I am about to begin the rebuild of an MG35 starter motor - the early one with the barrel commutator. Before I commit to a NOS armature in England can anybody tell me the minimum diameter of the commutator that it can be machined to and still provide a reasonable service life
The car is a 1966 English car and therefor must have this date and "Made in England" on the case. This precludes the use of a new replacement

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Mk1 Cooper S
Mini K Rally car

Lucas Australia armatures were dimensionally the same as the UK ones. If yours is worn bad, why not source a good Oz armature (everybody has a pile of these starters) and machine the minimum off it. Then fit it to your starter body.

To answer your question, I have machined .030" off the commutator diameter in the distant past.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

We are only talking a few thou of machining of the commutator here, and there is more than enough brush length to allow for it.

Make sure you ask them to undercut the machined commutator as well. There is mica insulate in between the copper strips, and they will machine your brushes down if they are left level with the machined commutator. They may do it automatically, but its not good if they omit to do so.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Mick I have seen these Commutators worn down 2 or 3 mm. Unless The Comm. is worn that much no need to replace Armature. I sell rebuilt Units ready to bolt on. With the correct date (When we have it) Or you can send me yours and I will rebuild it for you. I also rebuild Alternators, Generators and Wiper motors.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

The manuals I've read say DO NOT undercut the commutator? Which is it? Any advice on best way to compress the spring to fit the circlip on the end of the shaft on reassembly of the whole unit? It was a bugger to get off and I'm not looking forward to getting it back on!

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67 Deluxe
Dry suspension
1293cc, 11:1 comp, RE 282 camshaft, 45 Weber, extractors etc
4 synchro close ratio gbox
Cooper S brakes, no booster
Stealth box, as Doc says "goes harder and idles!"
RH tank cos I needed it crossing the Nullarbor etc..

I couldn't tell you why they say that, but they also say don't rebuild the steering rack or the brake booster as well...

If the mica pushes above the line of the commutator, then it will shred the brushes. They may be worried about a build up of carbon in between the contacts, so keep it minimal.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

To clarify what I said above, I was talking about `Mini' Lucas starters and armatures. Other brands run Lucas eg Hillman, Ford etc, their armature end details and length may vary.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Putting Circlip back is so easy when you have the tool that I have.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

To get the circlip out I made a little spring compressor out of 2 round plates of 3mm steel , each big enough to have 2 x 1/4 inch bolts 180 degrees apart on the outer edge of the plate . these bolts need to just clear the OD of the spring Nuts were welded on the bottom plate which also had a slot allowing you to slide the plate on between the spring and the pinion. The top plate had a hole in it to fit over the outer end of the shaft. This hole needs to be big enough to be able to get screwdrivers, pointed sticks or what ever to get the circlip out of its recess. Care here, the first on I did sent the circlip somewhere in the garage - never found it.
Bolts pass through the top plate to engage the welded nuts on the bottom plate.
Its not rocket science and took me 3o mins to make

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Mk1 Cooper S
Mini K Rally car

I machined up a brass bush tool to fit in the tailstock of the lathe, with a slot cut through it.
It was safe to use- just hold the armature in the chuck, wind the tailstock in (it acts as a mechanical press) then pry the wire clip off.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

So... Like Dr Mini is saying it is easy. All you need is a Lathe. Every one with a mini has one of those. Huh.

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DID I MENTION THAT I AM THE BEST SUPPLIER FOR MINI ELECTRICAL PARTS AND REPAIRS?
"Let us remove your "SHORTS"

I do some crazy and ill advised things with a vice and lots of levering. It works after a few attempts.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

Tony, my point was safety. You don't need a lathe, nothing is rotating here. I used it as a press, because I have it.
The pinion spring is stronger than you think, the stored energy when compressed can do damage if things let go.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.