A collection of knowledge about the MG Midget rear axle.
- Plugged breather (plastic "mushroom" located at the top of the axle housing, RH of the differential). Pull off the top "cap" and then unscrew the "stem" to clean.
- Bad wheel bearing (moving enough that the seal does not touch properly)
- Worn out hub seal (or dried by age, especially on a car not run for years)
- Hub seal dimensions OD 2.5, ID 1.75, seal lap 0.375".
- Worn housing (if you can feel a groove, install speedy sleeves, part # 99174, to restore the sealing surface.
Before: severe wear of the seal surface
After: speedy sleeve installed, sorry the photo is blurry
- Torn or re-used gasket (half shaft x hub). Min gasket thickness should be 0.010".
- The original O ring that supplements the gasket is 0.115", though a larger dimension could be tried if struggling with leaks.
- Make sure what the leak actually is from: Rear brake cylinders develop leaks (look for fluid around the dust seals), and are cheap to replace ($20). If the liquid is on the brake cylinder and under its dust seals, it is what's leaking. If it is around the hub (not easy for the brake fluid to get there), then the hub seal is leaking. If there is liquid all over everything, then either both are probably leaking, or the half shaft x hub gasket/o-ring are leaking.
- If you have a persistent leak between the axle and its mating surface or at the hub seal, your half shaft might even be bent
- When replacing the bearing, use a 207-FF (double sealed bearing), which then acts as a supplemental barrier against hub seal leak. You can purchase from any foreign auto parts store (also used in Subarus). If using a sealed bearing, be sure to lube the hub seal with some 90W before installing the hub to the axle, because it won't be getting the oil it normally sees.
Hub nut torque (not shown in the manuals): 140 ~ 150 ft-lb. This size of Grade 2 nut, diameter 1.5" - 14 threads/inch x 1/4" thick is physically capable of much more (actually, it is capable of more than 200 ft-lb of torque). Tool size to drive the nut is 1-7/8".
- Remember the LH hub nut has reverse thread (same reason as the RH wire wheel nut, but opposite side of the car, because of the rotation direction effect).
- Don't use a hammer and chisel to remove / install this nut. Get yourself an 1 7/8" socket, and if your nuts are really chewed up, replace them (2A7103 RH, 1G3584 LH).
Broken Half Shaft
- To "spin and snatch" a rear wheel on the shoulder under power can snap it
- Bad wheel bearing (excess play = overloading the shaft while cornering)
- Bent axle housing (have a machine shop confirm true)
- Inboard shaft spline worn by the differential gear's mating spline (if the wear on the shaft is more than +0.010" then it should be replaced. Also, the presence of this wear creates a stress riser at the most highly loaded portion of the shaft/spline)
Photo of worn shaft spline
Photo of unworn portion, for comparison. Note this heavily worn shaft has more than 0.020" of wear in the splines.
This not only reduces the width (strength) of each spline, it also creates a sharp stress riser right at the most highly loaded point on the shaft.
It is no wonder they break.
- Weak material (pre '67 shafts, "ATA" part numbers, used only EN7, but from '67 / 1275cc engine, BMC adopted EN17. Identify later shafts by a groove machined at the end of the differential splines. Steel wheels use BTA806, wire wheels use BTA 807. For reference, BTA 939 and 940 were factory competition shafts.
Photo of EN17 groove (note the groove, about 1/4" from the end of the splines).
This photo also shows well the spline wear mentioned above, just above the "oo" in "groove".
- Half shafts swapped R x L (reversing the loading reverses the twist in the material and could lead to failure).
- Double bearings have been used in an attempt to increase shaft life, especially for racing, but some have had sealing issues with them, and some racers say they still use stock bearings, without ever breaking a shaft. Growler's treatment (below) should be effective to minimize stress in the area where most fail (near the diff).
- Peter May makes EN24 shafts.
- Frontline makes EN40B nitrided shafts. Peter May says EN40B is too hard/brittle, and says "EN24 provides the best balance of strength and toughness".
- Both are about double the cost of new, stock shafts.
- When buying half shafts, or replacing the whole rear end, note that wire wheel axle housings are 1" narrower than steel wheel units, and have accordingly different length half shafts.
- Growlerize the shafts to minimize both the original stress riser where the spline cut ends, and also from the wear caused by the edge of the differential gear mentioned above. Growlerizing (named for the screen name of Grant Bowyer, the Kiwi machinist on the MG Enthusiast BBS / UK who brought it to wider attention) means to cut down the diameter of the shaft to the root diameter of the splines, so the land between the splines stick "out", rather than the splines themselves sticking "in":
- Start the cut 1" from the end of the shaft (so that the very ends of the differential splines are over air, not against the face of the splines), and provide a generous radius at the end of the splines (this photo shows too sharp a transition, a big radius would be better). Continue the root diameter for 4", then gradually taper over 6" back to the original diameter. Deburr and then shot peen (like is done with a connecting rod). Why does this work? The shaft itself is plenty strong enough to handle the engine torque, so reducing its diameter near the diff, although it gives up some torsional stiffness, is offset by the benefit gained by avoiding cracks at those two eliminated stress risers: the sharp corners at the end of the spline cuts, and where the diff splines end.
- Before doing anything with a shaft, have it crack tested first (no sense wasting time with a bum part). At that time, it is also a good idea to have the shaft stress relieved (before machining), for further resistance to possible cracking.
Rear End Noise
- Loud whine/howl at speed = ring and pinion are shot (gear tooth surface worn/damaged). Replace diff with a used one (typically, they last 150k miles with no maintenance, as long as they are never run dry, and never been exposed to broken half shaft fragments).
- Clunk noise:
- Worn U-Joint (there should be no play in either U joint)
- Loose brake back plate (confirm bolts tight and holes not oval)
- Shaft worn at diff (max 0.010" spline wear as shown above). The half shafts seem to be softer than diff gear, so the half shafts wear first.
- Wire wheel spline wear (new splines are squared off on the top, worn splines are pointed). Clean off the grease from both the hub and the rim and then check the play without the nut. Maximum 1/8" play at tire. Replace wheels and hubs at the same time (one new one will quickly wear the other). They are NOT cheap (new hubs and wheels), so don't do it half way. Think of how nice wire wheels look, while you are writing the check. Original hub spline outside diameter 2.450". Note, though, that a fair amount of spline wear can occur before this diameter decreases by much, hence the tire rotation test above, which directly measures the wear as a rotational free play.
- Worn splines encourage extremely tight wheel nut (pounding with a hammer to try and hold the wheel still by friction), which leads to spline bottoming out on the inner taper of the hub. So, spline marks on that taper can be an indirect indicator of worn splines.
Photo of extremely worn splines shown on right, brand new hub on left. Note that the teeth themselves "look OK". The big telltale is the large amount of free play (tire rotation check without the nut as described above) and the spine marks in the inner cone surface from a habitually over-tightened nut from someone trying to quieten down that "click/clunk" noise the worn splines were making.
Half shaft history:
- Frogeye = 2A7085 (? material)
- 1098 era = ATA 501 (EN7 material)
- 1968 ~ = BTA 806 (disc wheel) / -807 (wire wheel), (EN17 material, with Identifying groove on the inboard end)
- Pinion play should be 0.0 fore-aft, and max 1/4" rotational play (if much more than 1/4", diff is probably 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 upper RH surface of the diff housing.
Photo of 3.9 diff ratio stamp ("10/39")
- To replace pinion seal, ideally, you should remove and rebuild the pumpkin properly. But if you are a gambler, you can try to replace it on the car: mark the nut location/flat position of the nut before you start out, 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, which requires disassembly of the differential unit. If you try doing it on the car this way, make sure you also re-install the pinion input shaft on the same spline (otherwise, your mark you made to reinstall the nut won't be in the right place!)
- The proper way to replace this seal is to do it on a bench, and use a new crush spacer. Here is a photo of one good way to hold the pinion when torquing on a bench (by using a few feet of angle iron, some straight stock with some holes drilled and some nuts/bolts:
- The planet and sun gears have thrust washers that wear away over time, and can be replaced, but the ring gear (crown wheel in British speak) must be removed to access the lock pin to service 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 matched set, and the amount of shim offset to the right or left, all engraved on the back of the crown wheel.
- There are three numbers etched into the edge of the ring gear (crown wheel). The first one is the "match set number" that also is etched into the pinion. The second is the backlash (in this case, it is 0.005"). The third is the packing dimension for shimming the crown's cross car spacing to the pinion centerline.
- 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 / 0.99 liters). Run 100 miles and replace that with fresh gear oil.
- The drain/fill plugs have a tapered square hole that can be fitted with the square end of a 1/2" ratchet wrench, but the taper of the hole can make doing this problematic. Hardware stores sell the same size 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, if you want.