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MG MGB Technical - MGB GT Diff

Hi everyone.
I'm restauring a '67 MGB GT and everything have been taken apart - including the diff as seen in the attached pictures. I've purchased new washers for all 4 cogwheels, 2 fiberwashers and 2 copperwashers.
However the washers I've removed from the diff are all copper. So my question is, should I use the fiberwashers I've received or should I go for copper for all 4 washers?

Thanks in advance and a merry Christmas to everyone in here
Rgds
Torben




Torben Olesen

There should be copper (smaller) behind the pinions and fibre against the half-shaft gears, but the fibre can wear away to nothing. I've not heard of the larger ones being copper.
paulh4

"the fibre can wear away to nothing." Too true, and on mine the copper washers had worn down thin enough to be used as a razor blade.
Mike Howlett

Hi again

Understand that the two smaller ones are copper and kind of cup shaped, and that the bigger ones are supposed to be fiberwashers.
I've seen these fiberwashers being worn as thin as you both describe - even complete gone, and when looking in the exploded drawing in the Moss catalogue I find fiberwasers only (3 versions of different thickness). No copperwashers are shown i their drawing.
Beleive I'll go for the washers and simply forget about the copper ones.
I just wanted to be sure.

rgds
Torben
Torben Olesen

Torben
Maybe someone got sick of continually replacing the fibre washers and made up / found a suitable thickness set of metal ones -- If they're the correct thickness giving a good fit then I'd be using them again-
Most rear axles now a days use a hardened steel washer in that position as well as under the planetries

willy
William Revit

Copper washers were used in early B series banjo diffs. The banjo washers are the same size and part no as the tube axle diffs.
I've never worked out when the change to tufnell occurred.
Paul Walbran

Maybe they didn't change as such, Moss for example show three washers for both banjo and Salisbury as Torben wrote - but only one looks like fibre with the two over-sizes (with suffixes to the standard part number) looking more like copper at +5 and +13. Maybe a case of selective fit depending on how other parts are worn.

This https://www.youtube.com/shorts/Ek3bI6vKpnY is a 'good' example of wear with the top spider gear moving up and down as the half-shaft change direction.
paulh4

Both the original copper washers and the original fibre washers are 35 thou. The oversizes arent listed on the Austin Rover microfische, so presumably they are an aftermarket item aimed at compensating wear rather than manufacturer's variation. They are brass-ish in appearance rather than copper.
Paul Walbran

Torben-
Why are fiber thrust washer shims utilized instead of the more-common bronze-alloy-faced thrust washer shims? First, because they are oil-absorbent so that when placed under extreme pressure, they ooze hypoid gear oil in order to provide additional lubricant onto the heavily loaded running surfaces. Secondly, because, unfortunately, some owners would ignorantly use an EP90 GL-5 hypoid gear oil that had oil additives blended into it in it that would corrode bronze alloy, therefore fiber thrust washer shims were specified as a protection against such an unfortunate mistake on the owner’s part. I also suspect that the retention of the use of these quaint fiber thrust washer shims was a holdover from the days when lubricating oils were inferior to what is commonly available today. In addition, the cork gaskets that were in common use way back then would become sodden, and slow leakage led to differentials being operated with low oil levels as a result of a lack of vigilant maintenance. Way back when they were first introduced, they were seen as an engineering "insurance policy" to protect the running surfaces. The only time that they encounter excessive stress is when one wheel is spinning while the other wheel is at rest. Consequently, excessive use of the throttle when attempting to get the car unstuck from ice or mud will cause these parts to wear rapidly. A differential that is not exposed to these abnormal operating conditions should see its fiber thrust washer shims endure almost indefinitely.

After the top and bottom pinion gears have been removed, the other two gears can be removed one at a time, their worn fiber thrust washer shims removed and replaced using new fiber thrust washer shims, and then the top and bottom pinion gears can be re-installed. Note that the Original Equipment fiber thrust washer shim (BMC Part # ATB 7072) is of 0.034" thickness, and two oversize thickness fiber thrust washer shims are available (BMC Part # ATB 7072/40, +0.005" oversize, i.e., 0.039"-thickness), and (BMC Part # ATB 7072/48, +0.013" oversize, i.e., 0.047"-thickness), in order to make up for the years of wear on the gears. These are available from Brown & Gammons in the UK. Brown & Gammons Part # ATB7072040 is +0.005" oversize, i.e., 0.039"-thickness; Brown & Gammons Part # ATB7072048 is +0.013" oversize. i.e., 0.047"-thickness. Note that these are bronze thrust washer shims. Brown & Gammons has a website at https://www.ukmgparts.com. Measure the clearance gap between the output/side gears and the differential carrier by means of the use of flat-blade feeler gauges. Add the new fiber thrust washer shims to both sides until the clearance gap on each on the side of each axle is within a tolerance range of from 0.002" to 0.004" (0.051mm to 0.102mm).

The top and bottom pinion gears now need to be re-installed. Should they prove to be severely worn, the easiest way to accomplish this is to rotate the differential carrier until a hand can be inserted on either side of the differential carrier. Next, place the two pinion gears opposite each other, and then hold them in place and have an assistant slowly rotate the differential carrier again. You are aiming to be in a position to look down into the hole where the pinion pin secures the gears in place and see all of the way through. If you are a tooth out with the alignment, then one of the pinion gears will not align. Once the pinion gears are in their correct positions, slide the new metal thrust washers into place between the differential carrier and the pinion gears. Note that when installing a thrust washer, it must be installed with the oiling grooves oriented towards the rotating surface that bears against it. Once both of the pinion gears and their thrust washers are positioned correctly, drift the pinion pin back into position. Gently clamp the new roll pin in a bench vise, and then open its Inside Diameter to 7/64" by means of the use drill bit. This is necessary as the ends are crimped slightly as a result of the pin being cut to length. If a bench vise is not available, then hold the new roll pin vertically in Vise-Grip/Mole Grip pliers and drill downward onto a block of soft wood. Secure the assembly in place by means of the new roll pin. Insert a 7/64" x 2"-long cotter pin/split pin through the roll pin in order to ensure that it will not come out should fracturing occur. In addition, this extra thickness dramatically increases the shear strength of the roll pin, as well as assuring that the pinion pin will be prevented from rotating and breaking loose.

The halfshaft/quartershaft can now be felt back into position. When replacing the spacer, be aware that the spacer has both a concave face and a flat face. The concave face abuts against the shoulder on the halfshaft/quartershaft, and the bearing abuts against 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 having a very tight external radius between its face and its Inside Diameter, and the shoulder is machined having a larger internal radius. The concave face on the spacer has an even greater radius and this allows the faces on the shaft, the spacer, and the bearing to all abut together. If the spacer were not employed, then these radii would interfere, and the face of the bearing would not be against the shoulder. Making sure that the mating surfaces are thoroughly clean, install the end cap of the axle and the rear brake assembly backplate. Use the four SAE Grade 5 bolts to pull the whole assembly together slowly by tightening opposite SAE Grade 5 bolts a small amount at a time. Next, replace the hub and the machine grade castellated nut, followed by the brake drum, which needs to be secured by means of either the four machine grade stiff nuts in the case of wire wheels, or the two SAE Grade 5 countersunk retaining screws (BMC Part # CMZ 410, Moss Motors Part # 323-255) in the case of steel alloy disc wheels.
Stephen Strange

The final paragraph relates to the hub bearings, if the bearing spacer is fitted incorrectly you get a gap as in image 1. Correctly assembled there is no gap as image 2. If the bearing is not fully pressed onto the shaft you can get a gap either there or between the spacer and the bearing.




paulh4

This thread was discussed between 10/12/2022 and 15/12/2022

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