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Starter Motor Rebuild

on the 1981 300D (OM617)

by Army


OM617 Starter Motor Rebuild

Most fixing starter threads posted on this forum go through the process of removing a starter and then replacing it with another one – why? I don't know! Check out your old one first. If you haven't seen the insides before take a quick look through the photographs I've posted – I bet you'll be surprised how simple they are. I think it is worth while taking these things apart and assessing what needs to be done before junking them.

Before starting on a bit of a clean up of my starter motor I found some good information on In particular this pdf file.

Step one - take the starter motor off of your car – on a non-turbo engine just unbolt it and take it off – with a turbo have look through the wiki:-

Taking it to bits(1)

After removing the starter motor from the engine give it a wipe and start undoing the bits at the end.
Remove the wiring between the motor and the solenoid.

Remove the end cap that covers the end of armature axle.

Inside you'll find a rubber sealing ring...

...and a C clip that is used with several shims to set the axial end play of the armature axle shaft.

Taking it to bits(2)

Next undo the bolts that hold on the end cap...

...and if the cap is stubborn to remove use a puller.

Don't be tempted to pry this bit off. The main casing is steel and the cap is a nicely cast piece of aluminum alloy.
When you've got the cap off you will be able to see the commutator and the four brushes.

At the other end of the starter motor remove the nut and bolt that holds the actuating arm in place.

This actuating arm is the bit that gets pushed by the solenoid and engages the starter motor driving cog into the flywheel.

Taking it to bits(3)

Once you've got the actuating arm locating bolt out and the end cap off you can loosen the main outer sleeve. You might be able to pull it off – if not carefully persuade it with a screw driver blade (eeek be careful) – better still tap it with a rubber hammer!

You can now withdraw the mechanism that engages with the flywheel (the drive assembly) away from the nice aluminum alloy casting which also holds the solenoid.

I do like these castings! These Bosch starter motors are very well made – which is why it is a shame to junk them.
Take out the actuating arm and yoke.

You'll also find a rubber cover and a plate here.
You can now pull out the armature assembly away from the outer casing.

Inside the main casing you'll see four field coils.

To see how an electric motor works have a look here.

That's as far as I went – you can pull the drive assembly away from the armature – but mine was nice and clean – freely moving – so I left well alone. To see more on how to do this have a look in the pdf file – link posted above.

Taking it to bits(4) and cleaning it up

To remove the solenoid you in principle need to remove three 40mm M5 screws. Mine were stuck and needed to be drilled out.

The solenoid comes apart just like this:-


Cleaning it up a bit

As my starter motor was fully functioning before I took it apart I just needed to clean it all up a bit. I used brake cleaner to degrease the commutator and I very very lightly rubbed it with a plastic scouring pad to clean it up a bit. You are not meant to use emery cloth or anything too aggressive here. You want a nice smooth surface so that the brushes don't snag and chip.

If you find a worn / scored commutator you can get it turned (on a lathe) so long as there is enough meat still on it. Any decent machine shop should be able to help you here.
I chose to posh my starter motor up a bit and stripped and repainted the outside surfaces. I used a self etching primer for the aluminum alloy parts followed by Eastwood's under hood black, and for the steel parts I used plain old POR15.
I cleaned the brushes with brake cleaner and cleaned the four coils in the main cover the same way. Avoid drenching the whole assembly in brake cleaner. You don't really want that hanging about inside the parts you are cleaning. Think “cleaning delicate tender wound” not “let's pressure wash this muck off of the face of the earth”.

Reassembly (1)

Once you've taken one apart it is quite easy to see how it all goes back together again. After cleaning and painting parts I greased up the drive assembly... (Please note Bosch recommends oil on all moving parts - it was my personal choice to use grease)

...attached the solenoid actuating arm and placed it into its housing.

I applied grease to the bearing surface here too. I believe these bearings are meant to be soaked in oil prior to insertion to the assembly – I decided to use grease – but you could try oil with the bearings in place. Good luck with that!

(Yet again - Please note Bosch recommends oil on all moving parts - it was my personal choice to use grease)

Next I fitted the bolt that holds the actuating arm in place...

...and put the metal cover and rubber top in place.

Reassembly (2)

Fit the mounting bolts into the drive assembly housing and clamp the heads of the bolts in a vice so that the starter motor is upright.
To fit the main casing you ideally need to have four thin hands to get the brushes to make contact with the commutator whilst you lower the outer casing into position. Alternatively you can use some bent bits of wire – with protective duck tape – to help you along a bit.


Don't damage the brushes or the delicate housing whilst you are doing this. I found that once you've got the brushes on the lip just before the commutator you can hold the outer casing in a temporary position with the handle of a parallel pin punch (about 16mm in diameter) so that it does not drop down too far.
Make sure that you have the rotation of the brush-holding-assembly in the correct orientation so that the long threaded rods that run through the main casing go through the correct holes before you lift the brushes onto the commutator.

You can now fit the end cap...

...and tighten the nuts onto the ends of those long thin threaded bolts that run through the casing. The two aluminum alloy castings should now firmly wedge the steel outer casing in place.

Reassembly (3)

Now that the end cap is fitted you can go about setting the axial clearance on the armature shaft. I found that I needed to get hold of some new shims as I measured the clearance to be 0.8mm.

I don't know exactly what it is meant to be. The pdf file I refer to in post #1 says to remove the end play – I think strictly speaking that too is a bit extreme – the armature needs to spin easily but you don't want to motion of the engaging mechanism to promote a sort of whacking effect as it engages and disengages.

I sourced some new shims at a motor factory that also does repairs. The mechanic there was so shocked to see “such an old shim” that he emptied the remnants of this drawer into my eager hands and let me go without paying. I wish you all similar luck!

In the end with the shims that I have I set the end play to about 0.05mm. Basically this was achieved by adding all of the shims until you can just about get the C clip into place. Don't force it on there – these shims are delicate – you wouldn't stab feeler gauges between rocker arms and camshafts now would you? So just get the C clip on there and then remove one of your thinnest shims and that is the best you can do with what you've got. If, however, the shaft movement still seems excessive you need to measure it and get some more suitable shims. If you are smart you could even cut your own.

After the end play is set fill the little cap with grease – fit the rubber seal – and screw the cap into place, and then wipe away any excess grease that comes out.

Reassembly (4)

Lastly fit the solenoid. Give it a good clean – grease it up a bit.

If you've got knackered heads on the retaining screws replace them now. You'll need 40mm M5 bolts. I went for Allen heads instead of the slotted originals. At least if you have to drill out Allen heads they've kind of got a handy starting hole for your drill bit! Lightly smear with copper anti-seize grease before installing. If you have difficulty fitting these bolts you can use longer M5 bolts (say 70mm) to partially compress the springs in the solenoid so that you can more easily get a grip on the first correct length bolts. Fit the wire that runs from the brushes to solenoid.

Final comments

Well that's all folks. This shows what I did to rejuvenate my starter motor. I didn't really rebuild it as I didn't replace a single thing – except some M5 bolts, new paint and I added some extra shims. But from the photographs I hope that if you need to do more work on your starter you can see that there are only two bearing surfaces: one in the end cap and the other in the engaging mechanism's housing. You may need to replace brushes – they are held in place by springs. You may need to get the commutator turned. If the armature is damaged then I guess that it might be time to say bye bye – but other than that I think the majority of these units are salvageable. I'm not sure if you can do much with the solenoid. Perhaps someone can enlighten me?

Discuss this DIY here.


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