Help with delrin, silicone glue and 4-40 screws on HID light-head

[Rick Inman]

I'm about to do a little DIY repair/upgrade to my HID light. My question is, when I put my light-head together using the 4-40 screws, how much torque do I put on those screws? I'm afraid to strip the delrin threads if I bare down to tight, but I know they have to be good and tight to make the seal.

Also, the Vance book says to use black silicone to glue the parts together. Any pointers on using that glue in sealing the pieces?

 

[Steve in Seattle]

Delrin has a greater sheer strength than aluminum, so that shouldn't be a problem, but it doesn't corrode like aluminum so threads are always slick and easy to over-torque if you start wrenching on it... especially if you're using loc-tite or other liquid thread sealers.

If you can't find the spec you need, you can start with this general guide: Screw Torque

They suggest 2.9 inch-pounds for 4-40 screws in aluminum... 3 should be fine in delrin. YYMV.

 

[Steve in Seattle]

As for the black silicone... you're looking for a gasket-like seal between two flat machined surfaces.

Run a small bead all the way around both faces.
Smear it flat with a finger.
Give it a minute or two to "set up".
Seal it all up and wipe excess off while it's still wet. (tighten all screws lightly first time to ensure everything is flat... then tighten in a star/cross shape for even loading).