BritishV8 Forum

I'm in the late stages of an MGB V8 conversion with an LT77 transmission, and it's time to start thinking about the rear end.  Can anyone make recommendations, such as re-gearing the differential versus getting an entire axle, and describe what sort of modifications would be required to install a different axle?

The best diff gears to use are the 3.07 from an MGC. If you can find a complete C axle, apparently the entire carrier with gears will fit into the MGB diff.  Otherwise the B carrier can be machined to fit the crown wheel and pinion of the 3.07.

I'm going through this right now with my Rover 3.5 with LT77 drivetrain.  A few years back, I purchased a used C axle thinking it contained the 3.07 gears.  Had them installed and was none the wiser that I actually had 3.7 gears in there.  The C axle gearing options are the 3.7, 3.3 and the 3.07.  If you do find an entire axle, make sure you get the correct gears that you want.

The 3.7 works okay but with so little difference from the original MGB 3.9, I might as well have left the 3.9 in there and saved some cash.

Others will have suggestions for different axles from the MGB.

For our V6 conversions with 200 lbs of Tq, I like 3.3 or 3.4 ratio gears. It keeps the revs down to a decent number, even with a V6 T5 with a 3.76 1st and a 0.73 5th. I am unaware of what the LT77 has for ratios comparably.

As soon as you start to consider different items such as big brakes or limited slip, the MG axle no longer really has any true advantages. A limited slip for an MGB runs close to $2K USD. A LS for a Ford or GM rear end runs about $500. That extra $1,500 USD could go along way to altering a different rear axle.

If you build the axle correctly, there are no body or suspension mods to install a different rear axle.

-BMC.

From my point of view, the main advantage of keeping an MG axle is that you won't have re-engineer a parking brake solution and your brake bias remains unchanged from stock. On your 1980 MGB there's another big advantage: you won't have to adapt your rear anti-sway bar to a different axle. The MG axle is probably plenty strong for your application, but might not be strong enough for a stronger engine (e.g. most Ford 302s...).  

How to pick a gear ratio? The 3.909:1 gear ratio your car came with will let you drive your car, but you'll probably want a ratio somewhere near the range Brian suggested. Incidentally, what gear ratio is best for you depends on a lot on factors we don't know including your tire size, your camshaft, your driving style, what kinds of roads you drive, etc.  The 3.07 ratio Simon mentioned would have big advantages in highway fuel economy and probably in engine noise level - but you may not find it as sporting. Play with a gear calculator like this one to see what ratio seems to fit your needs:  http://www.britishv8.org/Articles/chart.xls You may also want to revisit the How It Was Done articles on this site because many of them include axle ratio advice. (There are also axle-related articles in the archive.)

What are the disadvantages of keeping an MG axle and just changing gears, as Simon discussed? MGC gears are a bit scarce and relatively expensive.

The MG axles all came with "open" type differentials, and you may want a limited slip differential. If you do, as Brian mentioned, the only one that's easily procured for an MG axle ("Quaife") costs a bloody fortune!

What issues come into play if you swap to an American axle? You just need to find a competent shop to narrow the housing for you, and they'll likely want to see your MG axle so they can duplicate the spacing and angle (relative to the pinion) of the spring perches. The actual axle shafts are best procured from a specialist like Moser Engineering. When you order them, you'll get to specify lug pattern. Something to note about MGB's - most of them were built with off-center axles to begin with! (This is pretty well documented - they had a wrongly-made jig in their shop and they never bothered to fix it because it didn't matter with the relatively narrow wheels/tires THEY were fitting.) You'll want to make sure everything is centered. While you're at it, I'd recommend having the new axle made narrower than stock-MGB so that you can more easily fit wider tires/wheels! (When I had my Ford axle narrowed, it came out a bit wider than stock and that gave me fits when I finally decided I wanted bigger tires!)

Great info, guys.  Thanks as always for your help!

New gear sets are still available from England. Try this site  http://www.autogear.co.uk/product/?groupid=5&model=&manufacturer=5

I have a 3.5 liter Rover mated to the LT77 gearbox in my B.  I bought the Clive Wheatley 3.07 R&P set from Performance British for about $600 USD.  

 http://www.performancebritish.com/products.php?cat=15

In order to install it on the stock 3.9 MGB diff carrier, the flange has to be machined.  If I recall correctly, I took .213" off the flange.

At this point, I've heard numbers from .213 to .220 removed from the carrier.  Does this depend on how much wear is already on the carrier?  Is there a target size that should be attained rather than focusing on the amount to be taken off?

No, it's not wear.  Nothing on the carrier itself is subject to wear.  It's a matter getting the ring gear centered enough to check the engagement pattern with the pinion.  I got a rough idea of how much I needed to take off the carrier by measuring the difference in diameters of the 3.9 pinnion vs. the 3.07 pinion and by measureing the difference in thickness of the two respective ring gears.  As I recall, the difference was about .440", so that meant I woud have to take half of that off the carrier.  I just kept cutting the carrier on the lathe until I got the ring gear centered well enough to get a decent pattern.  For the final adjustment, I shimmed the carrier over the last few thousandths.

I had a weblink on final drive set up for a Ford, the basics are the same for the Salibury axle.  I can't seem to find the link at the moment.  I'll do some digging around and maybe I can turn it up.