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MG Midget and Sprite Technical - Mk1 rev counter question

Having successfully passed my MOT today (after addressing detached anti-roll bar and leaking rear hub seal mentioned in a previous thread) I went straight for a country lane blast to give my Mk1 midget a good run. Everything was great until a very loud graunching noise came from the engine bay then silence and a dead rev counter.

Headed home without issue to find that the reduction gear had seized solid and started spinning the whole cable. A previous owner had cable-tied the alternator power and red light charging wires to the tacho cable as per picture. Luckily neither they nor the oil capillary were damaged though the wires were detached from the alternator. As both wires disconnected there was no indication of a lack of charge.

There seems to be no other damage. I've been for a drive since and all circuits seem to be working.

Questions: Is the reduction drive serviceable at all? It seems to be a sealed unit. A new one is more expensive that just fitting a s/h electric rev counter that looks identical in the cockpit.

If the reduction gear could be freed off the broken knurled screw as per picture could be wound in wire the epoxied as a crude but effective repair but if the drive is a known failure point I'm inclined to remove it altogether. Might even be a zero cost option by selling the mechanical tacho.

2nd question, will a negative earth electric tacho work on a positive earth system?

Thanks













P Peters

I don't know about the mechanical tach drive.

But on the pos earth question, you have three options.

1. Get a pos earth rev counter.

2. Convert a neg earth rev counter to pos earth. Not difficult.

3. Fully insulate the body of a neg earth rev counter from tgr chassis earth, wire it appropriately, and yes it will work.

Best option though is #1.

Edit. 4th option. Change your Midget polarity to neg earth. Then a neg earth rev counter will be fine.



anamnesis

I suppose one option is to fit the electronic tacho and, if you're worried about originality, refit the broken reduction gear and cable, but without the drive spigot.

If you want to use a negative earth tacho, you would either need to convert it to positive earth, or convert the car to negative earth.

There are positive earth tachos available.
Dave O'Neill 2

Is the reduction gearbox seized? If the cable spun I would have thought the gearbox OK but the cable inner had seized to the outer sheath.

I've not seen one of those gearboxes like that, mine came in two diecast halves held together with a few screws.
David Billington

You can re-wire the tach circuit internally to change it to positive earth. I did the opposite I.e. convert a positive earth tach to negative earth. You have to remove the instrument from its case to do this and do some soldering.
Bill B

All good to know thanks all. I'll get an electric tacho and convert to pos earth.

David B, the reduction drive is seized and the witness marks point to it spinning the cable around the 2 inch radius of itself. Ie the cable outer didn't rotate on its own axis, the reduction drive span it in a bigger circle until the knurled nut split and it fell off.

I'm not worried about originality at all really.

Anyone want the driven rev counter when I pull it out?

Might not be for a while as I have to work on another car and have spent quite a bit of unplanned time on the Midget. Also my fingertips are still recovering from fitting a new windscreen.

Actually thinking about it, if I went neg earth, what would need to be adapted?
Obviously, the battery gets flipped but
would the points fuel pump still work?
I have a dynamotor that looks like a dynamo but is an alternator. Two wires, power and warning light. The regulator is not used. I seem to remember something about flashing or energizing dynamos but alternators also?
Did I miss anything?

Thanks
P Peters

If you have a +ve earth dynamator you are stuck with a +ve earth midget. To go -ve earth you will need a new -ve earth dynamator. It should be easy enough selling the +ve earth dynamator especially as it has the tacho drive on it.
Rob
MG Moneypit

What you have there is a very early banded reduction gearbox. Later ones have the two halves bolted together so that the gears can be packed with grease. Yours may have a small screw allowing for oiling.
The band could probably be cut to release the halves for further inspection and carefully welded back together. The nut could probably be reproduced
I would certainly have a go at restoring it.
Alan Anstead

Thanks Alan, I've come across a cheap positive earth electric tacho on Ebay (£40!). I'll put the gear drive and rev counter in my box of bits I keep for the next steward of the car .. unless you or anyone on here has a use for them?

It's not concours or original - the engine has been changed at some point (still 948) so the number doesn't match, it had round arches at the rear at some point then changed back. It's had discs and anti roll bar added, uprated springs and non original wire wheels. It would be costly to return to original but I'm happy with what I have.

In several other non MGs I've changed the engine and while I have the itch to do that to the Midget, every time I drive it I find it to be such a nice feeling and sounding engine.

While this is my first Midget I had several Minors over the years and still go to the Minor National. Anyone that had experience of different Minors always preferred the 948 to the 1098. (no official 1275 but it was a popular upgrade). In the Midget I understand the charm

Actually how do the 1275 or 1300 variants compare to
the 948, other than more power, do they feel as nice and torquey?

A good friend says I should change my '61 Mg for a late model with Triumph engine and modify that instead of doing anything with mine as it's too original and patinated.




P Peters

I am lucky to have both 948, slightly modified to 62bhp, and 1275, (Sebring Rep) modified to 88bhp, from which I get great enjoyment.
I bought the 948 (Frogeye) in 69 and have travelled far and wide to take in Hungary & Czech Rep. I have been privileged to drive it, not racing, on many GP circuits and manufacturers test tracks. It did the opening sequences to ‘Wheeler Dealers’. I really like the 948 although because of the distances I travel it is now 5spd but still on drum brakes.
I am just south of the Dartford X-ing so you are welcome to drop in for a cuppa & a chat.






Alan Anstead

I didn't know your 948 was a star Alan.

How did that come about? Namesake connection per chance?


anamnesis

Anamnesis
Over the years I have been asked to provide a car for TV or magazines. On the occasion of ‘Wheeler Dealers” they wanted a Frogeye at short notice that was not red so I took the assignment. I have had to turn down displaying my car at Buckingham Palace due to a prior engagement but was able to supply an alternative. Sometimes it is just whom I meet along the way which is how my Frogeye features in Hungarian magazine (pic) and my Sebring Coupe in an American magazine: although not through journeying in the States. I have met, and spoken to, Ant Anstead once at a show but have no further contact.



Alan Anstead

Sounds like fun.

Do they pay you something for showing your Frog Alan? Are you registered with some kind of agency, apart from just chance meetings?

I could offer mine as an example of how to abuse a Spridget for almost 50 years. 🤣.

anamnesis

anamnesis
I did a day filming for a TV advert, many moons ago. The Advert was for the Anglia Building Socty and was to be broadcast in the Anglia TV region. I have never seen the add and failed to find anything via Google. I think I may have got £50 but magazines &c I might get lunch if lucky.

Next year will be 50 yrs since I appeared, with the Frog, on the cover of Classic Cars so I will write a bit about the car and send some pics to see if I can celebrate the fact with an article in the Mag. Paul Skilleter climbed onto my garage roof to take the shot.
Alan Anstead

Dr John E Davies checked out by rev counter reduction gearbox as part of his rebuild of my dynamo and control box. I am not sure what spares are available.

Cheers
Mike
M Wood

Dr J.E.Davies
I haven’t seen or heard from Dr Davies in a while. I donated a dynamo for him to repair and sell on as he donates to the RNLI.

Having a damaged dynamo pulley that I wanted to fit to aDynamator I bored out the centre and sleeved but then needed to cut the keyway. Dr D kindly cut the keyway as he held the appropriate tooling. I would certainly have a go at restoring that reduction gearbox.
Alan Anstead

@Alan Antstead,

That's very kind Alan, I may well take you up on that. I'm in Southend but often going through to London albeit not in the Midget.
P Peters

P.Peters
I am about 1/2 mile from Jct1.
Alan dot anstead at btopenworld dot com.
Alan Anstead

"Revenons à nos moutons" as they say across the channel. Here's a little video I did about butchering an electronic tacho I got from the US.

https://studio.youtube.com/video/OEOIq9qf4A0/edit

Best thing I ever did. I have also repaired the little gearboxes, even one that was assembled with the crimped band. That was really a matter of getting it apart, greasing it and crimping it back again. Much later I fitted a new brass bush to the input end, this time on a gearbox assembled with nuts and bolts. Yes I know it should be bronze but I didn't have any. Then eventually I got fed up with all this and fitted an ordinary Mini alternator and electronic tacho, which looks exactly original in the cockpit. I had actually invested in a new gearbox, but got so fed up with the unreliable dynamo that I got rid of all that. The car has been negative earth for about 40 years and it's probably the first thing you should do. The gearbox went on eBay.
Les Rose

This thread was discussed between 06/06/2025 and 13/06/2025

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