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Rear End Thrust Washers

Moss Motors
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albeegreen1 Avatar
albeegreen1 Gold Member bob tresch
bordentown, nj, USA   USA
1972 MG MGB MkIII "ALBERT"
Do them correctly and under normal driving conditions you wont be back there for another 50 years. thumbs up

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Steve S. Stephen Strange
Harrisonburg, Virginia, USA   USA
1972 MG MGB MkII "The Mouse Trap"
Just as a point of interest, note that the Original Equipment shim (BMC Part # ATB 7072) is 0.034" thickness, and two oversize thickness shims are available (BMC Part # ATB 7072/40 is +0.005" oversize, i.e., 0.039" thickness; and BMC Part # ATB 7072/48 is +0.013" oversize. i.e., 0.047" thickness). Why are fiber washers utilized instead of the more common bronze washers? Because BMC knew that part of the traditional appeal of their vehicles was that they were designed so that the private owner could do as much of the required maintenance himself. Unfortunately, some owners would ignorantly use a GL-5 differential oil that had additives in it that would corrode bronze washers, therefore fiber washers were specified as a protection against such an unfortunate mistake on the owner’s part.

Refill the rear axle with 0.85 liters (28.74 US Ounces) of EP90 GL-4 hypoid gear oil or until it oozes out of the filler hole, replace the road wheels, the ground (earth) lead, and then lower the car. Make sure that you use EP90 GL-4 hypoid gear oil, and not gear oil. There is a difference and it does matter. The oil is 'EP', meaning Extreme Pressure, because of the hypoid shape of the gear teeth that 'roll' past each rather than on a pin-point thin line. Be warned that EP90 GL-5 oils are unsuitable, as their high sulfur content will attack the bronze metals within the differential. It should be noted that dynometer testing by Peter Burgess has revealed that due to the heavier viscosity of petroleum-based oil, a rear axle that uses a petroleum-based hypoid gear oil can absorb as much as 3 HP more than one that uses a synthetic-based hypoid gear oil.

Usually, the only thing that damages the differential is letting the oil level drop too far. This often happens when the breather (BMC Part # 1H 3364, Moss Motors Part # 267-040) on the top of the tube on the passenger (starboard) side, right above the horizontal bracket, is plugged up with road crud. Air then is trapped inside of the rear axle. When the rear axle reaches its normal operating temperature, causing the air that is trapped inside to expand, the oil seals start to leak as a consequence of the build-up of internal pressure forcing the differential oil outward. As the rear axle cools, air is drawn in through the leaking oil seal, and the process of oil loss begins again when the rear axle warms up again. The process is repeated every time the car is run until the oil is gone, which usually takes a very, very long time. Rarely 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 then replace the drain plug securely.

Cleaning the two-piece rear axle 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. Its lift-off cap covers the two holes in the breather tube in order to reduce the chance of splashing water from getting in, and yet leaves a gap for airflow through the two holes in the side of the threaded breather tube. Just clean around the top of the axle housing with cheap carburetor cleaner so that crud will not get into the threads, unscrew it, and then spray it out with carburetor cleaner. Carefully clean the 1/8"-28 BSP (British Standard Pipe Parallel) threads with an old soft toothbrush, and then reinstall it after it has dried. It should be noted that the MGB GT V8 employs a newer, improved three-piece rear axle breather (BL Part # 21H 6060) for its Salisbury Tube-type rear axle that has an increased reservoir area and larger vent cap, but with a smaller orifice. This improved breather has a greatly reduced tendency to become plugged up. The air path is more convoluted, going up the side of the breather tube, and then through an insert instead of through holes in its side. The insert has a groove around its top, and the cap snaps onto the ridge around the body. This insert has a very small hole in its bottom that in fact does not allow water to run through under its own weight, although any that is lying in there on a hot axle will be sucked through as the axle cools. Consequently, it is common practice to attach a rubber line to the rear axle breather and then run it to a high point inside of the vehicle in order to prevent the ingress of water and dirt.

GlennMGB Avatar
GlennMGB Glenn G
Fort Worth, Texas, USA   USA
1967 MG MGB GT "Rose"
I recently replaced the fiber thrust washers with brass ones sourced from Simon at Colne Classics in England. He offered two thicknesses, .040" and .048". I ordered four of each, but when they arrived I found I had two .040, two .044 and four .048. This was not really important, as the tolerances are not critical and I was just guessing what I might need. Before removing the old thrust washers, I got rough measurements with feeler gauges and ended up putting in one .040 and one .044. This left about .003" of space on either side, not enough to allow clunking, but it should allow sufficient lubrication. I also replaced the dished copper washers.

I was hoping that this would resolve the axle gear noise experienced when cruising at speed with the driveline unloaded, but it didn't. It seems the only way to fix this will be to install a new pinion and crownwheel. Another possibility would be to try resetting the mesh, but the work seems extensive enough to warrant purchasing new parts beforehand. I'd hate to do all that work twice.

I wouldn't be terribly concerned about the effect of using GL-5 oil in an MGB axle. The only brass or copper parts in the axle are the thrust washers. They have a very easy job and will last a long time in GL-5. I ran GL-5 for four years (about 36,000 miles), and the dished copper washers still had plenty of metal on them. The fiber washers fared worse, as one was missing. Still, if you can get GL-4, why not use it? This time I ordered GL-4 from amazon.com.

Viscosity is not determined by the base oil. The viscosity for both natural and synthetic oils is measured using the same system. SAE 80W-90 natural gear oil has the same viscosity as SAE 80W-90 synthetic gear oil. Synthetics are available in a wider range of viscosities, so check the SAE viscosity rating if this is a concern.

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Steve S. Stephen Strange
Harrisonburg, Virginia, USA   USA
1972 MG MGB MkII "The Mouse Trap"
Glenn-
Perhaps I should have been a bit more clear in my statement. Yes, in a High School Auto Mechanics class the instructor always says that the SAE system merely indicates viscosity at cold and hot temperatures. However, the real-world operating difference in viscosity between petroleum-based lubricants and synthetic-based lubricants is at the normal operating temperatures, such as the differential in this case. Synthetics thicken less at low temperatures and thin out less at high temperatures, making classification and comparison with petroleum-based lubricants into a real Alice-in-Wonderland experience that only an engineer can hope to comprehend. For an explanation, here's a useful URL for beginning to acquiire an understanding of the mind-boggling, bewildering world of the SAE Viscosity measurement and its significance- http://www.jcmotors.com/images/understanding_motor_oil_viscosity.pdf You'll note that it addresses only the issue of petroleum-based lubricating oils, many of which differ over their temperature ranges.

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