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MG MGA - 3.9 rear conversion

I was under the impression that the MGB 3.9 rear was very plenyful and could be used on the MGA to helplower the RPM's at highway speeds. I have been monitering ebay for quite some time and never see any. I am concidering the conversion, has anyone done this and are they happy with it?
WMR Bill

Delighted with it. I also run the 5-speed box. The 3.9 comes off the mgb roadster, not the GT (different axle). They are not too frequent on our Ebay either.

Steve Gyles

I have a 3.9 set of gears.Good condition.I was going to install in my TD but used a set of 4.3 instead.
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conrad sanders

The 3.9 banjo axles can be hard to come by. You can take a GT or roadster complete Salsbury axle and install it in your car. The Salsbury gear set will not fit the banjo axle.
J Heisenfeldt

I install a complete '74 MGB rear end in my '57 MGA. Works great. The only thing I found is, it's about 1/2" wider from the spring to the outside of the brake drum, and is not a problem. I also have a Borg Warner 5 speed with a .63 5th gear with a 215 Buick V8. My fun meter pegs out. Click on Lyle Jacobson article for photos.

Lyle Jacobson

You have to be diligent, but one will pop up on ebay. Avoid the temptation to overpay for it though. They're not as plentiful as they used to be, but I've talked to many who have them in their basements, garages, etc... They never drag them out to swap meets because they're too heavy, is what they tell me. I did the conversion and I think it's great. If your motor's strong enough you don't really notice the higher gearing from a dead stop, and on the highway the car's really transformed. I have a 3.9 in both my A and Magnette.
Mark J Michalak

I have a 3.9 installed too. Makes quite a difference to the noise levels, but a little bit of the raw grunt has gone with it.

I'd hesitate before putting one in a standardr 1500 engined car.
dominic clancy

I'm going to hijack this thread for minute. Since installing the 3.90, when I'm coasting/engine braking down under 1500 rpm, I get some noticeable lurching. Usually the kind of luching you'd get when you're slowing to a stop in 4th gear, except now it does it in all gears. It didn't really do that with the 4.3. It's been a long time since I've driven a B regularly, but I don't remember that at all. The only thing I can think of is related to the engine. I have an 1800 with 9.5:1 compression, perhaps the taller gears are having a harder time driving the high compression motor while engine braking? What do you think?

Mark J Michalak

Could be more backlash in the differential. Put it in neutral, get underneath, and rotate the propshaft by hand. If the differential input flange moves more than about 1/8-inch at the edge you could benefit from replacing thrust washers for the bevel gears.
Barney Gaylord

If the diff isn't original and someone has replaced the ring and pinion at some point, the backlash between the ring and pinion may be set incorrectly. If you need to go in and change the washers as Barney says above, check the backlash on the ring and pinion and check the diff bearings for slop using a good size prybar under the ring gear.
J Heisenfeldt

Bill a good A will pull a 3.9 diff .I can't see any down side providing the engine has good compression and unworn carbs and distributor. Acceleration is still good, no matter what diff ratio you run the modern shoe boxes are still going to leave you behind at the lights. Fitting a B diff banjo requires the spider gears to be swapper over, easy, a total time of four hours.
S Sherry

I have a 3.9 in mine with an 1800 3 main and standard carbs and manifold. Mild tune cam and gas flowed head make the car sporty and able to cruise comfortably on the motorway (highway) at over 70mph. However, one thing I notice is that there is a big gap between second and third gear. I'm not sure if the p.o changed the ratios (he fitted new gear sets), does anyone else notice this gap?

Neil McGurk

I am running a 3.9 in my stock 1500 Coupe and find no problems. It run a lower RPM's which is great for the highway. In town I have good excelleration. The only draw back is on very steep hills. I find I am more likely to have to downshift sooner. Not sure if has improved my gas mileage. Gordon
Gordon Harrison

While the Salibury 3.9 makes sense in Lyle's car as it has a V8, it is also way heavier and is to be avoided if possible on installations with 4 cylinder power.

People with anything but Mk 2s should be aware that they will need to change axles or spiders.
Bill Spohn

a 3.9 diff - ment for MGB 14" wheels, T9 gearbox together with 15" wheels kills all the fun one has with an MGA.

anyone want to change his 4.1 diff to a 3.9 with quaife?


Bill, that's not entirely accurate. Late 1600's like mine have 25 splined shafts.
Mark J Michalak

With a 5-speed I now think I would be happier with a 4.1. While it is nice to cruse at 80 MPH @ 3500 RPM, with a 270 cam in an 1800 I would like a little more low end grunt.
Steve Meline

"anyone want to change his 4.1 diff to a 3.9 with quaife?"

I have an MkII and I'm 99% sure the differential is standard, so a 4.1. If you're serious, send me an email.

Darian Henderson

This thread was discussed between 09/08/2008 and 20/08/2008

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