Rebuilding an MG gearbox

This is an article written by Barney Gaylord (barneymg@juno.com) and reproduced with his permission.

Subj: Rebuilding the gearbox

This dissertation was intended for an MGA, but most of it applies to the MGB as well, exclusive of any overdrive unit.

When you open the gearbox, there are a few specific things to look for. First count all the teeth just to be sure that they're still all there -- that shouldn't be a problem.

Check the synchro rings after removing the side cover but before disassembling anything else, press the brass ring as tightly as possible against the mating cone surface, then try a 0.020 inch thickness gauge between the flat surface of the brass ring and the adjacent steel surface. If the feeler gauge fits without binding, the syncro will probably work OK for at least the next 40,000 miles or so. Check the brass rings again after disassembly. Look at the angled surfaces on the triangular ears, and compare the most worn ones (probably second gear syncro) with the least worn ones (probably fourth gear syncro). If you've lost nearly half the thickness of the triangular ears, it's time for new parts. I've never known them to wear out the conical working surface, just the outer ears.

When you disassemble everything, you'll probably find the lay shaft to be worn somewhat where the needle bearings run against it. You should probably replace the lay shaft, but it's a judgement call when you see it. If it doesn't look too bad, and you drive the car in a civil manner (yeah, sure), you may opt to put the original back in. But personally, I thrash the crap out of the thing, and I expect to keep it another thirty years or so, so I figured a new lay shaft and needle bearings to be a good investment. They're not the most expensive parts in there, and I've done it for two gearboxes now.

You may also consider replacing the big ball bearing on the input shaft. It's a pretty rugged part, but it depends a lot on its past history of maintenance and such. The best way to check this bearing is to run it before you yank the engine and listen for noise. With the engine running and the selector in neutral, depress the clutch pedal for a few seconds to let everything stop rotating and listen, then let the pedal up and listen again. If you hear a fairly loud hissing noise when you let the pedal up, that's the input bearing telling you it's tired. To muffle the engine noise, it helps to get another person to put their foot over the open end of the tail pipe and nearly stop it off, not enough to kill the engine mind you, but almost. When the parts are in your hands, try wiggling the outer bearing race. These big ball bearings do have a small internal clearance, so a little wiggle is OK as long as it turns smoothly without any noticeable scratching sounds. It should be very smooth to the touch when rotated with a little oil in there. If you find a lot of wobble or nasty noises, it's a gonner for sure.

Check the working surfaces of the brass shifting forks where they mate with the grooves in the sliding steel hubs. When new these fit almost like crankshaft bearings, just a couple thousandths inch of clearance. After 100,000 miles they get a little loose and wobbly but will usually still work just fine. Don't worry too much unless they're really sloppy. When my original gearbox has about 150,000 miles on it, the forks were working OK. I swapped them out for some younger ones recently, but only because I had some extra ones laying around and put in the best I had.

Look at third gear and second gear, on the main shaft immediately behind the input gear. Grab these gears and try to move then around. There should be little or no motion other than normal rotation. These two gears ride on bronze bushings on the third motion shaft, and there is a bronze thrust washer between these gears. If one gear (or both) feels loose and wobbly, the bronze parts inside will need replacing. These bronze pieces usually do not go bad. When the gear is engaged and driving, it is locked to the shaft and the bushing sees no relative motion. Any other time the gear just idles on the bushing with no load. Towing the car on the rear wheels for long distances can wear out these bushings, as they will then be running with no lubrication.

There are two sliding hubs. One called the Sliding Hub And Dog Assembly (or Sliding Hub Assembly 3rd & 4th Gear) is between the input gear and third gear on the main shaft. In 3-syncro boxes the other is the First Gear & Hub Assembly, the big one with straight teeth farthest back on the main shaft. In the 4-syncro boxes the other is called Sliding Hub Assembly 1st & 2nd gear. These hubs are in two pieces, inner and outer parts with splines in between. When you slide off the outer ring, there are three small steel balls with springs behind them. If you do not find the little bits immediately, search diligently under the workbench and behind everything - they fly a long ways on their own if you don't catch them.

Check the bore holes where the shifting rods slide in the aluminum housing. They should be a very close fit with little or no perceptible clearance. The ones inside of the main part of the gearbox are usually perfect. The rod in the remote control tower on the MGA gearbox can be a different story. This one is exposed to road dirt from underneath (housing is open on the bottom), and the aluminum bores can get pretty sloppy, especially the rear bearing hole. Also, check the spherical aluminum surface where the shift lever seats in the remote control tower. Somewhere way past 100,000 miles this surface gets badly worn, and the shift lever action gets sloppy. The flat top surface of the spherical part should sit about flush with a flat surface inside the housing. Of three gearboxes I have (or have had) I have only two of these housings. The one I use is really good, but the other is really badly worn, both in the rod bores and in the spherical seat. I could easily sleeve the bores, but fixing the worn spherical surface is cost prohibitive (too expensive). I think the only reasonable fix here is a new (used) aluminum part.

Check for wear on the selector parts, Selector-1st and 2nd gear, Selector-3rd and fourth gear, Selector-reverse gear, and Lever-front-selector; also the tips of the fingers on the Interlock Arm. If the corners of these parts are worn and rounded off, that's OK, it just makes it slicker shifting. But if you can't find any flat surfaces left (completely rounded off the parts), then the shifting will be quite sloppy. You should best check the feel of the shift lever before you start, while you can still drive the car. When you move the shift lever, if you have trouble finding the right gate or the throw seems to go too far, especially into 1st 2nd or reverse, or if it tends to get stuck badly in 1st 2nd or reverse, then the selector parts need to be replaced or refurbished. If you're a real craftsman type person, you may be able to weld up the worn surfaces and file them back to the right profile, but I think this would take a lot of patience. I would do it in a flash if I needed to, but so far I have enough good parts to pass.

Now referring to the MGA 1500 gearbox. This gearbox has a sliding spline joint where the propeller shaft mates to the rear of the gearbox. Inside here is a bi-metal bearing like a big copy of a kingpin bushing, bronze inside of a steel liner. To check this one you shove the drive shaft front yoke in to the normal working distance and try to wiggle it. When new the working clearance is just 0.002 inch like a crankshaft bearing, no perceptible clearance with oil in it. If the yoke can wiggle more than a few thousandths of an inch, it's a problem. It probably won't self-destruct in normal use, but it can cause a vibration in the drive shaft at speeds over 30 MPH. But what's also nasty is that it beats up the rear seal in short order and the oil leaks out. In one case when mine got especially loose, a new rear seal was completely shot in two weeks. Adding oil daily will save the gearbox but leaves a big oil puddle where you park.

The fix for a bad drive shaft support bushing is not easy. On new cars this is a non-serviceable part, only because the dealers don't want your car stuck in their shop while the aluminum housing goes out to a machine shop for bushing replacement. The parts books don't list the bushing at all, so you have to buy a new rear housing. The MGA books do detail this bushing as a replacement part, and Moss even has a number for it, but in the real world it's made out of "Unobtainium", so you can't buy one (I know, because I searched persistently for years).

When this bearing gave out on me, the only immediate fix was to get a new (used) rear housing from the British bone yard. I got the only good one they had out of six that they checked, and it only fit the later 1500 gearbox that belongs in my car (internal rear seal), but was still in need of a rebuild at the time. The early 1500 gearbox that I had rebuilt and had in the car at the time has a different rear housing (external rear seal). In a pinch I bought one of these also and installed it just to get back on the road, but with a sloppy bushing, the seal wouldn't hold up. I have since rebuilt the original gearbox and put it back in the car -- no more seal problem.

Now you want to know how to fix the bushing? I went to my local bearing supply house and bought plain bronze sleeve bearings. The original bimetal bushing is 1.375" ID x 1.500" OD x 2.75" long. The new bronze parts weren't available in that length, so I bought three pieces each 2.00" long. I cut one in pieces 0.75" long and install a 2.00" piece and a 0.75" piece end to end in the housing. The original bushing was a bear to remove, being in a bore with a shoulder way down inside. I had to make a thick double-D washer to put in behind the bushing and pull it out with a large threaded rod, washers and nuts. The second time I did one of these I just used a hacksaw blade to cut through the wall of the bushing and it came right out. Before installing the new bushing, drill a hole in one side to match the hole on the original bushing. Don't worry about the helical grooves, this bushing runs in oil all the time, the original bushing was a part designed for grease like a kingpin bushing. The new pieces should press in quite easily, being thin wall parts. They will not need to be honed to final size like the original parts, being precision parts to begin with.

Just on the side, the early 1500 gearbox needs a different propeller shaft as well. the output shaft in the early box is 1 inch 10 spline. The output shaft in the later 1500 box is 1-1/16 inch 10 spline. The early drive shaft is also 5/8 inch longer to match the shorter rear housing and output shaft of the gearbox. MGA 1600 and MGB gearboxes have a ball bearing in the back, and a flanged coupling bolted into the gearbox.

As for the rest of the internal stuff, just be sure that the parts are all there and nothing's broken. When you reassemble it, the front cover has a gasket and some thin shims for the front bearing. The bearing has an external snap ring. The idea here is to tap the bearing back to seat the ring solid against the housing, measure the extension of the outer bearing race from the face of the housing, measure the depth of the counter bore in the front cover and add the thickness of the paper gasket, then fit enough shims to take up most of the clearance. Leave a thousandth or two of clearance just to be sure the gasket seats properly. The book just says to reinstall the original shims, but I find that these often get mutilated somehow and need replacements. Stick them in place with some grease to hold them during assembly.

If your front cover is an original MGA 1500 or 1600 part, it won't have a rubber seal. But it is common practice to fit a front cover from a late 1600 MK-II gearbox which has a rubber seal. The early MGB front cover will fit on the MGA gearbox, but it has the pivot point for the clutch throw out arm in a different location to accommodate the thinner MGB clutch pressure plate. This cover uses the same seal as the late MGA part. Any cover with the seal and the seal itself will work with your original input shaft.

Now you remember that little split pin in the bottom of the bell housing? Good idea to ensure that it is still in place, and a little loose. A bit of vibration with the split pin keeps the drain hole open.

There are gaskets for the front cover, side cover, front housing to rear housing joint, shift selector cover, and two more for the remote control tower. Also a plastic washer at the speedometer drive fitting, and a felt seal on the oil dip stick (maybe not available separately). Gaskets are pretty cheap. Buy the whole gearbox gasket set if you can. You will also need the rear seal. The external seal for the early 1500 box is a financial rip-off but generally is still available. The internal seals for the late 1500 and later models are reasonably priced and easy to get. There is also a seal on the speedometer drive shaft, but you can change that one any time in the car.

You also want to check the rear rubber mount. These are only moderately expensive, but good piece of mind to have new ones in there. The MGA gearbox rear mount is a real pain to change, being a press fit steel shell in a thin aluminum housing. Be careful here, as the aluminum monkey ear on the rear housing is easy to break off. You surely can remove the old mount without a press if your willing. Start by drilling out the rubber all around and remove the inside steel tube. Then you can carefully hacksaw through the steel shell from the inside. Saw in the upward direction, toward the inside of the housing, so when you leave a small kerf in the aluminum part it's on top where it won't break out.

You may be able to tap the new mount into place with a hammer, but only if you can support the aluminum ring all around with a steel tube like a giant wrench socket. These things are supposed to fit really tight. They come new with a light coat of paint on the outside. I think you can chemically remove the paint, but if you sand it off the part may end up being loose in the housing. I had one that kept slipping sideways in use and had to wrap a bungee cord around the rear housing and frame to keep it in place. If you're the least bit queasy about breaking the aluminum monkey ear, take it to a shop with a press and the proper punches and dies. And it's best to do this when it's disassembled, so you only have to deal with the smaller rear aluminum housing. Oh! And be sure to put the rear mount in from the right side -- take a look at the picture in the book.

That's about it. The MGA and MGB gearboxes are pretty easy to work on. They only have two large ball bearings (three in the later boxes) and they're held on with jamb nuts, not a very tight fit. Just don't lose the spring loaded bits. It's frustrating to have to place an extra parts order for one little spring and two steel balls just because they went flying under the bench somewhere and you can't find them. And that's usually on the last day when you want to finish the job, so it makes you wait another week or more for the last few parts.

Well, trust me, it's not particularly difficult to rebuild a gearbox, and you'll get one hell of an ego trip when it's finished and running again.

Barney Gaylord