Loose Rotor Nut or Rotr Center: Triumph Service Bulletin Twin 9-70
Dave Porter, Customer and Tech Support at The Bonneville Shop
“When the rotor has been in service for many years, the magnetism can decline, which leads to weaker output from the alternator.”
This Triumph service bulletin, released September 2, 1970, was a reminder to dealers of the importance of a properly fixed magnetic rotor with the rotor nut. As the rotor is spun at upwards of 6500 rpm, it is critical that the rotor key, locking tab washer, and spigot nut are all in serviceable condition for the rotor to perform its job of revolving inside of the stator and creating alternating current. The stator and rotor together comprise the alternator.
The magnetic rotor is a robust component, but is not immune from trouble, which can lead to damage to the stator and other electrical parts. The rotor has a hexagonal steel center with a high energy magnet mounted on each face. Each magnet is keyed to a laminated pole tip and the complete assembly is cast in aluminum and machined to a smooth external finish.
When the rotor has been in service for many years, the magnetism can decline, which leads to weaker output from the alternator. Another issue that is common to the Lucas rotor is the loosening of the center hub, which allows the body of the rotor to rock on the hub. This a worrisome condition that necessitates the replacement of the rotor, as the specified air gap of .008” between the rotor and stator becomes erratic and can cause the spinning rotor to foul the stator, resulting in a short, and perhaps severe damage to the stator.
A loose center hub is often identified by a series of centerpunched divots around the outside circumference of the rotor hub, in a misguided attempt to stake the center to the aluminum body. The service bulletin suggests using medium-strength Loctite thread on the cleaned threads of the rotor nut and crankshaft stud, prior to tightening the nut to the specified torque of 40 ft. lbs.
If it becomes necessary to replace the Lucas 54213901-type magnetic rotor, we offer the Lucas reproduction rotor under part number 54202299 G, compatible with the unit-construction Triumph twins with single or three-phase charging systems.
