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MG MGB Technical - Rear Axle Noise

I'm trying to decide whether to do anything about the noisy rear axle in my BGT. There is considerable gear noise from the differential when coasting above about 40 mph. Any acceleration or deceleration makes it quiet. Do these symptoms indicate any obvious repair? The axle is from a later model car, probably a '75 roadster. Thanks. -G.
Glenn G

sound like gear chatter when off load. A minor overhaul will probably sort it. Check all the bearings move smoothly and replace the fiber thrust washers in the diff. Check the propshaft UJ's for play and replace any that show any rotational movement and all should be well again.
Nice easy project for the weekend.....

M McAndrew

I was thinking about carrying out the same minor overhaul on my rear axle as MGmike has suggested.

What bits would I need to order to do the job and how fiddly is it to get the diff pinions in and out if the diff?

Adrian & Glenn-
As with all work under the car, first remove the battery ground (earth) lead in order to prevent accidental starting. Next, chock the front wheels, and then lift and secure the rear of the car upon axle stands.

While you have the car up on the axle stands, now is the perfect opportunity to pull off the brake drums and check to see if the halfshaft (quartershaft) seals are leaking. You can also take a fast look at the brake linings to see if they are worn down close to the rivets that secure them to their steel shoes. Take the time to grease the U-joints (Universal joints), the driveshaft splines, and the handbrake cable, too. May as well get it all done at the same time while it is up on the axle stands, right? A Halfshaft (sometimes also called a Quartershaft or an Axle Shaft) is the shaft that transmits power from the differential mechanism to the drive wheels. The halfshaft (quartershaft) oil seal is the seal that keeps the oil inside of the axle from leaking out into your brake drums. Look on page 213 of your Bentley manual. The halfshaft (quartershaft) oil seal is #43. Just look and see if you spot any oil leaking or oozing out. The oil will ruin your brake shoes. Spray the brake system with CRC Brakleen and inspect everything carefully. While you have the drums off, you can clean off the rust and paint them with VHT engine paint. Remember, rust is a heat insulator, and it is heat that triggers the out-gassing that is the cause of brake fade.

Once the rear brakes are reassembled, set the handbrake so that the halfshafts (quartershafts) cannot move. That way your measurements will be as accurate as possible. Grip the differential flange that connects to the rear U-Joint (Universal Joint) of the driveshaft and rotate it to take up any freeplay, and then scribe a mark on its edge and a corresponding mark on the axle housing. Next, rotate the flange in the opposite direction and scribe another corresponding mark on the axle housing. If the marks are 4.5mm apart (60 degrees of rotation), you have a like-new differential. If the marks are 8mm apart (10 degrees of rotation), you have a usable differential. If the marks are 10mm apart (130 degrees of rotation), you have a worn differential. Note that this method of measurement can be misleading because holding the tire and rotating the pinion will show you several free-plays combined: wire wheel play, half shaft spline wear/play, and pinion/crown wheel play. Measuring the play of a differential using a dial indicator on the bench is the most accurate technique. The specification is engraved onto the back of the gear (each matched set will have a matching set number, and the crown wheel play for that set, and the amount of shim offset to the right or left, all engraved on the back of the crown wheel.

Of course, before you can do anything with the differential you will need to drain the oil out. This potentially simple operation is made more difficult because the British decided to use a 1/2” BSP square drive drain plug. This is a rather quaint plumbing item. Of course, the British like to do things their own way, so these are not compatible with any American threading system, plus their plug has a square hole in it for the wrench (spanner). The tool necessary to remove this charmingly quaint drain plug from the bottom of the differential housing is a 7/16” extension for a square pipe plug. Do not bother trying to use a hexagonal Allen wrench. You will just end up with a ruined Allen wrench trying to get the plug out. It is well worth purchasing the correct tool for this job as it makes life so much easier. MAC Tools makes them. I know because that is where I got mine. Sometimes you can get a cheap one from a plumbing supply house. Once you have gotten the drain plug out you have the option of swapping it for an American-made stainless steel ½” BSP plug from a hardware store / plumbers shop. They never rust in place, and to remove them all you need is a simple 3/8” Allen wrench.

Do not be surprised at what you see when you drain the old oil out. It is not unusual for this maintenance task to have been totally neglected by the DPO. The Owner’s Manual always said to “Check oil level, and top up if necessary.” Not a single word about how often to change the oil. Naturally, this led to neglect. It is entirely possible that the oil in it is the original oil. When you drain it out, it may look and smell like something that oozed up out of the ground, prompting you to expect to see old dinosaur bones floating in it, but do not worry too much about it. The Salisbury tube-type axle is a grossly over-engineered piece of design work, originally intended for use in light trucks and delivery vans. Usually, the only thing that damages it is letting the oil level drop too far. This often happens when the breather on the top of the tube on the passenger side (right above the horizontal bracket) is plugged up with road crud. Air then is trapped inside of the axle, the differential gets hot and causes the air trapped inside to expand, and then the gasket starts to leak as a result of the internal pressure. When the axle cools, air is drawn in through the leaking gasket. The process is repeated every time the car is run until the oil is gone, which usually takes a very, very long time. Once in a blue moon a particularly dedicated garage mechanic will check the level and top it off, so outright failures are unusual. Allow the oil to drain into the container and replace the drain plug securely.

Cleaning the breather is a simple affair, but most DPOs do not even know that it is there on the top of the right side of the axle tube. Just clean around the top of the axle tube with cheap carburetor cleaner so that crud will not get into the threads, unscrew it, and spray it out with carburetor cleaner, carefully clean the threads with an old toothbrush, then put it back in after it dries. Simple. Once that is done you can proceed with the replacement of the old cover gasket.

Getting the handbrake assembly off of the rear axle is actually very easy. Loosen and remove the self-locking nut that secures the compensating lever to the bracket on the differential. Disassemble, clean, and repaint the handbrake compensating lever mechanism. If it does not work properly, then the rear brakes will not apply equal force. See those two cables that traverse the axle and go out to the brakes on each side? They attach to the brake mechanisms. Look and you will see a clevis pin attaching the cable to each of the levers of the brake adjuster mechanism. Remove the cotter (split) pin that secures the clevis pin, then pull the clevis pin out and set it aside, along with its washer. Clean them both and carefully inspect the shank of the clevis pin for signs of wear. If you see any, it is best to replace this inexpensive part (BMC Part # ACB 8715). When you put it back in, be sure that it is pointing downward with the cotter (split) pin on the bottom. Use a stainless steel cotter (split) pin only. Installing the handbrake compensating lever mechanism assembly in the correct order is often a source of confusion and frustration for some people. Install the wavy washer first, then the flat washer with the larger hole, then the lever, then the washer with the smaller hole, and, finally, the nut.

In order to proceed with the replacement of the thrust washers it is necessary to move one of the halfshafts (quartershafts) by about six inches. Using a pair of pliers remove the cotter (split) pin of the main hub nut and use a 1 5/16” thin-wall socket to remove the nut. If your socket has a cupped mouth, you may need to grind it down in order to prevent it from slipping on the shallow nut. You will need to apply the handbrake in order to prevent the halfshaft (quartershaft) from rotating. Next, loosen the brake shoe adjuster by about three quarters of a turn and remove (wire wheels) the four nuts or (steel wheels) the two countersunk screws that help secure the drum in place. Release the handbrake, then gently tap the hub with a rubber hammer or a block of wood and pull it off of the halfshaft (quartershaft). The coned spacer can also be slid off from the shaft and set aside with it.

While it is possible to carry out the next procedure without disconnecting the braking circuit, it is inadvisable to do so. This is due to the high chance of damaging the brake line, which obviously has very severe safety implications. Therefore, first disconnect the handbrake cable from the lever. Remove the cotter pins from the clevis pins of the handbrake mechanism, and then remove the clevis pins in order to release the handbrake cable from the handbrake mechanism. Next, remove the brake master cylinder top, place some cling film over the opening, and then replace the cap. It is now possible to remove the rear brake line from the rear wheel cylinder with minimum loss of brake fluid. Once the two are separated, plug the brake line in order to prevent brake fluid loss as well as minimizing the amount of air getting into the hydraulic system. The four machine bolts that secure both the brake backing plate and the end cap of the axle in place can now be loosened and removed. Next, lift the backing plate away from the end of the axle. Finally, remove the oil seal collar, bearing hubcap, and the oil seal from the halfshaft (quartershaft). Inspect the oil seal for damage and replace it if necessary. When installing, be sure that the lip of the oil seal is facing inward when you do so. Look for a black wear ring near one end of the collar. If you should find such a wear spot, always use a new collar. In order to prevent leakage, be sure to smear some sealant under the square collar when it is reinstalled.

Ideally, a slide hammer can be used to release the bearing and halfshaft (quartershaft) out of the axle housing. If this tool is not available and the rear axle assembly has been removed from the car, replace the hub and its retaining nut back onto the halfshaft (quartershaft) and, using a block of wood in order to protect the hub, hit it with a club hammer on its opposite side until the halfshaft (quartershaft) releases itself. If the rear axle assembly is still on the car, another, better approach, is available. The books show fitting an old brake drum and beating the axle out of the housing, there is a much easier way: once you have removed the brake assembly, dust shield, and the axle bearing cover, replace the flange just far enough to thread on the large flange nut. Place a couple of large sockets between the flange and the differential housing, and then tighten the flange nut. A few turns and the bearing slides right out, no beating, no cussing. Once the bearing is out, set both it and its inner spacer aside. This spacer is essential to ensure that the bearing is the correct distance along the halfshaft (quartershaft), and hence the halfshaft (quartershaft) and hub will be in correct relation to both the differential and the backplate. The halfshaft (quartershaft) can now be pulled out about six inches by hand. Be sure to have the foresight to repack the bearing with fresh bearing grease before reinstallation.

Use a wire brush and cheap carburetor cleaner to thoroughly degrease and clean up the area around the rear differential cover plate prior to removing it. Make sure you clean both the bolt face and surrounding area of the axle casing in order to ensure that no dirt falls into the differential. Once the area is clean, release all of the securing bolts making a mental note as to where the handbrake pivot point is attached and the location of the top clips for securing the brake lines (pipes). The rear differential cover plate can now be gently pulled away. Carefully remove any remaining gasket material, and then clean it so that its sealing surface can be examined for signs of distortion. You may find that a DPO tried to cure a leak in this area by overtightening the securing bolts of the differential cover plate, resulting in distortion around the bolt holes. If you discover any distortion, then you will need to flatten the sealing surface by sandwiching the cover between to flat pieces of wood and pounding them with a stout hammer. If this should fail to correct the problem, a new differential cover plate fabricated by Steelcraft in the UK can be obtained via the British Motor Industry Heritage Trust (Steelcraft Part # MB 99).

You are now able to see the components of the mysterious differential mechanism. Clean everything with cheap carburetor cleaner so that you can inspect the gear teeth. Inspect the crownwheel (the large gear on the left of the differential cage) for any wear lines, cracks or chipping. If there is any visible damage you will need to seriously consider replacing the entire rear axle unit with a used one as that would be less expensive than replacing the crownwheel and its matching pinion gear.

First, rotate the differential cage around until it reveals the 3/16” X 1 1/2” roll pin that holds in place the mainshaft (third motion shaft) of the top and bottom pinion gears, and then drift the roll pin out. After the roll pin is removed, turn the differential cage again until the other end of the main pin is facing you. You can now start to drift the main pin out of the carrier. Take care not to push the pin too far through as it is very easy to jam the pin against the casing of the axle with no way of pulling it back, which would render your axle useless! Observe when the pin has started to move and as soon as it does, rotate the differential cage around again so that the pin can be pulled out from the top. Place a thin rod through the roll pin hole in the main pin and use this to pull the main pin completely out of the differential cage.

Upon disassembly of the differential, you may discover that one or both of the roll pins have fractured. Instead of simply replacing it with another roll pin, replace it with a coiled spring pin instead. There are two distinct types of spring pins: rolled and coiled. Both types share certain characteristics such as flexibility and the ability to accommodate wider hole tolerances than nonflexible fasteners such as solid pins. However, there are a few major differences between these two types of spring pin that must be understood. While the roll pin’s flexibility can reduce manufacturing costs by absorbing a wider range of tolerances, there are several disadvantages to roll pins that limit their applicability. The roll pin is significantly less flexible than the coiled pin, and it only flexes 180 degrees from its gap. In addition, this limited flexibility can result in plowing and debris generation during the installation process. Under load, the stress is concentrated 180 degrees opposite the gap in a roll pin, which is the origin of its breakage in differential mechanisms that are used for high performance applications. By comparison, coiled spring pins are shock-absorbing fastening components that provide flexibility, uniform strength, equal stress distribution, and closer diameter tolerances. In addition, coiled pins will continue to flex when a load is applied to the pin, thus providing excellent resistance to fatigue in dynamic applications. These are readily available from Fastenal. Fastenal has a website at http:/ .

Once again, slowly and carefully rotate the differential cage and watch the top and bottom gears move away from each other. One will come out at the front while the other tries to fall out at the back. Put your hand in to remove one along with its worn thrust washer and place them on a clean cloth in the same orientation as they were in when they came out of the differential housing. The other gear and its thrust washer should be removed in the same way. Remember that the gears have worn into matched pairs, so take care to keep the pairs separate from each other.

Now that the top and bottom pinion gears have been removed, the other two gears can be removed one at a time, their worn fiber washers removed and replaced with new fiber washers, and then they can be reinstalled.

The top and bottom pinion gears now need to be reinstalled. Should they prove to be badly worn, the easiest way to do this is to turn the differential cage until you can get a hand on either side of the carrier. Then place the two pinion gears opposite each other, hold them in place and have an assistant slowly rotate the differential cage again. You are aiming to be in a position to look down the hole where the main locating pin secures the gears in place and see all the way through. If you are a tooth out with the alignment one of the pinion gears will not line up. Once the pinion gears are in the correct position, slide the new metal thrust washers into place between the carrier and pinion gears. Once all of the pinion gears and washers are positioned correctly, drift the main pin back into position. Hold the new roll pin in a vise and open the Inside Diameter (I.D.) to 7/64" with a drill. This is necessary as the ends are slightly crimped as a result of the pin being cut to length. If a bench vise is not available, hold the pin vertically in vise grips (mole grips) and drill downwards onto a block of wood. Secure the assembly in place with the new roll pin. Insert a 7/64” X 2” cotter (split) pin through the roll pin in order to ensure that it will not come out. In addition, this extra thickness dramatically increases the shear strength of the roll pin and will prevent the pinion pin from rotating and breaking loose.

The halfshaft (quartershaft) can now be felt back into position and, making sure that the mating surfaces are clean, install the axle end cap and the back plate. Use the four machine bolts to pull the whole assembly together slowly by tightening opposite machine bolts a little at a time. When replacing the spacer, be aware that the spacer has a concave face and a flat face. The concave face butts up against the shoulder on the halfshaft (quartershaft), and the bearing butts up to the flat face. Upon reassembly, take care to ensure that the concave face of the spacer is facing the correct way. Because the bearing is machined with a very tight external radius between its face and its internal diameter and the shoulder is machined with a larger internal radius. The concave face on the spacer has an even greater radius and this allows the faces on shaft, spacer and bearing to all butt together. If the spacer were not used these radii would interfere and the face of the bearing would not be right up against the shoulder. Next, replace the hub and the castellated nut, followed by the brake drum, which needs to be secured with the two Philips screws. When reinstalling the splined hubs of a wire wheeled car, note that the hub with a stamped “RH” goes on the right halfshaft (quartershaft) and that the hub with a stamped “LH” goes on the left halfshaft (quartershaft). The mounting spinners for the wheels are also so marked. This is so that the mounting threads of the hubs will tighten the spinners when the car is moving forward.

The handbrake lever and its cable can now be attached and the brake pipe screwed back into the wheel cylinder. Release the cling film from the brake master cylinder and bleed the brakes. You can get any residual air bubbles in the brake lines (pipes) loose by tapping on the lines with the handle of a screwdriver. With luck you may only have to bleed the side you have removed the brake pipe from. However, if the brake pedal feels spongy, then bleed the whole system. Reset the rear shoes by using the adjuster on the back plate.
Stephen Strange

Thanks for the comments. I guess I will have to open the thing up and see what kind of shape it's in. -G.
Glenn G


Couple of good videos here on the very subject:
A Riddett

None of that has to do with the noise you asked about. The noise is because the pinion is not running true to the ring gear. Assuming it has not recently been run without oil, the usual cause is either a bearing failure, or more likely, the pinion nut was tightened incorrectly when the seal was replaced. When replacing the seal, the nut must be tightened to exactly where it was before. Once that is lost, it is guess or start from scratch. The nut is initially tightened to a precise bearing preload, against a collapsible spacer. If you overtighten and collapse the spacer too far, you take it apart and get a new one. The big problem is getting them; sometimes you can hammer them back out to "not collapsed" dimension, or use shims to gain some adjustment. If you have to take it apart, I suggest new bearings, and if you do not understand it, find somebody who does. Procedure is in the WSM, and it is just like almost every other diff in the world, so there are shops around.

FR Millmore

This sounds to me like a case where the oil level dropped to a dangerously low level and the excessive heat that followed damaged the case hardening on the ring and pinion. If this is the situation, a replacement set of gears or a complete used axle is in your near future if you want the noise to go away. When I purchased my '67 Tourer in '72, it had the same problem due to a failed pinion seal that went unnoticed. I lived with the noise for two years until I came across an earlier banjo axle that I swapped into my car. 38 years later and the only maintenance that has been required is the replacement of the oil every few years. All of the bearings and seals are original. RAY
rjm RAY

I do have some experience with rear axles, so Fletcher and Ray's comments seem to the point. When I got the axle several years ago, I removed the pinion nut in order to replace the pinion seal. Having worked on similar axles in the past, I did not think to read the workshop manual beforehand. Of course, when I was ready to put the nut back on and checked the manual for the torque setting, I discovered that I should have marked the position of the nut before removing. No problem, I got a new collapsible spacer and put the whole thing back together following the manual. Things are never the same as new, though. Either I did it wrong or the spacer I bought from Moss was not up to the job. Personally, I suspect the spacer. Rather than spend the considerable effort to redo the job with another suspect spacer, I think I will look for a replacement axle. Thanks again for the comments. -G.
Glenn G

If you did it correctly, to correct preload, then the spacer is not the issue - it cannot do anything other than collapse "more", and there is no force to make that happen. Most likely a bearing has died.

FR Millmore

This thread was discussed between 21/09/2012 and 29/09/2012

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