Chicagoland MG Club:Photos
Gearbox Tech Day 2001
St Charles, IL - July 14, 2001

The team attacks an overdrive gearbox.
When three or four people can't muscle loose the large nut on the output shaft,
you just know where that right hand with the glove is going.

Click for BIGGER pictures, average 40K.

With the team gathering at 9:00 am, first call for coffee and dougnuts, a quick inspection of some new parts procured in advance, and Steve Merical starts disassembly of his MGA 1500 gearbox. We all be ready for the technical feast, table cloth and all.

Starting with removing the clutch release bearing and arm. The release bearing was not nearly as worn as some of the parts inside, so someone was here before. Don't think we will be re-using that rubber boot. Pull the front cover, side cover, top cover and shift interlock arm, and the shift extension assembly. Then remove the rear housing (speedo gear first), extract the shifting rods in assembly with the detent block from the rear of the main housing, remove the shift forks, reverse gear and shaft. Then tap out the layshaft and insert a piece of steel rod in its place to keep the laygear thrust washers from falling out of place.

After extracting the main shaft assembly, the next problem is to remove the front thrust washer holding third gear in place (see picture on left). This one was being a bit of a problem, as the locking pin was frozen in place. The idea is to depress the spring loaded pin down into the base of the spline, then rotate the thrust washer to align the internal splines with the shaft splines so the washer can be removed. The final solution was to grind a sharp edge on a small flat tip screwdriver and drive it into the joint at the tip of the pin.

Once the front thrust washer is off, 3rd gear can be removed, exposing the bronze bushing and the thrust washer between 3rd and 2nd gears. In the second picture above you see the 3rd gear bushing partly withdrawn and the bronze washer slightly lifted. The 3rd gear bushing has internal splines that lock into the shaft splines to prevent rotation of the bushing. The washer and both bushings have interlocking tabs to prevent rotation of the washer and of the 2nd gear bushing. These bronze bushings also have a radial oil feed hole that must align with a hole in the shaft to allow oil feed from within the shaft.

In the 3rd and 4th pictures above you can see the 2nd gear brass syncronizer ring with somewhat worn ears. This was one of the primary parts which needed to be replaced.

Another part commonly showing wear in the MGA 1500 gearbox is the large bushing at the rear of the tail housing. This bushing is responsible for supporting the front yoke of the propshaft, allowing it to ride fore and aft on the output shaft splines, and maintaining alignment of the yoke with the internal oil seal. When this bushing gets worn the front yoke of the propshaft can wobble around and cause vibration when running, and the oil seal can take a beating and allow the oil to leak out of the gearbox. Unfortunately this bushing has been unavailable as a replacement part for the past quarter century or so, and the supply of used rear housings with a serviceable bushing is dwindling fast (like pretty much gone). The expedient repair is to use a plain 660 bearing bronze bushing available from most any bearing supply house. The required bushing size is 1-3/8" ID x 1-1/2" OD x 2-3/4" long. In this diameter the off the shelf parts are not available in that length, so a 3/4" long piece is installed first, followed by a 2" long piece to make up the required length.

Using the front yoke of the propshaft as a gauge pin, the bushing can be honed a little (if necessary) to allow a working fit. The proper clearance is about 0.001" on the radius, about the same as a crankshaft bearing or a rocker arm bushing. With a little oil applied to fill the space the yoke then has virtually no perceptable radial clearance in assembly. Putting the new bushing in is easy. Getting the old bushing out of the deep hole with the blind shoulder in the bottom is a long and somewhat painful discussion for another day. In this case a replacement rear housing was prepared with the new bushing in advance of gearbox disassembly.

Finally we get on with the reassembly, starting with transfering the selector shift rod into the new rear housing. This requires removal of both the front and rear levers from the rod to extract it from the housing. Reassembly is pretty much the reverse of disassembly, with just a couple of tricky moves. Replacing the laygear needle bearings is a real pain, because the internal snap rings have no provision for end grips, so you have to get creative and physical with a couple of small pointed awls or flat tip screwdrivers to get the snap rings out. The laygear goes into the box first, along with the two thrust washers, and a thin metal rod to hold it all in place in the bottom of the box, temporarily out of the way of the mainshaft. Then 18 small needle rollers get pasted into the back of the input shaft with grease to hold them in place until the mainshaft is installed. After the input shaft and mainshaft assembly are in place, then the layshaft has to be installed. This is easiest done if you stand the box on end so the laygear and thrust washers stay in place when you remove the small rod to insert the large shaft.

The rest is a lot easier and can proceed fairly quickly. Install the reverse gear and shaft, which is locked in place with a small bolt with a pilot nose and a lock tab washer. Then drop in the three brass shift forks, followed by inserting the three shift rods from the rear, bolting the shift rod detent block to the back of the main housing, and installing the locking bolts to secure the forks to the rods. If all has gone well up to this point you have not lost any of the ball and spring detent parts. Otherwise you may have a short delay occasionally while you try to find these parts wherever they flew around the shop, or you might have a much longer delay waiting for the new parts order to replace these lost bits. Smart money will have ordered a couple extra detent balls and springs prior to disassembly. Second picture above shows all forks and rods in place, and the rear housing almost fully in place, including a new paper gasket and a new rubber mount bushing on the bottom. Third picture above shows the new rear seal installed. This is easier done before the rear housing is installed over the output shaft. Final details include installation of the speedometer drive gear and spindle, the remote shift extension assembly, and the top, side and front covers.

There is a little fussing about with shims in the front cover to take out most of the end float in the large input ball bearing. Measurements must be precise to gauge the depth of the cavity in the front cover and the amount that the ball bearing race extends in front of the main housing in order to dertermine how much shim thickness is required. If you have done this in the past with the same unit you might justifiably assume that the pre-existing shims are correct. If you did not do the last prior assembly yourself it is only prudent to assume it's wrong, and proceed to do the proper measurements and shimming this time around. Stick the shims in place with grease to hold them while installing the cover. Late production MGA gearboxes had a rubber seal in the front cover, where the earlier ones did not. It is fairly common practice to install a later type font cover on the earlier gearboxes in order to use the front rubber seal. The clutch release arm should generally be treated to to new bushing, a new pivot bolt, and a new rubber boot.

Pictured above are the most common culprits of wear in the MGA and early MGB gearbox. The layshaft has very obvious wear at the points where the needle bearings run against it. especially for the rear needle bearing. With this much wear on the shaft the laygear will have the gear teeth misaligned enough to place the gear load farther out near the end of the teeth, making for more stress and increased risk of broken gear teath. When you hear these units whining in 2nd and 3rd gears it's prudent to open them up to replace the layshaft before you get broken gear teeth. The 2nd gear syncronizer rings get worn on the ears about the same time as the layshaft is beyond useful life, so these parts will likely need replacing at the same time. The 3rd and 4th gear syncronizer rings often show very little wear and can be re-used. If the 3rd gear syncro ring shows just a little wear, and the 4th gear syncro ring shows almost no wear, then is may be prudent to swap these two parts (same part number) for extended life, much like rotating tires to achieve even wear. The 3rd and 4th syncro rings will likely outlast the layshaft and 2nd gear syncro ring by a factor of 3 or 4 to 1.

Meanwhile and throughout the day part of the crew was working at swapping the speedometer drive gears between two MGB overdrive gearboxes. When the nut holding the rear yoke was being stubborn the manual impact wrench came into play (commonly known as the BFH). Removing the back of the overdrive housing was sufficient (and maybe not even necessary) to get at the speedometer drive gear. At least this provided us all a good look at the internal working bits of the cone clutch and planetary gear assembly.

Other small jobs done during the day were replacement of gaskets and seals and diagnostic inspections of a few other gearboxes, including one MG Midget gearbox. Gosh was that thing small compared to the full syncro overdrive MGB gearbox, and with the 3-syncro MGA gearbox about halfway in between.

©2001 Chicagoland MG Club, All rights reserved.