Gene:
Use a 2/56 nylon screw. It isn't metric but it is easily threaded into the mounting hole. It provides a solid mounting. I've done 2 S2s this way and have had no problems. You can also use a Kadee Delrin screw. See this link: http://www.tcsdcc.com/Customer_Content/Installation_Pictures/HO_Scale/Atlas/S2/Atlas%20S2.htm
Joe
I put NCE ATL-S4 decoders on my S2's. They come with the nylon screw, Kapton tape, etc. required to proper isolate the lower brush, and they run well.
I want to install a TSC AS6 decoder in my HO Atlas S2 switcher. The instructions indicate that you have to replace the motor mount screw in the bottom of the frame with a plastic one to isolate the motor, this has caused problems for me. The screw appears to be a 2mm wide by 5.1mm long panhead. As far as I can find none are available. Does anyone know of a source or has someone been able to work around this problem ??
Thanks,
Gene Perryman