Breech Entry Countersink Instructions
General
The front end shank portion of the Thompson bolt enters the breech through a
hole in the front end of the bolt cavity until stopped by the body of the bolt
hitting the front end of the bolt cavity. There is a fillet where the body and
shank portions meet. Clearance must be provided for this fillet at the entrance
of the hole by chamfering (countersinking) the entrance of the hole. The Breech
Entry Countersink is a specially made cutter having a pilot to guide it in the
hole and a 9/16" hex for use in turning it. The radius of the Countersink is
larger than the distance from the center of the gun bore to the inside top of
the receiver bolt cavity. Therefore, 1921-28 Thompsons use the actuator slot to
provide the extra room needed by the Countersink. M1/M1A1 Thompson receivers
have a clearance pocket cut into the top of the bolt cavity to provide the
clearance.
Setup
Insert the Countersink pilot end into the breech entry hole in the front end of
the receiver bolt cavity. Pass a 1/4" socket wrench extension through the pilot
hole in the back end of the receiver. Fit a 9/16" six point socket to the end of
the extension in the bolt cavity. Chuck the other end of the extension in a
drill motor. Fit the socket over the Countersink hex end.
Chamfering
Chamfer the hole edge in accordance with the Thompson receiver manufacturing
drawing.
Douglas W. Richardson
2100 McReynolds Road, Malibu CA 90265
Telephone: 310-457-6400
Website: www.ThompsonSMG.com