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MG MGB Technical - rear end clunk

my 1970 roadster has an annoying clunk coming from what im guessing is the diff?
its mostly noticeable when changing through the low gears up and down and when easing on and off the the throttle.
it as wire wheels don't know if that mater's
s truman

Two potentials. Splines on the wires are worn and the clunk is the wire wheels, or the thrust washer in the diff is bad. Check the wheels first, but thrust washer is a common cause.
Bruce Cunha

I think I would check out the U joints
MGBV81

Also, check the U bolts on the rear axle for tightness. If they aren't tightened periodically, the axle will move up and down with the torque of the engine. RAY
rjm RAY

how do i check the thrust washer and U joints.
s truman

The U joints are best checked by removing the drive shaft. Worn U joints are usually most noticeable on deceleration and are "felt" more than heard because the U joint end(s) that are loose in their caps are not under engine torque pressure and the driveshaft is then twisted or out of balance in its rotation. If you have worn U joints, they won't last very long, one or two miles at best. If they give out while the car is moving, a loose driveshaft in the tunnel is very noisy and the yolks at the end of the driveshaft, back of the transmission and front of the rear axle can be damaged and costly to replace. Remove the driveshaft, place it in a vise or grip it firmly and rotate the yolks at the end to insure they move freely in all directions without resistance. Hold the yolk firmly and notice if there is any play in the end of the U joints and their caps. See if the end of the caps are flat and smooth. There are several small needle bearings inside the cap. Think of it like a bucket lined with steel rods all around the sides standing vertically. Excessive wear often lets one or more of them drift downward in the cap and then you'll see a crack in the end of the cap. If all checks out, at least flush the joints with fresh grease. If you have worn U joints, you can replace them with a few tools and a good bench vise. The clunk sound is most likely from the rear end in my experience.
Rick Penland

well ive had this clunking for a while now so i dont think its the U joints as you said they would only last a couple of miles.

how do i check if its the thrust washer.
s truman

You can check the UJs by grasping the gearbox and axle flanges and twisting opposite directions, out of gear and preferably with the rear wheels off the ground and handbrake off.

You can check the diff by twisting the axle flange with the rear wheels on ramps or handbrake on. Some movement is inevitable, how much is too much is subjective. It'll clonk to the end of time without breaking.

UJ and diff clonk will occur *as soon as* you apply any torque at all to the prop-shaft from the gearbox, forwards and back. Spline wear clonk should only happen when sufficient torque has been applied to cause the tapered surfaces between wheel and hub to cause them so slip, as long as the spinner is properly tight. There is always some slippage back and fore because there has to be free play between them in order to get the wheel on and off without a press. Originally it is so slight you can't hear it, but over time the hammering back and fore particularly on the rear wheels (the front usually only have torque in one direction unless breaking hard in reverse) thins the splines and the gap increases, so the movement increases, eventually becoming audible. When the noise starts should should start to keep an eye on the splines. Originally they have a flat top, which thins to a point, then starts to bend in the direction of greatest torque, then shears. It's probably a good idea to change both wheels and hubs when the flat top becomes a point!

If checking play between the splines note that some movement of the wheel (off ground, handbrake on tight) can be caused by the shoes moving inside the drum, which is when the drum turns with the wheel. If the wheel turns *relative* to the drum then that is spline play.

You can also get play in the hub to half-shaft splines, some have reported that even with the hub nut tight the hub still moves on the half-shaft. It shouldn't as it should be clamped onto a taper with 150 ft lb.
PaulH Solihull

where and what are the axle flanges?
s truman

Have you removed, cleaned and regreased the wire wheel splines and hubs recently? I had a 70 with wires for 18 years and the wheels need to be serviced about once a year. If not, dirt and rust begins to wear the splines. You might try tightening the knockoffs and see if the sound is reduced. That would indicate it's play in the splines when you apply torque to the wheels and would be consistent with your first desciption. If it reduces the sound, remove the knockoff, raise one wheel at a time and rotate it to see how much play there is. The splines can wear to the point that your brakes lock the drum, but the wheel is free to continue rolling since the splines no longer meet up and you get a new and very different sound, a grinding sound followed by a thud when your knockoff on the right side of the car is unscrewed by braking and hits the road.
Rick Penland

s truman - Where the ends of the propshaft bolt up to the gearbox and axle.
PaulH Solihull

This thread was discussed between 09/09/2010 and 11/09/2010

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