Chicagoland MG Club
Spridget Rear Axle
by Norm Kerr - Ann Arbor, MI
pinion play should be 0.0 fore-aft, and max 1/4" rotational play (if 1/2" or more, diff is toast)
'78 ~ '80 Midgets came with 3.77 ratio (8/'77, from GAN6-200001), better for highway use than the 3.9 used from 12/'68 ~ '77 (but the 3.77s are rare, so more expensive). "58 ~ '68 (948 and 1098cc) used 4.22. The ratio is stamped on the outer edge of the diff housing.
==> photo of diff ratio stamp
to replace pinion seal, mark the nut location/flat position and re-install it with the exact same amount of rotation. Not doing this will mess up the pre-load and require replacement of the crush spacer.
planet and sun gears have thrust washers that wear away over time, and can be replaced, but the ring gear (crown wheel) must be removed to access the lock pin to access them (unlike an MBG diff which can be accessed directly).
trying to judge pinion free play on the car 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 diff on the bench is the accurate way, with a dial indicator. The spec is engraved on 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.
after a half shaft break, or any time the rear end has been apart, it is wise to assure that the insides are surgically clean for maximum diff life: refill the axle with kerosene, with the car up on jack stands, and run the engine in gear for a minute (i.e. no load) and then drain through a strainer. Repeat until the kerosene drains clean (may take several cycles). Fill with 80/90 gear oil (2.1 US pints / 0.99l).
the drain/fill plugs have a tapered square hole that can be fitted with the square end of a ratchet wrench, but the taper of the hole makes this problematic. Hardware stores sell pipe nipples with a normal, hex drive shaped hole to ease future servicing, or you can buy a ratchet extension and grind it down to match the taper of the original plug.
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