Chicagoland MG Club:Photos
MGB Differential Clunk Removal
April 19, 2003 - Naperville, IL

Well, guys? Ya gotta start somewhere.

A common problem for the MGB with Salisbury (tube type) axle is too much freeplay (backlash) at the input shaft caused by worn thrust washers in the differential. This results in a loud CLUNK on start, stop, backing up, or even just a change of accelleration while driving. Today we get to fix one, and it's not that big a job.

Click for larger images. - Larger pics average 33KB.

With temperatures sneaking into the low 70's most of the guys were overdressed. Evict the MGA and the John Deere for a few hours to yield the workspace for the BGT. Jack up the rear, get the body on stands, then drop the axle as low as it goes for best access to the back of the differential. Remove wheels, drain out the gear oil, and remove the rear cover. Using pin punch and hammer, tap out the roll pin holding the planetary gear shaft (also known as "pinion pin" by the book). Give the pinion pin a little nudge on one end to start it moving, then rotate the differential one half turn, and pull out the pinion pin. Do NOT push the pinion pin too far forward into the differential case, as it will jam inside the case and prevent rotation of the differential, making it VERY difficult to ultimately remove the pin. (We'll skip that part of the story).

As soon as the pinion shaft has been removed, rotating one halfshaft will dislodge the pinion gears so they can be withdrawn along with the spherical thrust washers. Then you can see the splined inner ends of the halfshafts protruding through sun gears (also known as "differential wheel" by the book). As the halfshafts are too close together here, one of them will have to be pulled out a bit to allow removal of the differential gears. Either left or right side will work as well, so take whichever halfshaft is most convenient for the workspace. Remove brake drum. Remove split pin and 1-5/16" hex nut, and pull the hub from the shaft. Disconnect hand brake cable and hydraulic pipe, remove four nuts, and remove the brake backing plate and the entire brake assembley as a unit (no need to disassemble the brake parts).

If you tap out the four bolts you can tap on one corner of the bearing retainer plate to rotate it slightly for ease of removal. Then extract the halfshaft. Pulling it out just an inch or two is sufficient for this operation. Then you can remove the differential gears. Notice in the last two photos above there were no thrust washers behind these gears. This may not be particularly unusual for the tube type axle. These are thin fiber thrust washers, and they sometimes wear and deteriorate away to nothing, which was the case here.

The thrust washers behind the pinion gears are smaller and spherical, also made of brass, but they also wear some. In the photo at left above you can see a small unworn ridge around the edge of the washer. This wear looks worse than it actually was. A check with micrometer and comparison of old and new parts shows this to be only 0.002 inch of wear. In the center pictures we have the new fiber thrust washer for the sun gear. Considering that it's made of phenolic and fairly thin, it's not surprising that they wear out. The picture at right above shows the differential gears back in place with the new thrust washers.

As soon as the differential gears are back in place you can reinstall the halfshaft and reassemble the hub and brakes at your convenience. Slip the pinion gears back in place paying attention to get them positioned in mesh exactly oppposite each other so they will both line up with the pinion shaft holes at the same time. Slip the new spherical thrust washers in behind the pinion gears, reinstall the pinion pin, and install a new roll pin to lock it all in place. Photo at right above shows it all reassembled with those beautiful new thrust washers in place, and the excessive backlash has been banished.

Finishing touches include reinstalling the rear cover with a new gasket, filling with gear oil, reconnecting the brake cable and the hydraulic line, bleeding and adjusting the brakes. Time stamp on the photos at left was five minutes before noon, or just two hours from our start time. So we were doing quite well considering all the time consumed by show and tell and taking pictures. This left plenty of time for a bit of natter 'n' noggin (not that we weren't doing some of that during the job), and off we went with another happy MGB with no more clunk in the rear end.

Shop facilities, photos and web page courtesy of Barney Gaylord.

©2003 Chicagoland MG Club, All rights reserved.