BOSCH starter motor fitting 1937 Bugatti Type 37A part # BOSCH BGB 0.6/12 Spiral Drive spring

a very rare and clever "Clutch" spring that expands inside of the starter drive gear&bore, and is engaged to the forward thrusting armature by drive-dogs at both ends of the spring- By UNWINDING the spring slightly- starter rotation expands the spring in the gear drive bore, and grabs the gear to be driven by the rotating motor armature. Get all that? pretty amazing engineering. Actually, crank-type window regulators work on the same principle- so that the weight of the window does not unwind the window crank. Here we used Silver alloy Brazing technique to repair a broken and worn out drive dog interface, by brazing the drive dog directly to the spring face.