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Re: Rear sight base threads
Posted by Pete Nelson on Monday, 14 February 2011, at 8:11 p.m., in response to Rear sight base threads, posted by Cary Meyers on Monday, 14 February 2011, at 4:40 p.m.
A couple of years ago I decided that it might prove helpful someday to have a table of the various thread sizes of all of the thread sizes (especially the smaller fasteners) made by the Springfield Armory in producing the various models of "TRAPDOOR ARMS". With the notable exception of the large square-form Buttress-type thread used on the Tenon of the Barrel where threads into the forward end of the Receiver (for some reason my table is missing that one), all of the other threads - - including the large thread on the Breech Screw where it threads into the aft end of the Receiver - - all of the other male and female threads are the no-longer-made 26-tpi thread form.
I measured all of the male threads using both Starrett 1-inch electronic micrometer and a Starrett 1-inch dial-type caliper,the flat round anvils of the micrometer giving the more consistant readings. I took quite a number of measurements on both the older-style slotless screws and the later slotted ones made for the Model 1884 Buffington Rear Sights, and the average of all the readings was 0.157"-26. In make a search of a number of tap-and-die manufacturers, I found none who made (or seemed the least bit interested in making) any of the now-obsolete 26-tpi special taps-and-dies. A few years ago "a certain TRAPDOOR dealer" was offering a 2-tap/2-die set (for $225.00 a set) which contained the 0.187"-26 tap/die (for the Hammer Screw; Sear Screw; Sear Spring Screw; and Bridle Screw) and the 0.195"-26 tap/die for the (2) Lock Assembly Screws (the ones which thread into the female holes on the Lock Plate).
If my table is anywhere near accurate (and I will re-check them all just to be certain):
The Stock Fore-end Cap Screw, like the Rear Sight Screws, is also a 0.157"-26.
The Trigger Pivot Screw is a 0.116"-26.
The Tang Screw (which passes down vertically through the clearance hole in the Tang of the Breech Screw, through the wrist of the Stock, and threads into the Trigger Plate just forward of the Trigger. It is a 0.215"-26 male thread.
And the thread on the nose of the Breech Screw is a 0.712"-26 (I will have to re-check the pitch on that one) and threads into the aft end of the Receiver where it forms the socket recess for the latching cam of the Breech Block Assembly. The is no thread which will successfully mate with the (2) tapped holes in the top of the Barrel to secure the Rear Sight in place, and the only one which is anywhere close (but still no cigar!) would be a #8-32UNF-2A fine series screw thread.
I finally bit the bullet and bought that little set of 26-tpi taps/dies, and it has proven worth its weight in gold more than once. Perhaps we "TRAPDOORists" as a group should approach Vermont or some of the better tool-and-die manufacturers to see what they would charge to produce a boxed set of 7 or 8 of these assorted sizes.
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