Lucas Starter Inspection And Maintenance
Tech Talk with Bob Shafto, Technical Editor
The Lucas starter (M35) was original equipment on all MGAs. They are simple and easy to maintain, but first we need to understand how they work. Most starters today are push to engage or outboard starters, but the Lucas M35 is a pull to engage or inboard starter, meaning the pinion gear moves toward the drive motor instead of away from it.
The pinion gear mates with the ring gear on the flywheel and both have a taper cut into the leading edge of the teeth (common to all starters and ring gears) to assist alignment and full engagement.
Inside the pinion gear housing, is a sleeve with a helical spline on the outer diameter (which acts like threads on a screw or nut and bolt) and a linear spline on the inner diameter that fits the spline on the motor drive shaft (Figure 1).
When the motor is energized, the sudden torque and (counterclockwise) spinning motion, causes the pinion to forcefully, propel (unscrew clockwise along the helical spline) toward the ring gear. The rotation of the pinion gear during travel is opposite the motor shaft rotation which causes the tapered face of the pinion tooth to meet the tapered face of the ring gear tooth for alignment and engagement at low speed. There is also a spring that will cushion the strike, if the pinion happens to strike a ring gear tooth head-on, allowing the pinion to rotate slightly and align. Once the pinion gear has reached the end of its lateral travel (full engagement), it is forced to turn with the motor shaft in the counter clockwise direction and against the non-tapered side of the pinion and flywheel teeth. This should take between 30ms and 50ms.
When the engine starts, the ring gear speed overcomes the pinion speed, screwing the pinion gear back to its disengaged position. An internal spring keeps it there.
Understanding how it works makes it clear that the pinion gear internal parts must be kept free of rust, dirt and grime to work properly. If the pinion does not slide quickly and easily, torque will be applied before total engagement and tooth damage will occur. The free movement pinion is the most important aspect of the starter. I recommend that the starter be removed, inspected and cleaned at each oil change.
The pinion lives in a somewhat secluded environment but still collects clutch and road dust. Cleaning should begin with a good blast from an air hose. Next, check operation by hand while turning the pinion gear so it travels full length, feeling for smoothness of travel and operation. When you release the pinion at the end of travel, it should return quickly and with an audible snap. Inspect the shaft and spline surfaces to be sure they are clean and smooth.
If further cleaning is necessary, use a thin spray lubricant like WD-40 to dissolve grime, then a good blast from an air hose. Repeat and continue until all the grime is gone and the drippings are clean, then do the hand test again. Do not use heavier lubricants as they will thicken, cause drag and collect dirt and dust. Cleaning seldom requires disassembly but for stubborn cases, soak in mineral oil over night then do the air hose and WD-40 process again. Always dry as best as you can with an air hose. This is also a good time to blow the dust out of both ends of the motor and add a few drops of 20 weight oil to the bearings. If the motor shaft and pinion show signs of rust that can’t be removed using this method, remove the pinion from the shaft and clean the shaft and pinion with 600 grit sand paper. You can securely clamp the motor to a bench and power the motor with a car battery while sanding the motor shaft just enough to remove the rust, then polish the shaft with 800 or 1000 grit sand paper. Caution: Do not run the starter more than 30 seconds at a time. Let the motor rest for a few minutes between each interval. Overheating can damage the motor.
Considering switching to a modern starter? First you should be aware that there is always a tradeoff.
Almost all modern starters operate using the same basic principles. They still use the helical spline which operates the same way for the same reasons, but they use a solenoid to move the pinion forward (push to engage). They also incorporate either a switch that applies power to the drive motor when the pinion is fully engaged or a 30ms to 50ms electrical delay between the pinion solenoid and motor activation to give the pinion time to fully engage before spinning the motor. This is an advantage over the original starter, however most modern engage to the opposite side of the flywheel and ring gear. This means the original leading edge tooth design is no longer used, so the pinion is no longer engaging a tapered flywheel tooth to help align and engage (Figure 2). It is instead, trying to mesh into the sharp corners of the ring gear teeth. Another issue is that the original ring gear tooth design has a reduced height on the engine side of the gear. This is the pinion engagement side when using a push to engage starter, so the flywheel tooth drive surface is as much as 50% less, doubling the point load on the tooth. Reversing the ring gear might be helpful by giving more drive area on the tooth and reducing point load, but the taper is on the wrong side of the tooth to be of much help with tooth meshing.
The ring gear teeth on modern designs have not changed and are always orientated to match the pinion engagement. If you are thinking that these starters will reduce tooth wear, it is unlikely.
If you can find a 120 tooth ring gear that fits the MGA flywheel and has the taper on the correct side for a push to engage starter, let us all know. Until then, I will keep the original starter working properly and repair it with original equipment when the time comes.