Question concerning this Zischang Sharps Borchardt, what is the screw in the top of the breech block for?
Hello LRF,
Michael is correct, the screw on the top of the rear end of the breechblock is a cap screw covering access to a much smaller adjusting screw contained within the firing pin, the adjusting screw which passes vertically through the firing pin contacts the top side of the sear. Turning the adjusting screw in, or out, will alter the depth of engagement of the sear in the sear notch.
A description of this adjusting screw and an exploded drawing of the Borchardt breechblock showing where parts have to be modified appeared in The American Rifleman for September 1946 pages 22 & 25 and was written by a Mr. Al Marciante, under the Title of 'Borchardt Trigger Pull'.
The article does contain a gross error in that the author recommends 'squaring up' both the sear notch and the sear tip. In fact the contact surfaces of the sear and the sear notch are angled at 103 degrees to the centre line of the firing pin. This angle is due to the geometry caused by the position of the sear pivot pin. This angle was known to Borchardt as it appears in his patent drawings, but seldom, if ever, in more recent drawings.
A simple grinding jig can be made to hold both the sear and the firing pin at the correct angle that can be used on a precision grinding machine. (See attached picture below). The firing pin in the picture is not finish machined, and is only shown in position in the jig for demonstation purposes.
I have not been able to trace any reference to this adjusting screw mechanism prior to 1946, and I strongly suspect that, whilst several of these sear engagement adjusting screws are known to exist on Zischang Borchardts, they were fitted long after the Master had passed on.
Harry
