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MG MGA - 3.9 differential question

I've found an MG rear axle on craigs list. The owner isn't sure what year its from.

What identifying marks should I ask him to look for in order to find out if it contains a 3.9 differential?


T McCarthy

You need to make sure it's from an early roadster - later MGB roadstr and all GT axles are a different type and won't fit the A housing. A midget diff is also smaller.

You need to look at the main wheel of the diff, where the number of teeth on the crown wheel and pinion will be engraved or stamped
dominic clancy

It should have an allow housing (Banjo Centre)

and should have


Stamped near the mounting flange.

43/11 = 3.9

You will also need to use your existing course spline spider gears if it comes with fine spline gears.

easy to do.

don't be tempted to change the large gears, as they are all setup with backlash, etc.

This is what Dominic is talking about

Mark Hester

All MGB axles will fit under an MGA. The early ones are visually similiar to the MGA units. To find the ratio, pull off the back cover and see the stamp on the gears, or rotate the input while counting the number of turns on the wheel, then divide.
SJS Steinhauer

A note: You don't have to change your spider gears if your car was late enough. A bit of info from Barney's website that was helpful to me:

All MGA 1500 had 10 spline half shafts.

MGA 1600 (chassis 68851 May 1959) had involute 26 spline until December 1959 (chassis 82748ww 82892dw May 1959)

MGA 1600 (chassis 82749ww 82893dw December 1959) had involute 25 spline, continuing through production of MGA 1600 MK II and including the MGB models.

I have a late 1600, so I didn't need to change the spider gears. All MGB axles had a 3.9 diff, but the early banjo style were the only ones that were removable as a "pumpkin" unit and compatible with the MGA axle.

Mark J Michalak

How long should it take to swap out a differential? To replace one pumpkin with another. I know the axle needs to be removed, just wondering how much to budget for.
Jim P


You do not need to remove the axle to swap over the pumpkin. Barney gives very good guidance on the process including swapping the sun gears to suit your half shaft splines:

I read his advice thoroughly, did a lot of planning, then completed the job in an afternoon - not difficult and most satisfying.

The error I made was not replacing the oil seal in the MGB pumpkin. I ended up with a significant leak. Recommendation: replace the oil seal when the pumpkin is still on the bench. Far easier than doing it lying under the car!

You will need a tool to hold the diff flange while torquing the pinion nut. I made my own. See here. 10th photo down. I think it was Mick Anderson who told me this trick:

Steve Gyles

The axle shafts need to be removed. Should only take you a couple hours if the new diff is clean and ready to install.
Mark J Michalak

Jim P.
I just cahnged my 4.3 for a 3.9 in my 1957 Mga Coupe 1500cc. The pordedure is a s follows

Jack up rear end / install jacks stands
remove wheels
drain differential
remove brkae drums
undo axles and remove them / do not mix them up
unbolt the differential and remove
clean up the diff opening
install new gasket and sealer
bolt in new diff
install axles with new gaskets
check out the brake pads while you are there
install brake drums and wheels
refill differential with 90wt oil
install wheels
remove jack stands while lowering the car

Parts axle paper gaskets/diff 90wt/diff paper gasket

Time factor...1 1/2 to 2 hours
Difficulty of work...easy
Cost under $ 30.00 Can

The websites say you will find 1st gear a little long with longer rev time but I am completey surprised how much better it is. Overall the differnce in car and shifting is nothing short of "perfection" I save about 3-400 RPMS on the highway and have gained a more suitable "cruising" attitude.
Gordon Harrison

What did you do to correct the speedometer/odometer to match the 3.9 ratio?

Ed Bell

Gordon, why is not mixing left and right axles up so critical?
Mark Mathiesen


You have to get the speedometer recalibrated because the rotations of the gearbox main shaft where the speedo is driven from no longer matches the expected rotations of the wheels, so it registers 3.9/4.3 times slower than you are really going.

Still waiting for my speedo back from Speedy Cables who have been everything but speedy - 7 months and counting...... though they say it has now been shipped.
dominic clancy

Mark on my car the axle threads are right and left matched. withe spoke wheels if I put them on wrong I could loose a wheel.

Ed so far nothing. I need to rebuild the speedo as it has a very bad case of the "jumpy....wavies"
Gordon Harrison

MoMa and Nisonger want something like $110 and $150 respectively just to recalibrate my speedo to 3.9, even though it's perfect otherwise and needs no other attention. For that price I'll do the math in my head, thank you. You can find your specific mileage offset pretty easily (tire size, rear end, etc...) just by taking your detroit iron on a 20 mile trip and then duplicating it in your A. Do the math based on the mileage difference and that gives you a pretty good idea.
Mark J Michalak

This thread was discussed between 19/06/2008 and 23/06/2008

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