The factory MGB GT V8 starter motor is the Lucas 3M100, a powerful pre-engaged starter sized to crank the V8's 3528cc engine reliably in all weather conditions. The starter mounts on the V8-specific bellhousing position, engaging the engine flywheel ring gear via a pinion that is engaged before the main starter motor circuit is energised. Pre-engagement avoids the gear-clash that would otherwise occur with a Bendix-type starter trying to engage a spinning flywheel.
Starter Operation
When the ignition key is turned to the start position, current flows to the starter relay, which in turn supplies the starter solenoid mounted on the starter body. The solenoid pulls the pinion forward into engagement with the flywheel ring gear, then closes the heavy contacts that supply battery current to the starter motor. The motor cranks the engine until the key is released, at which point the solenoid is de-energised, the contacts open, and a spring returns the pinion clear of the flywheel.
Starter Relay
The starter relay carries the heavy current to the starter solenoid, with a small switching current from the ignition switch operating the relay coil. The factory V8 used part number 142169 (rectangular type) up to 1976; cars built in 1976 used part number CHM68 (round type). Relay failures are a common cause of cranking problems, the relay clicks but the starter does not crank, indicating worn relay contacts or a failed coil.
Starter Solenoid
The starter solenoid is mounted on the starter body, integrated into the starter assembly. The solenoid both engages the pinion and closes the main starter motor circuit. Solenoid faults, burned contacts, worn pull-in coil, sticky plunger, cause cranking problems. The solenoid can be serviced separately or replaced as part of a complete starter exchange.
Service Notes
Starter motor service typically involves replacing the solenoid contacts (which burn in service), the brushes, and the pinion drive bearing as needed. Major rebuild may include the armature bushes and the field coils. Reconditioned starter motors are available on an exchange basis where complete replacement is preferred.
MGOC Spares
MGOC Spares supplies V8 Lucas 3M100 starter motors, starter relays in both 142169 and CHM68 specifications, solenoids and starter service components for the MGB GT V8.