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MG Midget and Sprite Technical - 1275. Alternator pulley diameter. What's yours?

I've just noticed that the alternator pulley on my 1275 midget and Sprite, are different diameters.

The diameter of the one on my Sprite is appx 56mm or about 2.2 inches. The one on my Midget is about 64mm or about 2.5 inches.

When the Midget first starts, if I let it idle, I have to blip the pedal to make the charge light go out. Whereas, the charge light on the Sprite goes out as soon as the engine starts. Obviously the alternator on my Sprite is rotating faster because of the smaller pulley, hence it charges from low idle speeds. But which is the correct one?

I've looked on the Moss website and they sell two different sizes, both bigger than on my Sprite.

12G1054 PULLEY, alternator driving, 2.75" dia. to 12V588F3192

BAU1461A PULLEY, alternator driving, 2.5" dia. 12V588F3193 on; 12V778

Does anyone else have the small diameter alternator pulley I have? it's been on there for years, and hasn't blown up the alternator, and as far as I can remember it came with the alternator the last time I bought a new one.

Bit of a mystery, can anyone solve it?
Lawrence Slater

didn't I mention this when you were trying to attribute a random belt you found the other week?
David Smith

Dont some race cars have smaller pulleys crank pulleys due to them operating at higher RPM's ?
Andy Phillips (frankenfrog)


Race cars - certainly Spridgets - tend to have larger generator and water pump pulleys, rather than smaller crank pulleys. It obviously achieves the same result.

Diesel engines generally have smaller generator pulleys.
Dave O'Neill2

Yes, you did indeed David.
"the Unipart book says it fits all sorts of stuff including certain models of Mini, 1100/1300, A35, Marinas, and late 1275 Midget from eng # 12V/588F/3193). Did they fit a smaller alternator pulley for the last few months?"

However, that fan belt won't fit my Sprite, even though that's the one with the small alternator pulley. And if later engines did have a smaller pulley, why isn't it listed in any parts suppliers I've checked?

Sounds like I have a very rare alternator pulley then.

Bit baffling this. Where did I get it? I didn't fit it on the alternator myself. It cam fitted. So if not a Spridget, what car is it for?
Lawrence Slater

I was given a new old stock unipart fan belt from a Midget from my neighbour who used to own one. He said it was from a 1974 1275 Midget. its smaller than the one i have on the car. The unipart part number for the belt does not appear anywhere...

Bob Beaumont

The plot thickens Bob. Was there a time that 1275 Spridgets were equipped with small alternator pulleys and small fan belts to match?

Does anyone have a lucas alternator parts cat of the time, listing applications and pulley sizes on 16/17ACR alternators?
Lawrence Slater

1973 Lucas catalogue:
Midget Mk 111 (early) - alternator 23717
Midget Mk 111 (late) - alternator 23750

but they came without pulleys, remember.
23750 uses pulley 54219467 and the note says 'order separately'
David Smith

BL parts book: alternator 12V engines:

12G1054 - use prior to 37H1061
37H1061 - 12V/588F/3193 onwards
...........12V/671Z/12846 onwards
...........12V/778F/101 onwards
David Smith

Thanks Dave. But mine's a '66 sprite. I probably got the alternator from a Mini in a breakers yard, but no idea which Mini. And that's been replaced at least once since then. I don't recall struggling to remove the pulley nut, and don't have air tools or an impact wrench, so it must have come complete with the small 2.2" pulley. It's a 17ACR as the tag shows.

My midget is a '73, but not the original engine, or alternator. It doesn't have an engine number on the block, and it's not even recorded in the V5 from what I can see. It's an LRA100 according to the foil sticker on the alternator, and has a 2.5" -ish pulley.

Lawrence Slater

Some info for your 23653 alt
Dave O'Neill2

According to My Moss catalogue (Yeah I know, not exactly gospel usually) "The pulley diameter was reduced in 1973 from engine number 12V588F3193 to increase the running speed. This improves charging at low engine speeds and is a good idea for earlier alternator fitted cars." This I believe is for 17 ACR units. Shorter fan belt required natch.


W Dunsmore

I ended up fitting the larger still dynamo pulley on my alternator as I got fed up of replacing the alternator. I was doing a lot of motorway miles and even with a 3.7 then a 3.9 it was taking its toll, no problems after fitting the dynamo pulley. The charge light would be on very slightly at idle but went off as soon as the revs increased.
David Billington

Bill, does your book say what size the pulley was reduced to?

Thanks Dave. Pity it doesn't say what the pulley size is.

That's how my Midget behaves with a 2.5" pulley David B. Not much use having the charge light come on at idle, if you've got your h/lights blazing. Yup you can raise the idle speed, but part of the point of an alternator, is that it charges from low revs, and I haven't noticed my alternators being cooked unduly. But then again, I don't drive around for hours at 6000rpm.

I've got a feeling that my very small 2.2 inch pulley isn't right, but if 2.5 inch pulleys result in the charge light being on at idle, that doesn't sound right either.

I'll ring Sussex tomorrow and ask what size the pulley is on the new alternators they sell.

The only other possibility is the diameter of the crank pulley, but I thought that was standard on all a-series engines.

Lawrence Slater


The book quotes two sizes: a 2.75" up to the engine number I quoted and a 2.5" from then on. A different pulley (no size given) is quoted for 1500s.
W Dunsmore

..which takes us to another question, what's the 'correct' measurement of a pulley diameter, OD or between the bottom of the V ?
David Smith

All the quoted measures are the "external OD", but your right, the V measurement would make more sense.
Lawrence Slater


The correct measurement is the pitch diameter for speed ratio calculations which is less than the OD by an amount dependant on the V belt section. Have a look here . That even says that the pitch diameter has been updated these days.
David Billington

Thanks David B.

Still doesn't tell me how I ended up with a 2.2" pulley though, or why a 2.5" pulley results in the charge light coming on at idle.
Lawrence Slater

sorry David it looks like you're gonna have to spell it out, good luck with that
Nigel Atkins

Have you compared the relative widths of the "V". If the 2.5" pulley is also slightly wider than the 2.2" one, the belt could run lower down in the V and in effect give the same diameter. I know the pulley sizes seem to result in the charge light staying on at higher revs but there are other factors that can influence this and it could be just a coincidence.

Apologies if you have already covered this point. I wasn't following this thread initially.
Guy W

I have in an indirect way Guy.
Apart from the obvious visual pulley size difference, the fan belt on my Sprite is shorter than the fan belt on my Midget. I put the Midget alternator on the Sprite, and had to swivel the alternator way down to get it on and adjust it. Both belts are the same width, and look to me like the same cross sectional shape. So I measured the od's of both pulleys, and got 2.2" and 2.5". But I could try a more scientific approach tomorrow I guess.

One sure way to check the charge light/revs, would be to swap the pulley on each alternator. I tried, but can't undo the nuts. I probably could, but not without damaging the pulley. Garage would probaly do it for a cost with an air gun, I suppose. But a bit of a waste of money.

I think there are some quoted figures somewhere, for the min rpm to produce a charge on an alternator, and that would verify the pulley size needed, as long as you also know the crank pulley size of course.

Lawrence Slater

OK, it was just a possibility. The other thing I have probably missed is whether both cars have always been alternator ones. Or has either been modified from a dynamo one?
Guy W

Midget was alternator from new (going by the wiring and date), but the Sprite had a dynamo originally.

Here's a couple of pics I just took. The belts are both running at about the same depth into the V. So I'm pretty certain this isn't a pitch issue.

Lawrence Slater

I thought that alternators ran at a faster speed than dynamos, so if the pulley on the sprite is smaller, it was maybe matched to the original dynamo?

If it wasn't sleeting outside, and I wasn't watching England overtaking France, I would go and check my Sprite. It is a 1971 car, but one of the last fitted with a dynamo. I changed to an alternator when my voltage regulator packed up (cheaper option). But I didn't change the pulley
Guy W

Yup alt's run faster than dynamos. The smaller the pulley, the faster the shaft will turn.

And now I've just noticed something else a bit odd.

In the fan belt thread on general a week or so ago, it was concluded that the standard belt length was 837mm.

But the belt on my Sprite, looking at that picture, is 864mm; Longer than a standard Spridget fan belt. And yet my Sprite pulley is smaller than the pulley on the Midget. So where is the slack being taken up?

Were there different water pump pulleys?

The problem is, my Sprite was for a very long time kept on the road using parts from breakers. Any part that would do the job, no matter what car it came from.

The VS864 on the fan belt seems to match a fan belt for: --

FORD CORTINA MkIII 1300, 1600 OHC (70-76) with Alt
FORD ESCORT MkI RS2000 (73-74) with Alt
FORD CAPRI MkII, MkIII 1600, 2000 (74-81) with Alt
FORD CORTINA MkIV,V 1600 (76-82) with Alt
FORD ESCORT MkII RS2000 Mexico (75-80)
FORD GRANADA 2000 OHC (77-85)

Maybe my alt, and or pulley, come from a Ford?

I've got a mongrel. LOL.

Lawrence Slater

<<The smaller the pulley, the faster the shaft will turn.>>

Wrong way round. On the driving pulley (i.e.the crank one) the smaller pulley moves fewer inches of the belt per revolution, so the driven pulley(alternator/ dynamo) turns slower.
Guy W

Sorry Lawrence, just been re-reading the thread as I hadn't been following the earlier discussion. I had wrongly assumed you were saying the two cars had different sized crankshaft pulleys. You are talking about the other end of the belt.

So yes, smaller one turns faster
Guy W

I did also ask about the crank pulleys Guy. But I thought they were all a standard size on the a-series.

Something very simple going on here, and I can't see it.
Lawrence Slater

Yup. Water pump pulleys come in 2 sizes. 3.87 inches and 4.75 inches.

I bet I've got a bigger water pump pulley, and that's why I've got a longer fan belt.
Lawrence Slater

Do both cars have the same sized crank pulley?
Guy W

You posted just as I did Guy.

Anyway I assume the crank pulley's are the same on both cars, but now I know there are two different size w/pump pulleys, I check it all tomorrow.
Lawrence Slater


The dynamo pulley is quite a bit larger than the alternator pulley, approx 4" OD IIRC. I have mine in the shed on the alternator but am not going to check its size accurately now as it's raining again , no real surprise and at least I live on high ground.
David Billington

No need to check David. I know dyno' pulleys are bigger than alt' pulleys.

Bloody weather's crap. Glad I'm off to the Phil's next Saturday. Mind you, they've just had amnother big typhoon where I'm going. So it'll probably be wet there too. At least it will be hot though.
Lawrence Slater

Your forgetting, on a dynomo the chargeing dosnt start till around 1200 -1600 rpm. on an alternator the charging kicks in at 800 rpm

I orginally tried to un thur the orginal regulator with a 3 wire alt....that was a night mare, I then went with a GM single wire alternator with a built in electronic regulator, and that cured all the issues, super easy

Prop and the Blackhole Midget


Did you say your going to the Philippines next week, how long, are you still going to be posting here as well ???

Prop and the Blackhole Midget

Nope not forgetting Prop. I'm talking about alternators, and your right, alt's start charging at about 800rpm on the crank. As long as you select the correct alt' pulley, that translates to a speed fast enough at the alternator shaft, to make the alternator output sufficient charge.

And that's why I'm wondering about the correct size for the alt pulley. My midget seems to need to be above idle(900 rpm) before it charges. Whereas my Sprite charges at lower rpms.

Yup 3 weeks in the hot semi-tropical sun. Nope not likely to see the internet for 3 weeks. Probably be too soaked in cheap booze, to see a computer screen and keyboard anyway. LOL.
Lawrence Slater

>>It is a 1971 car, but one of the last fitted with a dynamo.<<
that's wrong

my 1973 UK car (built 11 Jan '73) was among the early ones to get an alternator, IIRC they started from Dec '72
Nigel Atkins

Yes and no nigel

In the usa, the last year for the dynomo was 71, in the uk it was later around 73???

I want to say the differance in diameter of the water pump pully, was one is standard and the other is for high performance, the high performance pump moves slower to handle cavetation better

Interesting debate


Prop and the Blackhole Midget

as there was no mention of them being other than UK I discounted US, Can, Aus, etc.

also the car is referred to as a 1971 Sprite (and I may remember incorrectly that it's an Austin but that doesn't matter for the sake of this) so unless it's late dated it's a UK car, AH Sprites stopped being made for export a couple of years before IIRC

I've put UK for my car and the dates for UK Midgets, depending on when it was registered, Dec '72 on

Nigel Atkins

My car is 1971 Sprite and was one of the last ones fitted with a dynamo. They stopped making them in July 1971. I made no mention of whether dynamos or alternators were fitted to MG Midgets at that time, or later.

My car is an Austin Sprite. The bit that was dropped from the name for the last 1024 cars was the name Healey. Lawrence's is an earlier Austin Healey Sprite (1966 I believe). Both came from the factory with dynamos
Guy W

so what you meant was yours was one of the last cars so obviously was one of the last Sprites to have a dynamo as no more Sprites came after them so wouldn't have been fitted with an alternator

did you also have reverse gears on your bikes

don't worry we all make mistakes with our wording at times ;)

at least I remembered it was an Austin not AH that was good for my memory
Nigel Atkins

This is what the larger, later Mini & Metro water pump pulley looks like.

As used by many racers.

There is quite a noticeable angle to the sides of the bell, compared to the standard item.

Dave O'Neill2

Cheers Dave.

Mine must be the standard item then.

Well it can remain a mystery. I bought a smaller alternator pulley(new for 10 inc postage) to replace the one on the Midget, as I prefer it to charge at idle.

Lawrence Slater

This thread was discussed between 31/01/2014 and 02/02/2014

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