Eliminating Rear Axle Clunk

By John H Twist

A common problem I hear from MG owners is “There’s a clunking sound in the rear of my car!” The Salisbury, Tubed-type, or “GT” rear axle fitted to all MGB/GTs, virtually all MGBs from 1967, and all MGCs develops a clunk after as little as 50,000 miles. This clunk is evident when starting up, reversing, or changing gears. It sounds the same as a faulty U-joint without the ringing. The clunk occurs from too much free play in the differential pinions and wheels, or spider gears, which allow the axles to turn at different speeds (cornering). This type of clunk is particular to the Tubed-type axle, as the banjo differentials often travel 100,000 miles without developing excessive free play. 

To test for free play, jack up the rear end of the MGB and support it at the front of the leaf springs with stands. Have an associate depress the brake pedal, freezing movement of the rear drums and wheels. With the gearbox in neutral, grasp the drive shaft and rotate it through its free play. A movement in excess of 1/4″ on the circumference of the pinion driving flange is justification for repair.

The repair is straightforward, requiring the removal of only axle. The rear plate is removed, two pins driven out, and the gears literally fall out of the carrier. The washers behind the gears are renewed. It requires several hours for the home mechanic. 

ANTI-CLUNK TOOL KIT
  Jack

Jack Stands (Pair)

Lug Wrench or Wheel Nut Spanner

3/4″ Socket for Drum Nuts

Phillips Screwdriver for Disc Wheels

1/4″ Square Socket or Wrench for Brake Adjustment

9/16″ Socket and Wrench for Brake Backing Plate

1 5/16″ Socket, Six Inch Extension, and Breaker Bar for Hub Nuts (must fit inside the rear hub if you have wire wheels)

1/2″ Socket and Thin 1/2″ Wrench for Rear Plate

5/32″ Punch for Roll Pin

3/8″ Punch for Pinion Pin

Vise Grips for Pinion Pin

Hammer!

Brake Bleeder Wrench or Socket (1/8″)

Filler Spout for Gear Oil

7/64″ Bit and Drill for Tension Pin. The split pin must fit inside the Tension/Roll Pin

Silicone Gasket Sealant

Grease

ANTI-CLUNK PARTS KIT

Part	Factory #	Moss #
Gasket	BTB 674	296-210
Roll Pin (3/16″ x 1 1/2″)	BTB 715	267-125
Pinion Washers	1G 7445	267-130
Differ Washers	ATB 7072	267-140
Wheel Seal	GHS 179	120-700
Split Pin 7/64″ x 2″	NPN
Split Pin 5/32″ x 1 1/2″	NPN
Brake Fluid	LMA
90/140 Gear Oil	2 Pints

REPAIR TECHNIQUE

 1) Loosen the left axle nut ( 1 5/16″ ).

 a) Disc Wheels — Jack up the left rear only, and support on a jack stand. Keep the car in gear and the right rear wheel on the ground. Remove the left rear wheel, remove the split pin, have an associate depress the brakes o keep the hub from turning, and loosen the nut. Do not remove the nut yet.

b) Wire Wheels — Remove the left rear spinner or octagon nut. Drive the socket onto the nut, over the split pin. Have an associate depress the brakes to keep the hub from turning, and break the nut loose (by about 1/8th turn). Do not remove the nut yet. Remove the socket to allow the shorn split pin ends to fall away.

 2) Jack up the rear of the car. Place jack stands at the front of the rear leaf springs, chock the front wheels and drop the jack away. The rear axle will fall to its lowest limit. Rock the MG to ensure it is steady. Check  if the petrol tank leaks from the top (MGB gas tanks leak from the front top if rusty). If so, it will be necessary to drain the tank, or preferably, reassemble and drive the car until the tank is low on fuel. Leaking gasoline must be avoided. It is DANGEROUS!

 3) Drain the oil from the differential. Use a 3/8″ socket extension to remove the drain plug. 1980 MGBs have no drain plug, and the filler plug is a 1/2″ Allen hex. Drain the oil into a pan and properly discard it later.

 4) Remove the left half shaft.

 a) Remove the left rear wheel. Disconnect handbrake cotter pin by removing the split pin, then turning out with vise grips. A hammer or heat is sometimes necessary to free a stubborn pin. 

b) Remove brake drum fasteners.
 

 c) For Wire Wheels — have an associate depress the brakes to keep the hub from turning and remove the four 3/4″ nuts.

For Disc Wheels — Locate the Phillips screwdriver in the screw head, strike with a hammer, and unscrew. Do not round out the Phillips screws! If they will not come free, strike the screwdriver even harder!
d) Back off the brake adjuster (1/4″ square) and remove the brake drum. The drum sometimes needs prying or rapping to free it. Mark the position of the 1 5/16 cast nut and split pin before removal. Use a paint pencil to trace the direction of the split pin hole on the end of the half shaft / axel so you can line up the castle notches upon re-assembly. Remove the hub nut (1 5/16″) now and strike the cone washer with a punch so that it drops free.

e) Load the hub by pulling on it, and strike the edge with the hammer. It should pop free. Rarely is a hub puller needed.

f) Remove the brake backing plate. Either

 1) Remove the straps holding the brake line to the rear axle casing so that the plate can be pulled outwards over the end of the half shaft; or

 2) remove the brake line fitting from the wheel cylinder. Removing the line is more complicated, necessitating bleeding, at least. Sometimes the brake line twists and fatigues or breaks. If the backing plate is separated from the brake line, keep the brake pedal depressed with a stick so that the master cylinder reservoir does not empty.

g) Remove the four 3/8 bolts holding the backing plate to the rear axle housing (9/16″ socket / wrench) and tie the plate to the front of the leaf spring, or lay aside.

h) Refit the hub, cone washer, and nut to the half shaft. Then, rap the back side of the hub with a hammer to drive the half shaft and bearing from its housing. Turn the hub continuously while rapping! An alternative method is to use 2 threaded rods as spacers and use the shaft nut to draw out the bearing.

 5) Remove the spider gears

 a) Remove the rear plate. Be careful not to twist off the brake line held at the top of the unit. Plate the handbrake cable over the top of the diff or wire it away so it does not interfere with the internal work.

b) Rotate the carrier by turning the drive shaft so that the 3/16″ roll pin (tension pin) lies at an upwards angle. Drive this pin out using a 5/32″ punch and discard the roll pin. IT MUST NOT BE RE-USED!

 c) Again, rotate the carrier so that the roll pin end of the pinion pin is angled forwards. Use a large punch (3/8″) and drive the pinion pin forwards about 1/2″. Do not drive the pin too far forwards or it will be impossible to turn the carrier!

 d) Rotate the carrier to position the roll pin end of the pinion pin to the rear. Grasp the end of the pin with vise-grips TIGHTLY!, then twist and withdraw. The pinion pin is hardened and the vise-grips should not damage the surface.

 e) Remove the four gears and four washers. Often, the differential wheel washers have disappeared altogether.

 6) Prepare and Clean the Parts.

 a) Hold the new roll pin (3/16″ x 1 1/2″ in a vise and open the inside diameter to 7/64″ with a drill. This is necessary as the ends are slightly crimped from cutting. If a bench vise is not available, hold the pin vertically in vise grips and frill downwards onto a block of wood.

 b) Clean the old gasket from the differential and rear plate. A hand held propane torch will warm the plate sufficiently to allow the gasket to easily peel away.

 c) Replace the hub seal into its housing, if a new seal is fitted. Use a liberal amount of grease on the inside of the housing and the outside of the seal to facilitate installation. Tap the new seal into place using a wide faced hammer, or using a block of wood to keep the seal parallel with the housing base.

 7) Replace the spider gears.

 a) Liberally grease the new washers and gears, and place the differential wheels into the carrier.

b) Place the differential pinions onto the differential wheels, directly opposed, washers on the gears, and rotate the gears/washers so that the holes in the pinions line up with the pinion pin holes in the carrier.

 c) Replace the pinion pin, ensuring that the roll pin holes are aligned.

 d) Drive the roll pin through the carrier and pinion pin. Use the large punch.

 e) Fit the 7/64 x 2″ split pin through the roll pin, then splay the ends. This extra thickness dramatically increases the shear strength of the roll pin and prevents the pinion pin from rotating and breaking loose. THIS STEP IS IMPORTANT!

 f) Refit the back plate and gasket, using silicone RTV sealant — but use a thin film only.

 8) Refit the half shaft and brakes.

 a) Drive the half shaft and bearing into the differential housing. Position the castle hub nut on the half shaft so the hammer damages neither the shaft nor the nut.

 b) Plate a thin coat of RTV silicone sealant on the backside of the hub cap, use the 3/8″ bolts as guides, and tap the cap into place.

 c) Refit the brake backing plate. Reconnect the brake lines and refit the brake line straps.

 d) Refit the hub, cone washer, and nut.

 e) Fit the brake drum and tighten screws (disc wheel) or nuts (wire wheel) and adjust brakes.

 f) Bleed the brakes if necessary.

 g) Have an associate depress the brake pedal or temporarily tighten the brake adjust screw to hold the drum from turning. Tighten the drum nuts or screws.

h) Reconnect the handbrake cable (cable adjustment may be necessary.

 9) Refill the differential. Fill only until the oil is felt on the finger. The MGB should be level when filling, so leave the filler plug out until the car is level down on the ground. Refit the left wheel and drop the MGB to the ground. Allow the excess oil to drain from the filler hole. It is better to be a bit too low than a bit too high when refilling the differential.

10) Jack up the left rear, leaving right rear on the ground, and depress the brakes. Remove the disc wheel or wire wheel spinner or nut, and tighten the hub nut TIGHT so that the split pin holes align with the castle nut. Use your mark as a reference. Axle shaft nut 150 lb torque to next hole is specified. Get the rear nut as tight as you can! Fit and splay the split pin. Fit the differential filler plug. Refit the left rear wheel and set again on the ground.

11) Test the brakes and drive away WITHOUT the clunk!!

Video References:

“An MG is Born” Episode 3 Part 2 https://www.youtube.com/watch?v=z9YsTaaV-20

“An MG is Born” Episode 3 Part 3 https://www.youtube.com/watch?v=0JFrP1WdDE0