Re: base screw problem
The standard machine screws are 6-32, 6-40, 8-32, and 8-36.
But for fastening scope mounts to receivers, we use 6-48 or 8-40, which are non standard.
I buy the non standard taps, dies, and bulk screws from brownells, a gunsmithing supply house:
http://www.brownells.com/.aspx/pid=19929/Product/LONG_6_48___8_40_SCREWS
I often start out with long screws. I drill and tap dozens of different rifle designs and use many non standard or shop built scope mounts.
e.g. Today I am drilling and tapping a Sav 219 for a Weaver #74, which is made for a Sav 24.
e.g. Today I am also drilling and tapping the front mount of Rem510 on the receiver, not the barrel, so a Weaver #43 will be used, instead of a Weaver #42 barrel mount.
When the screw is shortened [with grinder or hack saw], the tip of the threads is fouled. It is very difficult to get a screw with a fouled tip to start in a threaded hole.
So I put the screw through a die or a pair of nuts before shortening it.
The die or nuts will be used to reform the threads on the tip, before they are completely removed from the screw.
The die or nuts also make a good way to hang onto the screw while shortening it.
Then I use the fine grinder on the end of the screw.
The screw is then backed off into the nut or dies and given half turns in and out while different angles of radial force are applied.
This raises the burr on the end of the threads.
I use the fine grinder again making a tiny bevel at the tip of the threads. This removes the burr.
With 6-48, there are not many nuts around, so we make nuts with Hex steel and the 6-48 tap.
For standard set ups, like VZ24s with #45 and #46 steel Weaver mounts, I have a 3/8" thick plate 1010 steel with drilled and tapped hole with the correct depth milled on the screw head side. The screw is screwed into the plate, and flush ground on the other side with the end wheel of the grinding wheel. The screw comes out of the plate already the exact correct length.
Without such a jig, I must count the turns.
e.g. If a rear mount screw interferes with the bolt movement, I back out the screw until the bolt moves freely or the screw looks like it is not protruding into the bolt bore. Then I screw it all the way in, counting the number of turns until the screw is tight. That number of turns represents the number of threads that need to be cut off the end of the screw.
The Tap Magic or Moly Dee or whatever cutting fluid is used to tap the threads, and what ever release agent is on the threads for glass bedding the mount to the receiver must be cleaned off before Loctite can be applied to the threads.
Loctite is like glue, and it does not bond through oil.
Steve Wagner has his own way of shortening screws:
http://www272.pair.com/stevewag/screwholder/screwholder.html
