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MG MGA - 3.9 rear swap

Thanks again to all the knowledgeable people on this BBS. I have a restored MG TD with the Skyhook 5-speed conversion and have longed to find a 4.3 diff for a swap. I have just started a frame-up restoration my 1960 MGA and may have an opportunity. I understand that the early MGB "banjo" 3.9 pumpkins are a direct replacement for the 4.3 in my MGA. (aside from the spline issue) How hard are these to locate and exactly what year/ model am I looking for? (and what is a reasonable price?)

Failing finding the early "banjo" 3.9, what about using the full MGB rear end/axle? I'm not looking for concours, just comfortable interstate travel across Texas.

I could also pick-up rear disk brakes?

Thanks again,

F. J. Bruns

I am told that 63 to 66 B rear ends are the easy swap for the MGA. If your car is a Mark II the splines are the same Or you could use the axle shafts from the donor rear end. I do not know about swaping the whole housing but it looks the same to me. Try it and let us know.
GK George

The early MGB differential assembly will fit right into your housing. There is a difference in the spline cound on the side gears so those need to be swapped for the MGA gears and then you can bolt it in. Swapping in the later tube type axle will require several changes to the parking brake cable and linkage as well as the drive shaft. You will need to use the MGB axle brackets and U bolts as the housing diameter is also larger than the original banjo type. Barney has information on both swaps on his site.
Here's the info on the gear swap.
Bill Young

I paid $150 for a complete differential housing with pumpkin. The swap will take about 4 hours if everything is ready. I leave the brake backing plate with brakes on the car and bolt it up to the new differential. That way I don't even have to bleed the brakes. Have a good day!

John Progess

I traded an early B diff with wires into my MK11. The donor was in Rocky MT. Vintage Racing and wanted my disc wheel setup. We both got what we wanted for free.

R J Brown

$150 is about the going price you should be able to find one in Hemmings
David werblow

RJ, "We both got what we wanted for free." That's the best way! Did you swap complete axles? Or are the B wire half shafts and A steel (wheel) half shafts the same length? or did you both need new half shafts? Or have I missed something?

Neil McGurk

I want to thank everyone on this board for your help.

I think now I have a plan. It looks like the 3.9 swap into the MGA is an easy DIY project, especially with the body now off the car -- just find a '63-'66 MGB "banjo" pumpkin and change out the sun gears to match the spline count on the MGA half shafts -- no adjustment necessary.

Moving the 4.3 into the TD requires a bit more machine work, but I think I can find the appropriate experience in the area.

Looking forward to moving into the fast lane (and using 1st gear in the TD again). I love it when a plan comes together!!
F. J. Bruns

Complete rear axles. I believe that axle length wire to wire and disc to disc are the same. If any know for sure please speak up.
R J Brown

The WW axle is about 7/8-inch shorter on each side to allow for the long splined hubs and retain same track width. Half-shafts are different length and cannot be interchanged. Bolt-on splined hubs aksu set the tires farther out near the fenders.
Barney Gaylord

"axle length wire to wire and disc to disc are the same." Yes.

FR Millmore

Too many words with too many meanings. I will try again for clarity.
I used a complete early MGB rear end assembly in my MK11. Including differential carrier, axle housing and axles.
FRM agreed with me and I think Barney misunderstood my question.
Are these 2 statements true:
#1 Early MGB wire wheel axles and MGA wire wheel axles are the same length.
#2 Early MGB disc wheel axles and MGA disc wheel axles are the same length.
R J Brown

Yes, and yes.
Barney Gaylord

This thread was discussed between 04/12/2009 and 07/12/2009

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This thread is from the archive. The Live MG MGA BBS is active now.