Old Britts – Overload Backfire Device Assembly and Adjustment
Download the complete PDF version using the green download button at the top right of this page for a detailed Old Britts workshop guide covering the assembly and adjustment of the Norton Commando MK3 overload backfire device. This important starter drive component is designed to slip during an engine backfire, helping protect the sprag bearing and associated electric-start gears from damage when it is correctly assembled and adjusted.
This technical article by Fred Eaton expands on the information provided in the MK3 workshop manual and explains a common point of confusion regarding the specified 50 ft-lb slip torque. The guide makes clear that this is not the torque applied directly to the retaining nut, but the amount of shaft torque required to make the overload device slip when properly adjusted.
Step-by-step photographs show the complete assembly sequence, including the circlips, thrust washers, backfire gear, ball bearings, carrier, disc springs and adjustment nut. The guide also explains the use of anti-seize on the contact surfaces, the correct orientation of parts during assembly and the final method for securing the nut after adjustment.
Contents Include
- Overload backfire device function and purpose
- MK3 starter train protection overview
- Correct interpretation of 50 ft-lb slip torque
- Complete backfire gear assembly sequence
- Circlip, washer, gear and carrier installation
- Ball bearing placement and disc spring orientation
- Use of anti-seize on operating surfaces
- Slip torque adjustment procedure
- Homemade torque-setting tool explanation
- Peening the nut flange and applying green Loctite
