Headspace Plug Gage Instructions
General
This gage is what is known as an “over-length” gage. It chambers like a
cartridge but is longer than the cartridge which prevents the bolt from fully
closing against the front end of the bolt cavity. By measuring the gap between
the front of the bolt and the front end of the bolt cavity and comparing that
dimension with the measured length of the gage, the headspace can be calculated.
This indirect method of measuring headspace is required because the Thompson
bolt does not lock into a fixed position when closed. It is either stopped by
the end of the receiver bolt cavity or the base of the cartridge.
Gaging
Remove the receiver assembly from the trigger housing assembly. Cock the bolt.
Chamber the Gage by hand as though it were a cartridge being sure to insert the
Gage, large end of the Gage first, into the chamber. Allow the bolt to move
forward against the Gage making sure that if the firing pin extends out the face
of the bolt, that it goes into the hole on the reduced diameter end of the Gage.
The Gage must be flush against the bolt face and the end of the chamber.
However, as described above, the Gage will not allow the bolt to completely
close. Measure the gap between the front end of the bolt and the front end of
the receiver bolt cavity. Gage pins are best for this but feeler gages can also
be used. If the headspace is proper, the gap measured will fall within the range
shown in the table below for the particular gun model being checked. All gages
shipped after August 1, 2008 have all been ground to exactly 1.000" long.
Model 1921-1928A1: Allowable gap is .098-.110"
Model M1: Allowable gap is .096-.112"
Model M1A1: Allowable gap is .107-.124"
Headspace Correction
There is no adjustment on the gun if the headspace is not correct. Headspace is
determined at the time of manufacture of the three components which affect
headspace. They are the receiver. bolt and barrel. If the headspace is not
correct, one or more of those three parts is defective. Aftermarket barrels are
notorious for not meeting Thompson specifications because the manufacturers
usually do not have the original drawings and, therefore, have no way of knowing
what the original specifications are. Original parts were all gaged at the
factory and do not change after use.
Douglas W. Richardson
2100 McReynolds Road, Malibu CA 90265
Telephone: 310-457-6400
E-Mail: ThompsonSMG@mail.com