as the "Army-Navy" aviation standards commitee worked out a mutually agreed standard which became known as the "Army/Navy" or "AN" standard. It described materials and fit specifications for aircraft hardware. As Aircraft became more sophisticated (and as the military services each became more involved in aviation) the standard was revised to include certain additional specialized fasteners, and a new standard known as the "Military Specification/Standard" or "MS" standard. As commercial aviation developed specialized needs for aircraft use, the "National Aerospace Standard" or "NAS" standard was defined.
Generally, AN bolts are approved for most applications, and most AN hardware also has "MS" definitions as well. The NAS hardware is usually more specialized, and almost always refers to hardware with higher strength materials and/or closer fit tolerances than ordinary AN/MS hardware. For that reason is is also more expensive.
Not the “official” definition…but …Hope this helps clarify.
The cautions against the use of “SS” hardware on aircraft is valid…but, in my personal opinion, unlikely to be a problem. The typical concerns are electrolytic or “galvanic” corrosion…. unlikely within the lifetime of your ownership of the airplane in most cases….. And the other concern is the use of powered hand-tools with “SS” screws, because it’s easier to damage the SS screw-slots than it is with “ordinary” AN or MS screws. (Once you’ve damaged the phillips-head of the screws they can be difficult to remove.)
Those who DO use “SS” or “Corrosion-resistant” (I.E. that’s what the “C” stands for in the PN) …. usually have no complaints…except from their A&Ps and IA’s because those guys spend their professional lives dealing with them after they’ve used their Makita or Milwaukee screw-guns stripping out those screw-heads. (I use the stainless “kits” I mentioned earlier on my airplane and have had no problems, and the reason I recommended the “kits” was because the excess amount of screws in those kits are helpful to already have on-hand if/when you do strip them.
A helpful little thing to reduce the “galvanic” corrosion issues is the use of nylon washers beneath the screw-heads when installing them. They are cheap and insulate the screw from it’s greatest electrolytic-contact-surface with the aluminum airplane and reduce the corrosion problem.
You can buy them from Spruce: If you use SS hardware I recommend them beneath the heads of the harware used on cowling and inspection cover applications…. I would not use the nylon washers in a structural application such as the fuel tank covers…. use “C” rated washers there if you use SS
MS27039C screwsl
https://www.aircraftspruce.com/catalog/ ... ey=3010471
YMMV