Go Brit Baby! - Tech Sessions

What’s the Diff?

Have you ever thought about swapping your stock differential for something else? Maybe you’re looking for something to give you a little edge at the autocross or for better cruising on the highway? TD/TF drivers seem to like the 4.3 diff from an MGA. Some MGAs want the 3.9 diff from a MGB.

Well, I certainly have thought about it and will be taking a crack at swapping out my MGA’s stock 4.3 differential. Below are the options and relative speeds each differential offers. Club Member and Endurance Driver, Paul Urquhart, provided these diff calculations. Paul based his figures on a properly inflated 24” tall tire. The tires commonly used today are not the same as the ones that came with the cars originally. So, these numbers may be a bit different than what you see published elsewhere.

’58 MGA ROADSTER
 DIFF    SPEED/1000 RPM   SPEED @ 3500 RPM
4.875   14.64 MPH       51.25 MPH     TD/TF
4.55    15.69 MPH       54.92 MPH     Magnette ZB/MGA
4.30    16.60 MPH       58.11 MPH     MGA standard
4.1     17.70 MPH       62.10 MPH     MGA Mark II
3.9     18.31 MPH       64.07 MPH     MGB standard

As you can see, the differential has a big impact in the way your MG drives. So, you want to make sure you pick the right diff for your driving requirements.

If you pull out the differential pumpkin, you will see some numbers on the gear work. It could be marked 10/43 (it’s also marked on the exterior of the carrier). This means that it is a 4.3 differential. 11/43 means it is a 3.9. Take the larger number and divide the smaller number into it. This will give you the differential ratio.

Differentials are tough and seem to rarely give a problem when properly maintained. Properly maintained? - Means put in oil. Remember when you bought your MG? Probably didn’t have much oil in the rear axle or the tranny. I know the cars I’ve bought were pretty dry.

Your car’s stock differential should be adequate for most driving experiences. But say you want set some great around-the-cone times on the local autocross circuit or you plan to do a lot of hill climbing; you might want to move up to a 4.55 diff. If you are looking for something to keep up with today’s modern highway traffic you might want to put in a 4.1 or 3.9 diff.

What does it take to do the swap? Well, if the differential gear set and housing (pumpkin) is out of the same model/year car - all you really need to do is pull out the shafts in the rear axle housing and replace the pumpkin. Easy. If you have a pumpkin from a different model/year it still isn’t that bad but it does require just a little more tool wrenching.

I’ve selected a 3.9 differential from an early MGB and will be putting it in the place of my MGA’s stock 4.3 diff. On the outside, the pumpkins look identical with the only difference being the numbers stamped into the top of the pumpkin. However, the inside gear work for axle shafts are different. The early MGA has coarse 10-spline shafts while the MGB has much finer splines. So the axle differential gear work on the 3.9 diff need to be swapped out with the 4.3 axle gears in order to be compatible with the MGA axle shaft splines.

Once you’ve removed the differential pumpkin; you can follow the procedure listed below that I learned from Barney Gaylord.

1. Drill out the peened over end of the hole for the locking pin, tap out the locking pin
2. Tap out the pinion axle, and all of the bevel gears and thrust washers come out.
3. Reassembly is the reverse of disassembly. Use a center punch to peen the edge of the hole to retain the locking pin when finished.
4. Place gasket and pumpkin back in place and tighten the pumpkin down. Don’t crank the nuts down too tight. You might warp the mounting area.

So, what’s the price for this highway cruisin’ MGA? I’ve spent $50 on the MGB pumpkin at the CMGC’s February swap meet and another $1.50 for the housing gasket. So, about $52 total to go from about 58 mph per 3,500 rpms to 64 mph with the new diff. The engine won’t have to work nearly as hard to approach 65 mph on Chicago’s tollways. I think the longer legs on the highway should be a real nice upgrade.

One thing to note for anyone interested in this conversion is that you will need to recalibrate your speedometer to match the new rear axle ratio. Your local lawman will be happy to remind you if you forget to have this done.

We will be having a tech session on this diff conversion June 3rd at 9 am in St. Charles, IL. Please join us for this differential swap.

Want to experiment on your own car with different wheels and differentials via the internet. Check out: www.dirtroad.com/gears.htm. (no such host 7/30/2013). It allows you to plug in different options and see the results. Lots of MG differential fun.

Steve Merical
merical@aol.com   630-443-4985