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MG MGA - Conversion to B engine and trans

I've removed the 1600 engine from my '59 A and have a '65 B engine and trans. The B trans mount differs from the A mount point and wonder what others have done to fit the B trans. It looks like I will have to cut the A's trans mount "ears" and construct an adaptor for the B mounts.

Is there a single best solution or setup for this?

Thanks much!

JM Morris

Gday Jack, I cut into the A's box section welded in a flat steel plate to keep the rear of the gearbox the same height then welded the B's gearbox mounts in place. Wouldn't a "A" gearbox bolt straight to 65 B engine? Bob PS I'll try to post a pic later

Jack, I have a 5 main bearing "B" motor and "B" gearbox in my car. No modification needed. It all just bolted in!
Barry Gannon

So, Barry, you drive an MGB? Can't wait to hear how the MGB gearbox was a bolt-in for the MGA.
Barney Gaylord

take a good look at the two gearboxes.
They are virtualy identical.
Slide off the MGA gearbox rear section (if it the later type that is) and slide it onto the MGB gearbox (if it's 3 syncro and non overdrive)... Job done....
I've done it several times....

MGB ratios with MGA mounts.
Slightly over simplified but you get the point.

If only it was this easy to convert an MGB engine to a twin cam !!!

Mark Hester

Okay, so you have the late MGB engine (5-main) with the early MGB gearbox (3-synchro), and small flywheel (1965-1967 issue). It does bolt together that way, if you have all the right parts. Not sure you are justified in calling it a bolt-in MGB gearbox. What you really did was to convert the gearbox to MGA 1600-MK-II specification, then install the MGA gearbox in the MGA. Did you also swap the speedo drive gears in the process?
Barney Gaylord

Is there a place where I can get some information on doing this swap? I have heard so many things about three main, five main and early and late transmissions. What is the correct way to do this?
GK George

If it's the trans. that you're really trying to replace, if you have the money, the Hi-Gear 5-speed is truly the way to go!
David Werblow

OK--I feel a little sheephish about this. Barney--as usual--covers it on his website.

I hope it's as simple as sliding the rear sections off
and simply swapping, assuming the tailshaft nut is the only thing preventing a slide out (other than the bolts holding the rear section to the main case). Correct?

I couldn't make heads or tails out of what goes where from the exploded view gearbox illustrations in the Haynes manual.

For those who think the mounting points are the same, see attached. The B trans is at the top.

Thanks to all for your kind help!

JM Morris

Barney covers it.

Won't work with a sliding spline tailshaft MGA trans, but will on any of the flanged tailshaft boxes (1959 15GD power unit or later). Just need to swap out the tailhousings, and as Barney noted, the plastic speedo drive gear.

It gets more complicated if you want to fit an OD MGB box in an MGA.....but it isn't necessary even then to start hacking up the MGA frame!
Bill Spohn

Dave Godwin has done the B OD transmission conversion, and if you don't modify the chassis for the mount, the tail end is very high and the propshaft angle very acute. The suspicion is that the UJs will wear very quickly.

I have the OD conversion documented with other gearbox conversions here
dominic clancy

This is how mine turned out. I think it's similar to the one on Dominic's link


Additional followup: The A tailsection has now been swapped over to the B case. (By the way, a B tailsection won't fit on an A case, which I attempted for storage purposes of the leftovers. Or at least, I wasn't able to find a way to do it.)

Since I'm also putting in a B rearend (3.9), I'll need a different speedometer drive gear. What are the popular sources for this?

Thanks all!

JM Morris

Pretty sure there is no factory original combination of parts that will make your odometer read right. MGA odometers are geared for 1450 0r 1450 turns per mile for the 4.3 final drive and 15 inch tires. MGB odometers are geared for 1280, or 1200 or 1000 TPM with 14 inch tires and 3.9 final drive. This is why I asked if you changed the speedo drive gears in the gearbox while converting it from MGB spec to MGA spec.

The only way(s) I know to do it with original looking (Jaeger) speedometer is to install the MGA speedometer drive gears in the tail of the gearbox, then either change gearing inside the speedometer for 1325 TPM, or install an in-line speed change gearbox for the speedometer cable with 11/10 speed increase ratio. The speedometer fix is maybe easiest, changing two odometer final drive sprockets from 58 teeth to 53 teeth.

If you use the early MGB (Smiths) speedometer to match the MGB speedometer drive gears in the gearbox, you still need a calibration change to account for the larger tires (15" vs. 14"). If you change tire size, all bets are off, and you start measuring distance and turns before recalibrating anything.
Barney Gaylord

Thanks, Barney. The early B speedo cable drive gear will not fit into the A tailsection, so I had to use the A drive gear. But that's temporary until i get the corrected gear. Since I'll be changing wheels and tires, I'll wait until I've measured distance and turns. I have both the '59 A speedometer and an early model B speedometer.

But in the meantime, what/who is the source for custom drive gears?

Also, what is the minimum roll distance I should use to guarantee an acceptable accuracy for the number of cable turns? I'm guessing 21.12 feet would be fairly easy to measure off accurately (which is 500th of a mile) and is long enough to eliminate significant error?


JM Morris

52 feet 9-5/8" is 1/100 mile, and very accurate. Measure and make two marks on the pavement, push or drive the car that far while counting cable turns to the nearest 10th (disconnected from the instrument), and slip the decimal point over 2 places.

I had mine re-calibrated by Moma Mfg. who mysteriously have whatever gears are needed (but might not sell them retail).

The primary reduction is driven my a single pitch worn gear. It is my understanding that the driven gear is limited to either 25 or 30 teeth (as original factory issue parts). The secondary reduction is driven by a ratchet pawl, one tooth forward for each turn of the fist reduction. Ergo, 25x58=1450 TPM, and 30x48=1440 TPM. The final drive sprocket might be any number of teeth from about 40 to 70 perhaps.

When I changed from 4.300 to 3.909 final drive, the last gear in the odometer was changed from 58 teeth to 53 teeth. Magnetic field in the spinning iron bar magnet can be adjusted (increased in this case) to make the speed needle read correct speed. There are some notes on my web site on these points.

I think I saw somewhere just recently that someone does supply the odo drive gears. I must have been momentarily distracted, as I didn't get it recorded.
Barney Gaylord

I googled "Moma mfg" and came up with nothing. I then tried "Moma speedometer" and got Mo-Ma Speedometer Repair in Albuquerque, NM. I also found a speedometer repair in Nashville, TN.

I've also used some incorrect terminology. The speedometer drive gear is the one mounted on the shaft inside the tail section and the driven gear is the one that spins the cable. I've incorrectly referred to a drive gear above when in fact I've been concerned with replacing the driven gear. (And may not have to do that--just the speedometer gearing, depending on what I find out with the measurements). I think you (Barney) refer to the drive gear as the "single pitch worm gear". I think I'm on the right track now.

Many thanks to all!


P.S.--David Werblow--I hear ya! But I just can't make myself part with $3,000+ for one more gear. It's the priciple of the thing, not the money. (Yeah, right!) :)
JM Morris

Moma Mfg
1321 2nd St NW
Albuquerque, New Mexico 87102 USA
Phone: 505-766-6661
Fax: 505-766-5419
(no web site)

The first reduction gear inside the instrument is a single pitch worm gear. See here:
Barney Gaylord

Another picture.

Barney Gaylord

This thread was discussed between 11/05/2011 and 19/05/2011

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