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MG MGA - 3.9 Differential On Ebay

Spotted this one. Not mine.

Item number 300544533403

Steve Gyles

Damn - you've told everyone - I had hoped no-one else had noticed!!
Cam Cunningham

Ooops, sorry Cam. How is the 5-speed conversion going? If you do that, plus a 3.9, you will be quite highly geared - like mine. It's great for long stretches of open road, the likes of the M6, but tends to be a bit too high round the smaller lanes near you. I am thinking that I may revert back from the 3.9 now I am down south and not doing the long stints any more.

Steve Gyles

I'm still looking for a 4:1

John Bray

What set up do you have John?

In my car I have a bored out MGB engine combined with a 5 speed box and a 4.3 diff.

The 2nd gear in the Ford box, although a little higher than in the std 4 speed box, is still pretty low and I find that once I get the car moving I never select it again except on the steepest of hills

My gut feeling is that the low down torque of my engine would very comfortably drive a 4.1 diff even in 5th gear and maybe even make the 2nd gear high enough to be more usefull.

But that fitting a 3.9 would most likely just be a diff too far!

However,it may just be that the loud exhaust on my car helps to give me this impression of to many revs in top gear and that I am better off with my 4.3 diff.

Any suggestions on this anyone?

Colyn Firth

I tried a 3.9 when on the African trip (5 bearing motor coupled to a 5 speed Sierra gearbox) and found it too long... At the slightest gradient I had to change down one gear. I used a 4.3 for the China to UK trip and that was perfect. I have a 4.1 in my race MGA but feel that it would be too long in RIP (but have not actually tried it).
David Godwin

A 4.1 differential in combination with the Ford 0.82 top gear in an MGA with its 15" wheels gives almost identical gearing to an MGB in overdrive top with its 3.9 differential and 14" wheels. To me, this feels "spot on" with both my O/D Mk1 MGB and my MGA.

Speaking of gearing, I took the taller first gear option offered by Pete (Hi Gear ) Gamble for my MGA T9/5 speed conversion. I HIGHLY recommend this modification, as the MG gearbox first gear always felt too "short" to me (and the standard Ford T9 has the same ratio first gear). The taller first in the T9 now gives a lovely progressive spread of ratios with a functionally useable first gear.
T Aczel

I have the 4.1 in my Mk 2 with the Ford 5 speed The 1622 motor has been mildly tuned ( plus 40 overbore mild cam etc 9.5 CR) and seems ideal
P D Camp

Other than the 1600 Mk2, are there any other cars that used the 4.1? i.e. for anyone trying to source this ratio it would help the search.

Steve Gyles

I have 3.9 lsd diff with 1800 3 brg motor (mild cam, extractors, aluminium head, lightened flywheel) and C/R MGA gearbox that is great for cruising but I am going to go back to a 4.3 diff so I can try and catch my mates in motorkhanas, hillclimbs and sprints! (What I lack in skill I'm am trying to make up with technology - maybe the gap is to big).
Mike Ellsmore

I have recently changed from a 3.9 to a 4.3 coupled to an 1800 and a type 9 box and i must say I am happy with the improved bottom end albeit in my opinion not that great a difference ,I do agree with T Aczel though that in his mind the 1st gear on the type 9 feels a little short, Vin
Vin Rafter

It is kind of difficult not to listen to the advice of someone who has had so much long distance driving experience as Dave Godwin.
If he can drive half-way around the world with a 5-speed / 4.3 set up then I suppose my occasional 200 miles a day trips with mine should be quite easy!

According to my sat nav, I am doing 80mph at 4000rpm and so I suppose this isnt particularly excessive and maybe a quieter exhaust ( or earplugs!) may be the way to go.

Like the idea of a higher 1st and (if possible) a higher 2nd gear though.

Colyn Firth

I do actually enjoy the 3.9 with my 1800 and 5-speed. But I guess it is up to the individual what he wants out of his car.

My type of driving conditions have changed down in the Surrey hills, with the windy, hilly and pot-holed roads. I certainly use the gears more, but that is not a problem for me. Not so much top gear, mainly 3rd and 4th. Performance is very good, just get in the correct gears.

Overshadowing all this is the fuel cost. It is quite noticeable to me how everyone has backed off by about 10mph. Drivers are switching off at traffic lights, even for just 20 second waits. Maybe the 3.9 diff is good on fuel efficiency, but not sure on this one yet.

Steve Gyles

I am running a 4.1 on my Twin Cam. It seems like i want a 5th gear but i won't fit a hi speed. I have a 3.9 which would take a couple of hundred rpm off @ 50 mph, but i am not convinced i might lose a bit of 'sparkle', even though i do not tear away from traffic lights, plus i wonder if i can be bothered for such a small revs drop.

C Manley

Just for Colyn and others who may be interested. Colyn's 80mph at 4000rpm with the Type-9 box would in fact increase to 94mph with the 3.9:1

Full list which I have previously measured on my car:

1000 rpm = 23.5 mph
1275 rpm = 30 mph
1700 rpm = 40 mph
2000 rpm = 47 mph
2125 rpm = 50 mph
2550 rpm = 60 mph
3000 rpm = 70.5 mph
3400 rpm = 80 mph
3800 rpm = 90 mph
4000 rpm = 94 mph
4250 rpm = 100 mph

Steve Gyles

There is some interesting info here - thanks Steve et al - perhaps a 3.9 is not the way to go as I quite like nipping along the country lanes , only hitting the M6 occasionally - and the 5 speed soon to be fitted - should help there.
PS - saw Vin's "clockwork" MGA in the recent MGCC "Safety Fast" - fame at last albeit as an April Fool joke.
Cam Cunningham

Story of my life Cam, according to Colyn its better than a picture of me from the front, I think its just a wind up
Vin Rafter

I still have the orginal MGA (close ratio)gearbox in my car. I find the 3:9 is just a little over-geared even with the 1800 engine.

John Bray

The MGA is an amazing car that enjoys being driven, and responds accordingly,so using the gearbox and keeping the engine revs on or around the cam, is a real joy, The car can be driven on the throttle through bends and roundabouts as it tends to power oversteer even with a front Anti roll bar,great fun. All my many MGA's since 1976 have had a 3.9 rear end, and a stage 2 MGB 1860cc (the push rod cars always lacked power) and it will cruise on the motorway at 80+ all day, and potter through towns at 30mph in third, great sound.

Perhaps the cars are too valuable now and a little over restored which is a great shame as in my opinion this detracts from what they were intended for.

Sorry guys just wandering dowm memory lane
Trevor Jones

T E Jones

"My type of driving conditions have changed down in the Surrey hills, with the windy, hilly and pot-holed roads. I certainly use the gears more, but that is not a problem for me. Not so much top gear, mainly 3rd and 4th. Performance is very good, just get in the correct gears.

Overshadowing all this is the fuel cost. It is quite noticeable to me how everyone has backed off by about 10mph. Drivers are switching off at traffic lights, even for just 20 second waits. Maybe the 3.9 diff is good on fuel efficiency, but not sure on this one yet.



Not sure how the 3.9 would improve fuel efficiency if you are "Not so much top gear, mainly 3rd and 4th..."

Neil McGurk

So far no one has touched on the best reason for selecting the appropriate final drive ratio, and why the factory used the original gearing sets (although a few comments have getting close). From the 1920's through 1980 the four cylinder MGs were always somewhat underpowered (by modern standards). Final drive ratio was always selected to match the peak of the engine power curve for best top speed (without over-revving the engine). Ergo MGA 1500/1600 = 4.3, MGA 1600-MK-II = 4.1, MGB 1800 = 3.91. For slightly improved fuel economy and quieter engine at casual cruising speed, an overdrive unit was optional for the MGB. The more modern equivalent is the 5-speed gearbox with overdrive ratio in 5th gear.

When using 5-speed or OD, the original final drive ratio is retained for the original reason, best top speed in 4th gear. The overdrive gear ratio is selected to provide lowest engine speed commensurate with being able to pull up a mild grade or drive into a strong head wind at normal road speed without downshift (a problem previously noted).

If you do some performance upgrade to the engine, then you may want to lower the final drive ratio. Some time ago I treated my MGA 1500 to a slightly over-bored 1600 block (1625cc) and early MGB head (larger valves). Being equal to the 1622 engine configuration, best final drive ratio would be 4.1. However, I also installed a fast street cam to bump up engine torque another 10% or so (close to 100 bhp), resulting in capability to run about 5% higher top speed (about 108 mph). This would result in running the engine well past red line in 4th gear (verified with the first fast expressway run). Solution was to drop the final drive ratio to 3.91. Now it will cruise 105 mph at 5600 rpm. A nice side effect is now running 75 mph at 4000 rpm rather than 4400, or alternately running 82.5 mph in the fast lane at the same 4400 rpm (but it could still handle an OD gear at those speeds).

Stock engines do have enough torque to pull an OD ratio at normal cruise speed, as long as the OD ratio is not too aggressive. I.e. original final drive and 0.80:1 OD or 5th gear works well for casual cruise speed. Lowering final drive ratio 10% along with 0.80 OD ratio. is equivalent to 0.73:1 final drive, in which case the car has a tough time handling a mild grade or head wind without a downshift. If you increase engine torque you can use lower gear ratios (my example in the prior paragraph). Finding the appropriate final drive gear set for best top speed in 4th gear is fairly easy for MGs, as there are lots of optional gear ratios. Finding alternate OD ratios in 5-speed units is somewhat tougher, but even then there may be a few options.

If you somehow manage to increase engine torque (and power) by at least 30% (120 bhp or more), then you might think about final drive gearing for best top speed in 5th or OD (common with modern cars).
Barney Gaylord

After reading Barneys excellent post Im beginning to wonder about myself a little.

My previous MGA was a 1961 MkII Coupe that I bought in 1971 for 3OO and it was the best sports car I could afford at that time. I loved the car, it seemed fast, comfortable, great fun to drive and an ideal daily driver for a 6 ft 1 inch tall 30 year old. We even toured in it to Switzerland for two weeks and had a great time.

My present roadster is in vastly better condition, has much more power has better seats and holds the road much better too.

But today, after 100 miles or so I almost have to to be prized out of the car as I have a numb b*m, my right leg has "gone to sleep!" and my ears are ringing from the exhaust noise.

The MGA is still basically the same car but obviously 40 years have changed me quite a lot an I wonder if I am subconsciously trying to make the car more of a sports tourer, God forbid!
I think I had better keep the standard diff, fit a V8, a supercharger, nitrous oxide injection and a roll cage and just go back to being the speed merchant that I was!

Otherwise I will just have to swap it for Hillman Minx or something similarly comfortable.


Colyn Firth

John I have a 4.2 if you wanted to try it awhile and see how it goes. (well was told it was 4.2 but could so easily be .1 or .3 as I didn't count!) I live not far so you'd be welcome and then can decide. My email is Rachel-macbathotmail.codotuk if you do.

Reading the input from rachmacb has made my day it typifies the helpful and unselfish views of the people that use this forum, good on you, vin
Vin Rafter

I suspect the 4:2 is from the 'A' series engine. But I have sent you a PM to arrange a visit.
John Bray

I agree with you completely Vin.

The irony is that in my racing days I had a 4:1 diff which I lent to a fellow competitor during a weekend race meeting. Unfortunately he sold the car a few weeks later with my diff still in it!

But as you suggest its a great community and I still frequently lend tools and parts.
John Bray

I agree Vin, you cant fault an offer like that, there are some great people on this forum.

I was looking through this thread again and re read Colin Manley's post about his Twin Cam.

Colin, if originality is the reason you want to stay with your standard 4 speed box, well thats understandable and quite right too.

But fitting a Hi Gear 5 speed with your 4.1 diff wouldnt change how your car accellerated away at all, it may even improve it a little as the gears are slightly better spaced. (4th gear is one to one ratio like your standard box) The bonus would be that you would have your overdrive 5th gear to reduce the revs on motorway runs for instance. It could even make your twincam engine last longer which isnt such a bad thing.

You would also have the benefit of a first gear with synchro which I have found surprisingly useful.

Not trying to convince you, just information really.

Colyn Firth


When I first saw your post, I thought "Good lord, Neil has used the the same words as me!"

I did say, I was not sure about the fuel economy aspect. That said, the fixed aspects are the windy roads, hills and potholes. The variable is the diff ratio - hence the rpm:distance travelled ratio. 3rd and 4th (primarily 4th) with the type-9 takes me comfortably round all these local roads. I suspect I would be in the same gears with an MGA box. To me that says increased fuel economy - maybe.

I run a very accurate MPG spreadsheet, so I can tell you down to however many decimal places you want my average fuel consumption since I moved to these parts a year ago. The only way I can prove or disprove my theory would be to put a 4.3 back in and continue my analysis for another year.

Steve Gyles

Sorry, I am confusing myself. I did not mean 'MGA box', I meant 4.3 diff.

Steve Gyles

As usual, Barney's comments are correct. I also always presumed the final drive ratio was chosen so that the car reached its maximum road speed at approximately its maximum engine speed in top gear. However, the MGB ran a 3.9 differential not because it had more torque than an MGA, but because it had 14" wheels instead of the MGA's 15".
The MGB (3.9) turns 17.9 mph/1000rpm in direct top.
The MGA 1600 Mk2 (4.1) turns 18.1 mph/1000 rpm in direct top.(Vitrikas quotes 18.3)
The earlier MGAs (4.3) turn 17.3/1000 rpm in top
With a 4.55 differential, 16.8 mph/1000 rpm

According to what I've read, the MGA's have less frontal area area and less wind drag than the MGB. Until the 1800 engine appeared at the last minute, Abingdon was facing the potential embarrassment of the 1622cc MGB being slower than the car it was superseding.
T Aczel

Thanks Barney - that confirms that a 3.9 diff needs an uprated engine to make it work comfortably with a 5 speed box - so I won't be going for a 3.9 as I don't intend to beef the engine up.
Steve - looks like the 5 speed should be enough to make the M6 driveable at last.
Cam Cunningham

Steve, you asked the question if any other car used the 4.1 diff.

I went through all my old microfiche for old austin morris cars of the era, and can confirm that it fits nothing else!! Except for the Mk2 MGA.

Now I find that difficult to believe, and would love to be proved wrong!

When I bought my dog of a car, Mk2, guess what was missing? Correct!

I do now have a 4.1, but despite this being a friendly forum, no you cannot have it!


Colin Parkinson

The Austin A90 Westminster had a 4.10 rear axle, as did the Morris Isis and the Austin A105 (as reported in "Motor" 1956 road tests) ... in other words, a standard fitting for the "bigger" BMC cars at that time.
Geoff E

Geoff E, -- Those were much heavier cars with larger 6-cylinder engines. I don't have those listed for the Austin B-series rear axle. Do you know what rear axle they used? If it was not the B-series, then those would not be a source for 4.1 gears for the MGA.
Barney Gaylord

Hi Barney - I'm afraid I have no idea what *type* of rear axle they had. That's not the sort of data that road tests usually give. I was just commenting on the 4.1 ratio. Interestingly, the stated oil capacities were 3 (imperial) pints for the Austins and 3.75 pints for the Morris. This is larger than the MGs, isn't it?

All 3 models had the 6 cylinder 2639cc engine.
Geoff E

Austin B-series rear axle used in MGA and early MGB holds 2-1/4 Imp. pints, 2.7 US pints, 1.28 liters. I suppose that means the larger cars used the larger C-series rear axle.
Barney Gaylord

Hi Barney,

Not strictly true.

I've taken and still have an Austin Cambridge Diff in it's cast iron (not alloy) housing which is perfectly interchangeable with the MGA / MGB.

I don't profess to know much about the Cambridge but the one or maybe two I removed the Diff's from were 4 cylinders. (In Australia)


Mark Hester

My thoughts are somewhat different to most expressed above. I have a 1600 basically tuned 1600, would do 100 MPH fairly easily I suppose.

I take the view hat if you want a car that can accelerate quickly then o not buy an MGA it is low by most standards. That said it is a joy to drive as we all know and lovely on country lanes. However I like to go touring in mine and at many times I am on main roads wanting to travel at around 75MPH for hour upon hour.

With that in mind I love the 3.9 diff I have in my car and would go longer if I could find some thing suitable Unfortunately for me a 5 speed is out of the question, I need to keep my car period for classic rallies. I tried to bid on a diff a year or so ago it was if I remember correctly a 3.5 but I was easily outbid but I would have happily fitted that.

I accept Barny's point that over gearing could reduce top speed as it can move ultimate power out of the right area but equally it would move max power to around 80MPH which would suit me fine. Equally the cruising RPM at 70 would be reduced with a 5 speed and 3.9 and that would be marvelous for most. Don't think there are many owners who want to be driving a screaming B Series engine in an MGA doing 100MPH but I amy be wrong.

Did anyone here buy that diff last year on ebay?
Robert (Bob) Midget Turbo

Mark, -- This is getting a bit outside my area of expertice. I try not to spend to much time researching the Austins when it is not directly related to MGA. I know the A90 Atlantic 1949-1952 was a 2660cc 4-cylinder and the A90 Westminister 1954 on was a 2.6 liter 6-cylinder.

I don't think I'd want to put an iron differential carrier in the MGA, but perhaps the ring and pinion gears might work. If anyone knows for sure if these would fit, do tell.
Barney Gaylord

I have been trying to find information on other cars that may have utilised our rear axle, but so far without much luck. I did howver find some information on the MGB BBS about suppliers of crown wheels and pinions for our diff. This is what was said in August last year:

Newly-manufactured crownwheel and pinion gearsets for the Hardy-Spicer banjo-type rear axle of the MGB MKI Roadster are currently available in:
3.7:1 (available from Autogear, Part # CWP033)
3.909:1 (available from Moss Motors, Part # 267-165; and Autogear, Part # CWP034)
4.1:1 (available from SC Parts Group and Cambridge Motor Sports)
4.3:1 (available from SC Parts Group and Cambridge Motor Sports)
4.55:1 (available from Moss Motors, Part #267-185) and Cambridge Motor Sports
4.875:1 (available from Victoria British, Part # 5-1049)

Steve Gyles

According to the "Cambridge-Oxford Owners Club" the A40 used the "new hypoid-bevel B-Series back axle unit. This drive line had already been seen on the new MG Magnette ZA and was the real foundation of BMC's 'building block' strategy for the 1950s and 1960s." -

When I then looked at the A50 and the A55 on the same site it said the running gear remained the same.

Steve Gyles

Steve - the diff finally fetched 259 on the ebay auction !!
Cam Cunningham

Obviously sold mine far too cheaply last year... Someone here took it as far as I remember.

dominic clancy

Good lord. I paid 50 four years ago. There again, like any auction, it depends who is wanting at any one time. If 2 bidders get locked in, the price can go stupid. I see it with Shelley jacks all the time: 100 in one auction and 4.99 the next.

Steve Gyles

This thread was discussed between 06/04/2011 and 13/04/2011

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