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MG MGB Technical - Excessive play in differential

I have 255,000 miles on my diff and there is excessive play and a mild clunk sound when starting off.The MGB is a 1970 w/wheeled car.The rear hubs are brand new. Would this be the crown gear and pinion,spider gears or both? I assume the diff must be removed for this repair?
Jon Rosenthall

Jon-
Nope. Clunk repair is easier than you think. 1968-1980 MGB MKII roadsters and virtually all MGB GTs were fitted with the Salisbury tube-type rear axle. Because axles are sometimes changed, an easy way of telling what's fitted to your car is to look at the rear of the axle housing. If there's a sheet metal cover bolted onto the back side of the differential housing, then your car has a Salisbury tube-type rear axle. If there are no bolts on the rear but a series of nuts at the front of the differential housing then the axle is the earlier Hardy-Spicer banjo type rear axle.
After many tens of thousands of miles, wear in the differential is inevitable. This wear normally manifests itself as a 'clunk' being both felt and heard initially upon acceleration and decelleration. If your MGB has wire wheels, first check that the hubs and splines are not worn, as this can also be the origin of a 'clunk.' A diagram showing profiles for various degrees of wear can be seen at http://www.britishwirewheel.com/faq.htm . If your splines and hubs are sound, or if the car is fitted with steel wheels but the clunk persists, then the condition of both the universal joints of the driveshaft and its splines are worth investigating. If these and the driveshaft are sound, then the noise may be due to wear in the thrust washers of the planet gears of the differential. If care is taken, replacement of these inexpensive thrust washers is quite straightforward. Doing so will help to prolong the life of the rear axle.
As with all work under the car, first remove the battery ground (earth) lead to prevent accidental starting, chock the front wheels, then lift and secure the rear of the car on axle stands.
While you've got the car up on axle stands now is the perfect opportunity to pull off the brake drums and check to see if the halfshaft seals are leaking. You can also take a fast look at the brake linings to see if they're worn down close to the rivets that secure them to their steel shoes. Grease the U-Joints, driveshaft splines, and the parking brake cable, too. May as well get it all done at the same time while it's 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 seal is the seal that keeps the oil in the axle from leaking out into your brake drums. Look on page 213 of your Bentley manual. It's #43. Just look and see if you spot any oil leaking out. The oil will ruin your brake shoes. Spray the brake system with CRC Brakleen and inspect. While you've got the drums off to inspect the seals, you can clean off the rust and paint them with VHT engine paint.
Grip the differential flange and rotate it to take up any freeplay, 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. When you make the marks on the rear axle housing, be sure that the parking brake is engaged so that the Halfshafts (rear axles) can't move. That way the measurements will be as accurate as possible. If the marks are 4.5mm apart (6 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 10 mm apart (13 degrees of rotation), you have a worn differential.
Of course, before you can do anything with the differential you'll 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 plumbing item. Of course, the British like to do things their own way, so their plug has a square hole in it for the wrench. The tool necessary to remove this charming drain plug from the bottom of the differential housing is a 7/16" extension for a square pipe plug. Don't bother trying to use a hexagonal Allen wrench. You'll just end up with a ruined Allen wrench trying to get the plug out that way. MAC Tools makes them - I know because that's where I got mine. Sometimes you can get a cheap one from a plumbing supply house. Once you've got the drain plug out you have the option of swapping it for an American-made stainless steel 1/2" 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. It is well worth purchasing the correct tool for this job as it makes life so much easier.
Don't be surprised at what you see when you drain the old oil out. It's not unusual for this maintenance task to have been totally neglected. 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's 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 and you might expect to see old dinosaur bones floating in it, but don't worry too much about it. The Salisbury tube-type axle is a grossly overengineered piece of designwork, originally intended for use in light trucks and 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) gets plugged up with road crud. Air gets trapped inside the axle, the axles gets hot causing the air trapped inside expands, then the gasket starts to leak. 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 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 don't even know that it's there on the top of the right side axle tube. Just clean around the top of the axle tube so crud won't get into the threads, unscrew it, and spray it out with carburetor cleaner, clean the threads, then put it back in after it dries. Simple. Once that's done you can proceed with the replacement of the old cover gasket.
Loosen and remove the self-locking nut that holds the compensating lever to the bracket on the differential. Disassemble, clean, and repaint the compensating lever mechanism. If it doesn't work properly the rear brakes won't 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'll see a clevis pin attaching the cable to each of the levers of the brake adjuster mechanism. Remove the cotter pin that secures the clevis pin, then pull the clevis pin out and set it aside, along with its washer. When you put it back in, be sure that it's pointing downward with the cotter pin on the bottom. Use a stainless steel cotter pin only.
In order to proceed with the replacement of the thrust washers it is necessary to move one of the halfshafts by about six inches. Using a pair of pliers remove the split (cotter) pin of the main hub nut and use a 15/16th UNF thin-wall socket to remove the nut. You will need to apply the parking brake to prevent the shaft from rotating. Next, loosen the brake shoe adjuster by about three quarters of a turn and remove (wire wheels) the four nuts or (disc wheels) the two countersunk screws that help hold the drum in place. Release the parking brake, then gently tap the hub with a rubber hammer or a block of wood and pull it off of the halfshaft. 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 high safety implications. Therefore, first disconnect the handbrake cable from the lever. 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 fluid. The four bolts that hold both the brake backing plate and 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 hub cap, and the oil seal from the halfshaft. Inspect the oil seal for damage and replace if necessary.
Ideally, a slide hammer can be used to release the bearing and half shaft out of the axle housing. If this tool isn't available, replace the hub and retaining nut back on the half shaft and, using a block of wood to protect the hub, hit with a club hammer on the opposite side until the shaft releases itself. Once the bearing is out, set both it and its inner spacer aside. The shaft can now be pulled out about six inches by hand. Be sure to repack the bearing with fresh grease before reinstallation.
get the handbrake assembly off of the differential, it's actually very easy. Loosen and remove the self-locking nut that holds the compensating lever to the bracket on the differential. Disassemble, clean, and repaint the compensating lever mechanism. If it doesn't work properly the rear brakes won't 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'll see a clevis pin attaching the cable to the levers of the brake adjuster mechanism. Remove the cotter pin that secures the clevis pin, then pull the clevis pin out and set it aside, along with its washer. Examine it closely for signs of grooving and replace if you find any. When you put it back in, be sure that it's pointing downward with the cotter pin on the bottom. Use a stainless steel cotter pin only.
Use a wire brush to thoroughly clean up the area around the rear differential cover prior to removing the cover plate. Make sure you clean both the bolt face and surrounding area of the axle casing to ensure that no dirt falls into the differential. Once the area is clean, release all of the securing bolts making a mental note to where the handbrake pivot point is attached and the location of the top clips for securing the brake lines. The rear cover can now be gently pulled away.
You are now able to see the components of the differential mechanism. Clean everything with cheap carburetor cleaner so that you can inspect the gearteeth. Inspect the crown wheel (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 crown wheel.
First, rotate the differential cage around until it reveals the roll pin that holds in place the main shaft for the top and bottom planet 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 and use this to pull the main pin completely out of the differential cage.
Slowly and carefully rotate the differential cage again and watch the top and bottom gears move away from each other. One will come out at the froits worn thrust washer and place them on a clean cloth in the same orientation as they are in when they come 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 keep the pairs separate.
Now that the top and bottom planet gears have been removed, the other two sun gears can be removed one at a time, their worn fiber washers removed and replaced with new fiber washers, and then the sun gears can then be reinstalled.
The top and bottom planet 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 planet gears opposite each other, hold them in place and have an assistant slowly rotate the differential cageagain. You are aiming to be in a position to look down the hole where the main locating pin holds the gears in place and see all the way through. If you are a tooth out with the alignment one of the planet gears will not line up. If the planet gears are in the correct position, then slide the new metal thrust washers into place between the carrier and planet gears. Once all of the planet gears and washers are positioned correctly, drift the main pin back into position and secure in place with a new roll pin. Insert a cotter (split) pin through the roll pin to ensure that it will ncome out.
The halfshaft 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 bolts to pull the whole assembly together slowly by tightening opposite bolts a little at a time. Replace the coned spacer, hub and the castellated nut, followed by the brake drum, which needs to be secured with the two Philips screws. The handbrake lever and cable can now be attached and the brake pipe screwed back into the wheel cylinder. Release the cling film from the master cylinder and bleed the brakes. With luck you may only have to bleed the side you have removed the 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.
You might also want to inspect the pinion seal for signs of leakage and decide if you want to replace it while you still have the car up on the axle stands. If you choose to do this, be aware that although this can be a mechanically risky undertaking, if proper procedures are adhered to, there should be no problems. Mark the flanges of both the driveshaft and the pinion to ensure correct reassembly, then disconnect the driveshaft. Measure and record the torque required to rotate the pinion with the wheels removed from the rear of the car. Preventing the pinion from rotating, remove the flange retaining nut and its washer, then remove the pinion flange. Remove and throw away the old oil seal. Closely examine the the oil seal track area of the pinion flange for damage. Grease the periphery and the sealing lip of the new oil seal and fit the seal flush into the axle casing. Refit the pinion flange and washer. At this point it is necessary to proceed strictly according to procedure. Screw on the retaining nut, tightening gradually until resistance is felt. Rotate the pinion to settle the bearings and measure the amount of torque necessary to rotate the pinion. If the amount is less than that which was previously recorded prior to the removal of the oil seal, tighten the nut a very small amount, then resettle the bearings and recheck the torque reading. Repeat this procedure until a torque reading equal to the recorded amount but not less than 4 to 6 in-lbs is attained. e.g., if the Original Recorded Figure = 9 in-lbs, then adjust torque to this figure (9 in-lbs). If the Original Recorded Figure = 0 in-lbs, then adjust torque to 4 to 6 in-lbs. Caution: Preload buildup is rapid, so tighten the nut with extreme care. If an Original Recorded Figure that is in excess of 6 in-lbs is exceeded, then the axle will have to be disassembled and a new collapsable spacer installed.
Ensure that the mating surfaces of the axle and its cover are cleaned by removing all of the old gasket, dirt and grease. Hopefully the old gasket will come off easily. Don't be surprised if it comes off in sections and pieces. In order to have a leak-free rear axle, first you've got to get all of the old gasket off and have a clean, oil-free sealing surface. A razor scraper that uses single-edge razor blades does nicely at this task, but use it patiently or you'll snap off the blade. Whatever you do, don't make the classic Beginner's Mistake of spraying any of it with solvent. You'll remove the oil and it'll be as hard as a rock, forcing you to shave it off a few hundredths of an inch at a time. If you make this mistake, you'll end up going through a box of single edge razor blades by the time you're done. Once you've got all of the old gasket off, clean the metal face of both the differential housing and the sealing flange of the sheet metal cover with good old fashioned rubbing alcohol. Next, check the sealing flange of the cover for distortion. Whenever a leak develops, the common tendency of DPOs is to put a wrench on the cover bolts and tighten them down to three grunt-pounds, crushing the gasket, distorting the cover, and thus worsening the leak. Drip, drip! Use the old Petroleum-Jelly-and-Mirror technique to check the sealing flange of the cover for flatness. If the sealing flange is distorted (and it often is), you can usually flatten it out by placing it on a flat surface and putting a socket open-end-up over the bulge, placing a piece of wood on the socket, and gently striking it with a hammer. Make sure that the socket is the same size as the bulge. Don't hit it too hard or you'll thin out and displace the metal, creating a warp that you won't be able to get out. You'll have to purchase another cover from Victoria British (Part# 5-1030, $69.95) if you do. As always, work slowly and carefully and you'll be fine. Hopefully your cover won't be distorted, but don't count on it.
By all means, replace those nasty old nuts and bolts. A "torque reading" is really just a measurement of friction between threads. You really can't get a worthwhile reading if the threads are dirty, rusty, or deformed. Personally, I like to use stainless steel nuts and bolts on the underside of the car. I get them at a hardware store because the quality is higher and the price is lower than at an auto parts store. Be sure that all of the bolt holes on the mounting face of the differential housing are clean. Get one of your kid's old tiny toothbrushes and some carburetor cleaner and clean them all out really well. Do it right the first time and it'll never leak again. I put everything together with antisieze compound on the threads so that if I ever have to take it apart again, it'll spin right off. Don't worry, antisieze compound isn't a lubricant. Once properly torqued down, it won't come loose.
The differential cover gaskets from Moss Motors and Victoria British are about as thick as a piece of typing paper. They're typical junk gaskets. I make my own from the best gasket material that I can find at the local auto parts suppliers. They're not hard to make. All you need is a sharpened pencil to trace around the outside of the cover and to draw the circles for the bolt holes and a cheap extendable razor knife cutting tool. You can pick up one of these at any Home Depot type store for very little money. If you consider that to be too much hassle, you might try getting a new gasket from Brit Tek as theirs are of decent quality, but not near as good as you can make yourself. When you total up shipping and parts cost, you're no better off financially than doing it yourself. Just tell the guy behind the counter what you're going to use the gasket material for and tell him that it needs to be thick (a little compressibility is always good for getting a better seal).
Using the new gasket and sealer, smear a thin layer of Permatex onto the outer edge of the gasket to glue it into place on the differential housing. Apply the Permatex only to the outer half of the gasket. Why? When you torque the cover down you don't want the Permatex to ooze into the inside of the differential housing where it can break free and damage the internals. Next, apply the gasket and again smear Permatex onto the exposed outer half of the gasket, replace the cover and torque the bolts in an alternating pattern a few ft-lbs at a time to no more than 14 ft-lbs. Any more than 14 ft-lbs will deform the cover and it will leak. Be sure that you've cleaned all of the threads or you'll get a false torque reading. When you replace the axle cover, remember that the brake line clips at the top and handbrake pivot on the left.
Refill the rear axle with EP90 hypoid gear oil until it drips out of the filler hole, replace the road wheels, earth (ground) lead, and then lower the car. Once the car is on its wheels the hub nut can be fully torqued up to 150 ft-lbs and a locking split (cotter) pin bent into place.
Clunking from the axle should now be much reduced, or barely audible. However, if there is no improvement, then providing you have checked the hubs, wheels, and the driveshaft universal joints, you may need to consider a replacement axle.
You might think that painting the axle while it's on the car is the easy way out, but you'll find that it's a pretty miserable experience. Nobody ever tries it again once they've done it before. Do it the easy way. Take the axle off to clean it, be sure to plug all of the holes so the POR-15 won't get in (once it's in, it's there to stay), and paint it in the light so that you can do a proper job.

Steve S.

My God Steve, did you type all that, or was it a cut-and-paste job? Whatever, you are to be saluted. Very comprehensive.

Mike
Mike Howlett

What do you do in your spare time Steve?....LOL....great post
Pete
P J KELLY

Steve, thanks for the detailed explanition. I plan on doing this job this coming Saturday, only I'm going to have the rear axle assembly out of the car. I have John Twist's procedures, but I believe yours are more detailed.

Thanks again,

Ken
Ken Waringa

Steve,many thanks for the responce.Wether you pasted this info or typed it out,the deatail and simplicity is really great. It is guys like you that these threads so important and succesful.Neville is at the 'salon'(body shop) for a few months ,but when he returns I will attack the rear end and armed with this info ,it should be a snap.
Cheers Jon
Jon Rosenthall

Steve- Clearly prescient by birth you, I just took on the much dreaded "MGB REAR END MINI-REBUILD" this weekend cause I'm on a roll after a really satisfying finish on the complete front end rebuild on the 1980 last week. And, this morning, I was about to order odds and ends via Moss, and found Dr. SteveS tome.
Too much and I mean that in a wholely good way.
If anyone was a scosh unclear from the quik-over in the Haynes Restoration Manual, they gotta read-read this thread.

One short point: IF you do drift the main pin too far and lock the carrier ( guess how I know this), you can use your drive punch through the split pin hole in the main pin to wiggle it back down and release the carrier. Even with this side road, start to finish, without the paintwork it was 1/2 day. And yep, the old washers were worn thin, and the clunk is outta there. I used REDLINE 75/250 rated Shockproof GearOil for the lube.
vem myers

I have been working on an axle overhaul and have a comment to make about half shaft removal.

As recommended by Steve and others who have written on the subject, I put the flange back on secured by the nut and hit the back of it. In fact I kapt on hitting the back of it harder and harder until my bit of wood splintered. So I stopped for a rest and looked at the problem.

I then loosened the nut and flange so that the nut was only on a few turns. This enabled a pair of 1/2 inch drive sockets to be nipped between the flange and the bearing housing. Tightening up the nut gradually releases the bearing from the housing.

This seemed so much better that if I ever have to do the job again I will use this method first and leave the hammer in the tool box.

David
David Witham

Steve and all,

It appears from this thread and prior responses from Steve that we have a true "Mechanic for the people".

Really, really good info Steve and in layman's terms for us week-end mechanics.

Jon's a very good freind of mine and I'm sure we'll be talking about this today.

Now when are you going to publish this information in softcover Steve?

Well Done, Dino
Dino

Great Job Steve! And just think, you didn't insult anyone! How refreshing!
You really should consider publishing your knowledge.
William

Great overview Steve. I have one question though. What size thrust washer does one use, and how to know if the correct one is installed? I replaced the whole axle this summer cause i was intimidated by the job. Thanks Jay
JLG Galbreath

Jay-
I hate to tell you this, but the thrust washers only come in one size for each application. It's the distance collars behind the differential bearings that come in multiple sizes, but they should be able to be reused.
Steve S.

I have just completed this exercise and can offer a further tip. I found that the (near-side) half shaft only has to be eased out a short way to allow the diff. gears to be removed from their splines. I used 2 back plate bolts plus nuts/washers between 2 diagonal bearing housing holes with the bolt heads pressing outwards onto the temporarily refitted hub + nut. Tightening the nuts evenly against the axle housing with a washer in the way pushes the bearing about 2/3rd from the housing (no need to take it all the way out) until the clearance inside the diff. is obtained. It is an easy matter to tap the bearing back home once the diff thrusts have been replaced.
n macpherson

Good deal on the updates I'll forward this to a buddy that has this project ahead of him.
JLG Galbreath

In addition to the above information see the following for some photos and notes on declunking an MGB differential. FWIW, Clifton
http://chicagolandmgclub.com/photos/b_clunk/
http://www.mgdriversclub.com/tech/rear.axle.clunk.html
Clifton Gordon

Clifton- Great contribution. We are all going a long way in dispelling the myths here ( see GLG above). One more small note-SteveS recommended blocking the master with a vac top of cling wrap, which worked somewhat. After I removed the brake line from the wheel cylinder, I pushed the thread bush back and slipped a vac plug over the end. That stopped it altogether. On the other side, I had no more vac plugs in the right diameter, so I cut off the thumb on a used set of nitrile disposables, and with a rubber band wrap around the end, fully stopped off the other side.
vem myers

Clifton- Great contribution. We are all going a long way in dispelling the myths here ( see GLG above). One more small note-SteveS recommended blocking the master with a vac top of cling wrap, which worked somewhat. After I removed the brake line from the wheel cylinder, I pushed the thread bush back and slipped a vac plug over the end. That stopped it altogether. On the other side, I had no more vac plugs in the right diameter, so I cut off the thumb on a used set of nitrile disposables, and with a rubber band wrap around the end, fully stopped off the other side line.
vem myers

opps
vem myers

I have replaced the bushing and the spider gears. My differential is with proper specs. However, I still have the "whine". The whine occurs in fourth gear and sometimes in third gear going about 30 mph with moderate rpms. It sounds like truck tranny. When accelerate just a little, the whine goes away. I think that the "whine" is normal, any one notice the "whine"? Am I whining about the whine???
Steve Slett

Steve-
The whine is always present, but it shouldn't be obtrusive or annoying. If it is, then the pinion gear and the crownwheel gears need to be properly spaced (see above).
Steve S.

"Vac plug", "nitrile disposables"? Don't have either one of those high tech pieces of equipment in my shop (really I just don't know what they are). Anyway,I use a 1" long piece of rubber hose with a screw stuck in one end to plug it off. Just push the other end over the brake line that's disconected. Just thought I would mention that as another way that may help someone out FWIW.
Paul
paul S. Canup

Paul- Same idea, perhaps ezier
vem myers

This thread was discussed between 29/11/2003 and 07/12/2003

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