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MG MGB Technical - Rear end/differential fluid change

73B Roadster

Thought I'd change the rear end fluid prior to a trip to VT next week. Never done this sort of thing.

I would like to assume there is a drain plug on the bottom or lower back plate somewhere?

What fluid should I ask for at NAPA?

From where and how high do I fill it? I would imagine I could cause some seal damage (34 year old seals I would imagine) if I overfill it.

I know this sort of info is in the factory manual but I find it to be a bit hard to follow at times for the novice.

Thanks,
BH
BH Davis

Before you empty the rear end (there is a drain hole on the bottom), make sure you can get the filler nut off. It is a recessed nut that can be loosened with a ratchet and faces the rear of the car about half way up the unit. I found it to be a hard job to get this off. I used 90 weight gear oil.

Where in Connecticut are you?

Jim
JMcHugh

I made a tool for removing the differential drain and fill plugs fron a 5/8" bolt. I used a bench grinder to make it fit the square hole in the plugs. A cheap 1/2" drive extension also works. The holes in the plug are tapered so the tool need to be tapered to fit.

The drain plug is on the bottom of the houshing and the fill plug in in the rear differential cover. Just fill it to the bottom edge of the fill hole. Caution, it's close to the gas tank so you need a dispenser to fill it. Some people use a had pump that fits the plastic gear oil bottle. I usually use 75w-90 gear oil. Any brand is OK in my opinion. If your rear is a little noisy as mine is a 75-80w-140 gear oil helps. These items should be on the shelf at NAPA and other stores.

Clifton
Clifton Gordon

Thanks!

Jim, I'm in Thompson in the northeast corner, about 20 minutes south of Worcester, MA. From Hartford it's just about an hour.

What about you?
BH
BH Davis

You can get a properly fitting drain plug set fairly readily for a few bucks. 80-90W gear oil here.
Curtis Walker

While your under there, make sure the axle breather is clean. It could save your halfshaft oil seals. Chris.
Chris82

I use a universal tool, it has 4 plug fittings on each end one always does the job and it did not cost much. It needed a good belt with a hammer to shift the plugs the first time but they come out with a good heave now. The EP90 should last for ever, but checking the level is a good idea once a year or so. The plastic breather cover just pulls out and you can shove a bent wire into the hole to ensure it's clear, use one that cant possibly fall in!
Stan Best

BH-
The potentially simple operation of draining the oil out is made more difficult because the British decided to use a ˝” BSP square drive drain plug. This is a rather quaint 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. 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 ˝” 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. 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 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 then is trapped inside the axle, the differential gets hot and causes the air trapped inside to expand, 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 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 axle tube. Just clean around the top of the axle tube with cheap carburettor cleaner so that crud will not get into the threads, unscrew it, and spray it out with carburettor 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.
Steve S.

Steve S,

Thanks a bunch for taking the time to write that out. Very detailed and very much appreciated. I acutally printed your descrition out and will have it with me in the garage this weekend when I do fluid change.

BH
BH Davis

I've used a 1/2" socket drive for 17 years with no problem but this year just could not shift the level plug in the roadster. Like Clifton I made a tool out of basically a bolt, and that works fine. I also reckon a 50mm 1/2" wobble extension would work well, it has to be a short extension because there isn't much room between axle and tank how I do mine, which is backed up on ramps on a downward sloping drive, which makes the car level. I could have bought a tool with umpteen different heads on it, but my toolbox weighs enough and is full enough as it is. One thing to remember with the modified bolt is not to overtighten it as when it comes to undoing it next time the bolt isn't as hard as an extension (or the multi-headed tool for that matter).
Paul Hunt 2

This thread was discussed between 30/07/2007 and 02/08/2007

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