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MG MGA - Diffs and 5 speed boxes

Have just bought another engine to replace the one that has a broken crank following the prang last week. I now have to get it collected and shipped to me here.

I've also decided to fit a 5-speed after the test drive in Vin's. I actually drove one a few years ago at Lawrenz in Bavaria, but the car was generally such a heap of sh*t it was hard to make a decision as to whether it was any good or not. Vin's car definitely shwed the benefits of the gearbox, (even if his car is a bit slow :-) )

Question is: I have a B 3.9 diff and an A 4.3 diff. I fitted the B diff to get the cruising revs down, bit then lost out on some performance. I'm thinking of putting in the A diff again, because with the 5th gear the revs will be much lower anyway. But how does an MGA with a 5-speed and a B diff perform? Flwheel is lightened anyway, and I use a B clutch.
dominic clancy

I do not appreciate the fact that the second bracket on the smiley face looks suspiciously like one of my chins but ill let it go this time, I dont know if you remember our conversation but your thread reminded me that you were going to tell me how to identify the ratio of the diff,cheers, Vin
Vin Rafter

Hi Dominic
I too am interested in the 5 speed gearbox and diff ratio subject.

My MGA has a Sierra 5 speed with a 4.3 diff and runs a bored out 3 bearing MGB engine (1850cc approx) It goes well and will pull 5000rpm in 5th gear (on private roads of course). I have been told that the gearing is 20.73 mph per 1000rpm in 5th gear but I dont know for certain if that is correct.

Driving the car I feel that maybe the 1st, 2nd and 3rd gears are a bit on the low side and possibly a higher 2nd gear would help to close the gap between 2nd and 3rd. Also if 3rd gear was a little higher ratio too it would be even better as an overtaking gear.

The engine has lots of low down torque and once the car is on the move I tend to just use 3rd, 4th and 5th which kind of indicates that 2nd at least is a little too low.

(Apparently it is possible to select the gear ratios you prefer if you buy your 5 speed gearbox from High-gear Engineering, probably at a price.)

So thinking about it, the fitting of a 4.1 or a 3.9 diff to my car would effectly raise all the gear ratios and probably get me the gearing I would prefer using my existing 5 speed.

My feeling is that fitting a 4.1 diff would be about right as it could be that the 3.9 diff combined with the overdriven 5th gear would be a step too far.

Let us know what you decide,


Colyn Firth

Hi Vin

Being well brung up I was taught always to use bracket in pairs, so I sincerely apologise for not getting it right this time, (I think this one is then more to your satisfaction as the distance between the brackets has been appropriately amended :-)) ) Will that do sir??

Glad that you are safe and sound back home. If you make it to Abingdon tomorrow, say hi to all from me.

I'm just about to go on meolides, so buzz me again in about two weeks and i'll go and take a photo of what there is to look at.

dominic clancy


the 5-speed box is available with different gears for 1st, 2nd and 3rd gear, so you should get information here. With the 3,9 diff it takes out the fun of driving, except on the Autobahn. It is very difficult in the mountains, say Stelvio, thte right hand corners are a hell to drive. I'm looking for a 4.1 but no luck so far.



I run the 1800 with the 5=speed and 3.9 diff. I haven't measured my performance against other cars so I am hesitant to say it's the best thing since sliced bread. Obviously it is more highly geared throughout the range so it does take a bit of getting used to for best performance. Essentially it makes all the gears much longer. So in general town and country road use you can stick in 3rd and especially 4th at much higher speeds and in a rev band that is comfortable and also gives good performance.

I should have let you have a drive last Christmas!


PS. Now in my Farnham abode, but still awaiting telephone and broadband. It's internet cafes and sympathetic neighbours at the moment.
Steve Gyles

The best set up would be a higher ratio diff than stock (I'd use the optional 4.55) in combination with the 5 speed, but to make it work you need a longer 1st and 2nd gear, which may not be available in the box you use.

The absolute best set up I have ever run in an MG is in my race car with the early non-synch close ratio gears, a 4.55 and an OD trans.

Not sure what you can get in the Sierra boxes.
Bill Spohn

Bill, I think your set up with a 4.55 diff would be ok for circuit racing but a bit revvy for more relaxed road use.
The 4.55 is a "lower" ratio than the stock 4.3 and would increase revs at all speeds.
I presume that you meant to say that your car was modified by fitting a 4.1 diff.

My car had a full race engine with a 4.55 diff as it was set up for hillclimbing, racing up hills.
It was ideal for racing on shorter circuits but not great for long distance travel when it was would have been revving far too much for comfort.

It now has an MGB which is tuned for lots of torque and really doesnt need to rev over 5000 rpm. The 4.3 stock diff is much better suited to the lower revving motor but I think that just maybe a 4.1 diff could be even better.

I suppose Dominic and I are seeking the ideal set up (if it exists) with reasonably good accelleration through the gears combined with a good top speed at slightly lower revs.

Has anyone else out there tried this set up

Compared to the standard MGA box, the sierra gearbox has slightly higher ratios in 1st 2nd and 3rd. 4th gear is 1 to 1 like a std MGA gearbox with 5th being like an overdrive.

Colyn Firth

Sorry, the above should read " it now has an MGB engine". Otherwise it just sounds plain stupid!
Colyn Firth

My MK2 Coupe has the standard 4.1 diff with the original 1622 rebuilt engine with a 9.5 comp. ratio and mild road cam. I have had the sierra gbox fitted and delighted with the response good acceleration through gears and at 4500revs in 5th is doing around 95 mph( the fastest ive tried)at 70 its showing around 3500 revs
P D Camp

Im sorry to say that I dont know what diff i have fitted but with the B engine and sierra box at 3000 revs I show about 72mph can anyone shed any light on this, Cheers Vin.
Vin Rafter


I get 70 at 3000rpm with the 3.9 diff.

The standard diff would require 3300 - 3350 RPM.

Steve Gyles


I have just looked up my previously recorded data for the Type 9, 5-speed - all subject to instrument error (speedo and tacho).

4.3:1 diff gave the following

40mph = 1880-1900rpm
50mph = 2350-2400rpm
60mph = 2820-2900rpm
70mph = 3300-3350rpm
80mph = 3750-3850rpm
90mph = 4230-4350rpm
100mph = 4700-4800rpm

3.9:1 diff gave the following:

40mph = 1700rpm
50mph = 2125rpm
60mph = 2550rpm
70mph = 3000rpm
80mph = 3400rpm
90mph = 3800rpm
100mph = 4250rpm

The wide rpm bands quoted for the 4.3:1 were due to speedo gauge wander, whereas I fitted the right angle speedo drive to the gearbox with the 3.9:1 diff and all the wander ceased.


Steve Gyles


This is still (just) on subject but will you be fitting one of those harmonic crank balancer pulleys to your replacement engine?

I have heard of a number of broken MGA crankshafts over the last 3 years and recalled seeing the balancer advertised in "Safety Fast".

If it works as advertised it may well justify its price tag of 149 from SC Parts Group Ltd.

Colyn Firth


I suppose it comes down to what you want to do with the car as to which diff you fit - 4.3 for acceleration or the 3.9 for cruising. I've got a 4.3 on my 2.1 big bore, as I'm not interested in top speed, only acceleration.

Going back to Siggi's point, Hi-Gear and some other people do differnt gear ratios that you can request. There are also (I believe) a couple of different "standard" type 9 gearboxes out there - petrol and diesel have slightly different ratios.

Good luck and sorry to hear about the prang,

Grant :-)

PS Colyn, is that crank balancer the same as the pulley fitted to the 5 bearing in the MGB?
G Hudson

Have to admit that I dont know much about the crank balancer, I just saw it advertised and thought that it may help. It looks quite similar to the crank pulley that is fitted to my 3 bearing 1850 engine.

Colyn Firth

I actually have three options on the diff : I have 3.9, 4.3 and 5.1 diffs on hand - the 5.1 will produce blistering acceleration, but I doubt that it will be practical for everyday use.

As for the harmonic balancer, that can't be fitted with the 1622 engine, and it also can't be used with the Judson 3V pulley that I have, nor with the standard 2V pulley that is used in addition to the normal MGA crank pulley.

Thanks too for various pointers to engines. Chris Betson is doing me a balanced special with low comp pistons and a Stage 2 head and hot cam ground for the Judson. That combined with the Jeff Schlemmer dizzy, a car refurb, new vanes for the Judson and the new box should liven things up again. I have lined up another complete running engine as a spare for rebuilding for fifty pounds, so two hundred starting price for an unknown quantity on ebay is a bit too much.
dominic clancy

Colyn, I did mean the 4.55.

With close ratio gears the lower gears aren't too low even with the 4.55 diff, you have fantastic acceleration and with the OD you also have a relaxed crusing speed. Best of all worlds.
Bill Spohn

Bill, where would I come accross a 4.55 diff, Cheers Vin
Vin Rafter

Vin, we will have to get together to see if we can measure what diff ratio you have fitted to your car.

There are a few ways to do it and most of them require two people.

I know you suspect that you may have a 3.9 fitted to your car and that ratio is probably the highest you can comfortably run.

My car has a 5 speed like yours but on a 4.3 diff and so you really ought to drive my car to see how they compare.

I honestly think that a 4.55 to 1 ratio would be too low for you as it would increase your engine revs to significantly more than you would be happy with.

(I believe the MG Z series Magnette is the source for the 4.55 diff but I understand that many Magnette owners fit 4.3 or higher ratio.)

I would go for a 4.1 probably (with my MGB engine set up) but I hear that they are quite rare now.


Colyn Firth

Dominic, I'm running the 5 speed with the 4.3 rear end in my MGA Coupe and love it. The engine is a std 1600 bored out to 1622 + 30 thou (1650 high compression)and goes great. The engine is very happy on the freeway between 3500-4000 rpm which puts you in the mid 70 to 80 range. I think it's the perfect set up for acceleration and cruising. I also have the MGB crankshaft damper/front pulley.


Andy Preston

This thread was discussed between 15/07/2010 and 29/07/2010

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