Gemini Starter Motor for Twin Cam / Kent Blocks

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Gemini Starter Motor for Twin Cam / Kent Blocks

Post by Boris67 » Fri Mar 04, 2016 2:33 pm

Thought I'd do a write-up on the replacement of my aging Lucas inertia starter motor. This will probably be relevant not only for the series 4 but any Kent based Seven.

I started having problems with the Lucas starter not long after the rebuild of the twin cam in my Series 4. I took it to bits and found that it was in pretty good nick internally, but the pinion was starting to wear. So put it back, hoped for the best but it slowly got worse and worse. And as I live in Perth it's not like there are that many hills to park on, besides it's not fair to expect your significant other / child / passenger to push start the thing! Well, not too many times I guess...

I thought at first I'd just shell out a load of cash for a bespoke starter but after doing so, being promised it would arrive the next week and then two weeks still no starter motor I cancelled and started to think laterally. I happened to come across this article on swapping a standard Lucas inertia starter motor in a Mini Moke for a Holden Gemini unit: ... ni-starter

The Lucas unit looked a lot like the one from my car so started to look very closely at the possibilities of doing this swap. From this site: ... page&id=71

I found that Minis came standard with four types of starter motor, all of which were interchangeable. These are the model / part numbers:

M35H, M35G, M35J or M35K (the last apparently only used for a very short time)

25071A/F, 25075, 25083J and 25147B (do not correspond directly to model numbers above)

The model number of my starter is M35G, part number: 25022F. So then found this (what I'd consider reasonably definitive) parts list for Lucas starter motors: ... motors.pdf

sadly it only lists the 25022, 25071 and 25075 part numbers. Which was okay because the Mini units are interchangeable. Checking the vehicle application guide is fun, so many old English cars!

From the spare parts lists for the 25022 and 25075 units I found that the pinion / sleeve / barrel are the same part number, bazinga! 250698. The 25071 lists a 54250372, which I'm guessing is an uprated 250698, they certainly look very similar.

The major difference in parts appears to be the drive end / fixing bracket, which you'd expect. Anyway, buoyed by that information and also finding some people on the net who'd had positive results doing the same thing I decided to get hold of a Gemini starter motor and have a crack.

I bought one from a 1984 Gemini off Gumtree (similar to Trademe), with the first thing being to check it had a 9 tooth pinion - it did! Yay! I took it apart and cleaned it as it seems possible the Gemini it came from was pumping out more oil than my twin cam - impressive! A good scrub revealed it was in reasonable nick with the brushes and commutator looking fine. While I had it apart I lengthened the slots, as per the Moke guide. I also test fit once I'd done that (with just the fixing bracket) and found that I needed to grind off a bit of the bracket where it was hitting up against the block. I put the starter motor back together and tested the solenoid, it was pushing the pinion out so all good. I then bolted the complete starter motor into the car, which fitted snugly:


You can see where I needed to grind away some of the fixing bracket, next to the top bolt.

It was getting late at this time so knew my tinkering time was nearly up, but was keen to know if it was going to work... connected up the power feed (needed to eek the cable forward an inch from the back of the car which was not too hard as plenty of slack in the boot) and earth strap. Then with trusty screwdriver in hand I gave it a whirl... Eureka! It sounded great and the engine was turning over well (and apparently in the right direction!).

The next thing was to lengthen the wiring so it could connect directly to the new starter, rather than the old remote solenoid. The wire running from the coil to the starter was easier, just need to get it out of the wiring bundle and down to the new starter. However this wire made me scratch my head a little, on the remote solenoid there are effectively four connections:


From the wiring diagram this wire provides 12V directly to the coil, so for cold start the coil is getting lots of voltage. I decided to wire directly to the ignition wire and see how that went. The moment of truth came and I turned the key, it all turned over well and tried to start, however once I turned the key from position III (start) to II (run) it immediately cut out. Obviously something was not quite right! I had a good look at wiring diagrams, googled the net and talked with mates. Mr Adams suggested disconnecting the 12V wire to the coil and see how that went. I did so and success, well sort of, the engine turned over but only started when the key went back to the II position. So obviously the 12V feed was needed but something was going wrong.

After more discussion I ran the multimeter across the old solenoid and found the problem. When the key was in position II the ignition wire went to earth, whereas the 12V wire did not. With the 12V wire going to earth the coil was being shorted out. How to solve this and still get 12V power to the coil for startup? Thanks to the genius of Mr Adams (and his Datsun forums) the answer was a DIODE! Ah, electronics, how we love you. Simply stop the current flowing back from the 12V wire to the new starter motor solenoid and the problem was solved. At present the wiring in of the diode is pretty makeshift, but it works.


And I now have a car which seems to start before you get to position III with the key... :-)

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