Should I be using Nylon or fiber washers on my spinner screws? What about on my inspection access panels?
Search strings: nylon washers, stainless steel screws, AN525, AN526, AN526C
Technical Editor:
The correct Nylon washer from Spruce, for size -3 (10-32) screws, is P/N 04-00218. Most spinner screws are size -3. See below:
Spruce Nylon washers
Note that if you just search Spruce for ‘Nylon washer’, the wrong ones will come up. The sizes you most often need, for spinners and access panels, are #08 and #-3 (8-32 and 10-32 screws).
For -3 (10-32) spinner screws, you can use AN526C-1032-8 MilSpec stainless steel truss-head screws from Spruce:
High-strength SS truss-head screws
This same type of screw (with Nylon washer) can be used in the forward LH and RH cowling joint, on the 1970 and later two-piece cowl; but you must use the longer -10 (5/8″) length. Inspection panel screws are the -832-8 size (again, with Nylon washers).
Note that, unless the connection is kept treated with Corrosion X/ACF50, SS screws should not be used directly against bare aluminum, unless a Nylon washer is used. Cad-plated hardware is designed so that the Cadmium is sacrificial; otherwise the aluminum would cause the steel to rust. This is why it is so important to stay ‘caught up’ with hardware that is showing signs of rust; it means that the Cadmium plating is failing (used up). If you put SS directly against bare aluminum, the galvanic activity is in the opposite direction; the SS will cause the aluminum to corrode. The Nylon washer not only protects the paint from damage; it interrupts the electrical flow that can lead to panel corrosion.
If your plane has little spots of corrosion around many of the panel holes, now you know ‘why’. Buy a few hundred of the Nylon washers, and make sure that they get used. Or you can switch back to the standard AN525 Cad-plated washer-head screws. But they won’t protect the paint; they cost much more; and you’ll still wind up changing screws out due to rusting. Note that there are structural panels inside the plane that must use the AN525 screws, for strength reasons.