The tube type rear axles fitted to all MGB GTs and MGB Roadsters since 1967 are notoriously prone to developing an annoying clunk after as few as 50,000 miles. The proper rectification of this problem can easily be completed in a few hours and should cost less than $20.00 in parts.
Before giving us a call to order up the required parts, spend a few minutes diagnosing your particular problem and eliminating other possibilities by checking the following:
l.U-Joints: failing U-joints also have an audible clunk in their earlier stages of demise, but the particular sound is more of a ringing clunk. A really well thrashed U-joint will cause the drive shaft to vibrate considerably. Check the tightness of the joint by hand and replace if any play exists.
2.Wire wheels will clunk (or fall off!) if loose. Be certain wire wheel nuts are tight but avoid over tightening, as this also can damage components. If loose, check hub and wheel splines for wear and replace effected parts if serious.
3. Loose rear shocks, shock links or rear axle U-bolts can cause a clunk or acceleration or deceleration but will generally rattle over bumps as well. Be sure rear axle U-bolts are moderately tight as a loose rear axle assembly will also cause a disconcerting handling twitch on acceleration/deceleration.
4. A clunk could possibly develop due to a loose or worn pinion flange or worn axle to hub splines. However, these are remote possibilities, hardly worth pondering.
5.The likely culprit can be isolated with a quick final check. Jack up the rear of the car and firmly support with Jack stands. With the emergency brake firmly on and the gearbox in neutral, rotate the pinion flange and take note of the amount of free play. Anything in excess of 1/4″ free play at the circumference is grounds for rectification.
The actual clunk is caused by wear in the thrust washers immediately behind the pinion and differential side gears.(Illustration 65 & 67 on page 31 of the Moss Motors MGB catalogue.) Access to these thrust washers is via the rear inspection cover. The L/H axle hall shaft must be removed, which entails removing the L/H brake gear and the brake
backing plate. A slide hammer is not normally required in order to extract the hall shaft, but reinstallation of the rear hub alter the brake backing plate is removed provides a surface to gently tap against.
To remove the pinion gears roll the center carrier around to place the small roll pin, (which secures the main pinion shaft within reach), and use a small drift to remove pin. The pinion shaft can now be removed by use of a drift and vice grip pliers, which should not mar the hardened steel shaft. With the shaft extracted, the gears are easily removed and thrust washers replaced. Although not normally a problem, check the fit of the differential gears against the hall shalt splines and replace if any wear and or play is evident. When re-assembling always use a new steel roll pin as your entire rear axle could sell-destruct if the pinion shaft works loose. An English mechanic’s trick is to fit a 7/64″ x 2′ cotter pin through the center of the hollow roll pin. This will not only strengthen the roll pin slightly, but will also insure that the pin does not move once installed. It may be necessary to slightly drill out the center of the roll pin, which should be done prior to installation.
Reassembly should be straightforward, but don’t forget to bleed the brakes, replenish the rear axle assembly with 90 wt. hypoid gear oil and reinstall the cotter pin securing the axle half shaft nut.
Anti-clunk Repair Parts List
2 x 267-140 – DHL Gear Thrust Washers
2 x 267-130 – Pinion Gear Thrust Washers
1 x 267-125 – Roll Pin. Pinion Shaft
1 x 296-210 – Inspection Cover Gasket
1 x 120-700 – Rear Hub OH Seal
By Chris Nowlan