Product: SP4137 Frizzen Spring Screw (repro.) for the M1816 and M1821 U.S. Musket
From: Robert Flowers
Date: Saturday 29 April, 2017
This US Arsenal screw thread measures 4.83mm x 1.0M, (SAE callout would be a #10 machine screw with 25.4 tpi). It was copied exactly from French musket threads to keep the Revolutionary War Charlevilles working, and used because it worked on later muskets. If the original 1816 frizzen screw cannot be found, a sear-spring-screw from a trapdoor springfield model 1873 will fit, but the head is a different size. If making this substitution, trim the threads flush with the inside of the lock plate after installing to avoid interference with the Mainspring. In ANY event, because the 1816 predated interchangeable parts and ALL parts might have to be hand-fitted, verify the installed threads are flush with the inside of the lock plate.