Problems With Inertia Drive Starter Motors

Problems With Inertia Drive Starter Motors

This guide captures ‘bygone’ advice on Problems With Inertia Drive Starter Motors. It is part of a series I am building on Starter Motor Problems 

To see all my ‘bygone’ advice guides please visit Classic Car Maintenance DIY Guides.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.





Common Starter Motor Problems

Below is a quick diagnosis chart for most common starter motor problems. All of which will be covered in detail within Starter Motor Problems . This particular page focusses on a seized inertia drive.

Common Starter Motor Problems

Problems With Inertia Drive Starter Motors

It is by no means certain that problems with the operation of the starter are always caused by faults in the Bendix gear mechanism.

If the starter motor does not turn when the ignition switch is operated, it is important to check first exactly what is causing the malfunction.

If there has been no abnormal behaviour previously, the most likely source of the problem is a flat battery or a fault in the wiring, earth connection, switch or solenoid.

Exploded Diagram Of An Interia Starter Motor


If the starter sometimes works normally and at other times fails to engage, jams in mesh or makes unusual noises, then the Bendix gear needs cleaning or replacing.

In some cases only the cushion spring may be broken.

If the starter motor fails to turn, first switch on the head-lights. If they glow brightly, indicating a healthy battery, try the starter again and if you cannot see the glow of the lights or a reflection, get a helper to watch or operate the key for you.

If the lights dim and the starter still does not turn, the starter pinion is most likely jammed. Usually the accessible end of the armature shaft is square, so that the pinion can be wound out of mesh with a small spanner.

In some cases this square end is hidden under a metal cap, which can be pulled off with pliers or prised off with a screwdriver blade.

Although this remedy is relatively quick and means that the starter will not jam again immediately, it does not often form a permanent solution and stripping is advised at the earliest opportunity.


How The Inertia Drive Works

Although there are several different types of inertia drive starter motor, the operating principles are the same.

Before starting work on an overhaul, it is important to understand the functioning of the system.

The pinion gear, which meshes with the much larger ring gear on the flywheel, is mounted on a screwed sleeve carried on the splines of the armature shaft.

When the starter switch is operated, the armature rotates under the influence of the magnetic fields set up, together with the screwed sleeve.

Because of the starter pinion inertia, the screwed sleeve turns inside the pinion gear, winding it forward into mesh with the ring gear.

When all the available travel is used up, the pinion gear is locked to the armature shaft and the motor turns the engine via the flywheel at a speed sufficient for it to fire.

The reaction from the shock of engagement causes the sleeve to move back against its cushion spring, reducing the magnitude of the stresses imposed.

As soon as the engine fires and starts to run, the flywheel (and hence its ring gear) start to rotate faster, which winds the pinion gear back along the screwed sleeve out of mesh.

A light spring is fitted to prevent the pinion vibrating back into mesh when the engine is running normally.

Variations in the basic design are confined to details like the fixing of the retaining collar for the pinion gear assembly and the addition of refinements like the Eclipse drive which uses a strong torsion spring to transmit the drive for further cushioning of the engagement.


Free A Sticking Inertia Drive Bendix Starter Motor

If the starter spins without engaging with the engine, the drive probably needs cleaning.

With the starter on the bench, check that the pinion moves freely on its sleeve.

If it sticks, try wiping any dirt from the spiral screw groove with a clean petrol-soaked rag or work the pinion back and forth in a shallow bath of petrol or methylated spirit.

Wipe dry and check the operation again. Oil sparingly with a light (SAE 5) lubricant.

While examining the drive, make sure that there are no obvious signs of damage to the springs or pinion teeth.

If so, the drive must be stripped off the armature and the broken or worn parts replaced.

Dismantling An Inertia Drive

Before you start make sure that a spring compressor tool is available (fig. 8).

Removing The Inertia Drive Spring on a Starter Motor
The starter. showing the inertia drive. The compressor tool is necessary for easy removal of the drive

The drive assembly is secured to the armature shaft by a device which can vary according to the type of starter and when it was made.

Early types are held on by a special nut, locked by a split-pin.

Later designs use a spring clip retained in a groove by the pressure of the cushion spring.

Early Type

To remove the special nut, first pull out the split pin and grip the square end with a spanner. The nut usually has a left-hand thread and when it has been removed the buffer spring,’ pinion gear, screwed sleeve and anti-drift spring can be drawn off in turn.

Later Type

With the more modern spring clip type of retainer, the cushion spring must be compressed to release the clip.

The cushion spring length differs from model to model and you should try to get the appropriate compressor. If you can only obtain a tool for the longer cushion springs it can be adapted for a short spring using a ‘C’ spacer.

Make up the spacer from a piece of pipe which just fits over the spring clip and bears against the end of the cushion spring. Cut out enough of the pipe so that the retainer will slide out when the spring pressure is released.

Hold the starter body in a vice with the armature vertical and the pinion gear uppermost. Do not overtighten.

Attach the spring compressor with its lower end plate under the pinion gear and screw its threaded portion down until there is just room for the C-shaped spacer and a piece of flat plate to spread the load evenly.

Compress the cushion spring (fig. 9) and ease out the retainer with a small screwdriver.

Compress the Bendix Spring on a Starter Motor
Compress the spring until wire circlip is visible on the shaft. Carefully prise out the circlip from its groove

Release the compressor and dismantle the pinion gear assembly. While the motor is dismantled, check that the armature shaft is not bent. If it is, a new replace­ment could be costly, so check second-hand sources initially.

Remove The Interia Drive on a Starter Motot
Having pulled off the spring, pull drive assembly off shaft. Check that the shaft is straight and free of burrs
Worn Pinion on Starter Motor
A worn pinion assembly is shown on the left. a new assembly on the right. The worn pinion should always be replaced

Renew any broken springs or the pinion if it is worn, and re-assemble in the reverse order making sure that all parts are meticulously clean. Grease the splines on the armature shaft and use thin oil (SAE 5) for the helix in the screwed sleeve. Slight damage remove with a small oil stone or fine file.

Replace Spring on Inertia Drive Starter Motor
Replace pinion assembly. As you release the compressor take care that the circlip is fully home

Check for free operation before refitting.

Lucas Inertia Drive Starter Motor
Check that all parts move freely and refit the starter. If jamming still occurs check flywheel ring gear for wear

What Next ?

Not found what you are looking for ? Head over to my starting page on Starter Motor Problems.


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