Thread pitch for a sight rail?

Armen · #1 11-25-2010, 11:39 AM

I'm building an S+W model 64 into a target gun. After I install the Clark stainless bull barrel, I'm going to mount a sight rail on top. The rail is a Bo Mar type rail I bought used with no screws or identification. The two mounting holes in the top are drilled for #6 allen or cheese head screws. I should be able to get .300 of thread engagement in the barrel.
So, the question is-what thread pitch to use? Figure I'll do the drilling and tapping on a milling machine, and go three steps on the tapping (taper, plug, bottoming).
Should I use 6-32, 6-40, or 6-48?
If it was really hard steel (it isn't, it's stainless), I'd do the 6-48. If it was aluminum, I'd do 6-32 (corse thread in the soft stuff).
Probably anything will work, but one has got to be better.
Suggestions and reasons?
thanks,
-Armen

tomcatt51 · #2 11-25-2010, 12:12 PM

The "industry norm" is to use 6-48 or 8-40.

Armen · #3 11-25-2010, 12:45 PM

Looked at it a bit closer. Maybe upping the screw size to 8-40 is the way to go. Easy enough.
Thanks for the info.

tomcatt51 · #4 11-25-2010, 01:49 PM

The factory drilled and tapped S&W revolvers are 6-48.

scooter123 · #5 11-25-2010, 02:39 PM

Finer pitch threads offers an an increase in holding power and resistance to vibrating loose, which is why S&W uses the #6-48 thread for the newer mounting scheme. I would be inclined to use the # 6-48 simply for that reason, however these are sizes that are NOT easy to find. Before you make your final choice I would suggest that you check Brownells and some other sources to make sure you can find screws that fit the holes you have tapped.