General rule for ball and roller bearings is that the rotating race of the bearing should be a press fit, and the non-rotating race should be a slip fit. For front hubs you should need a press or a hammer and punch to R&R the outer bearing race in the hub, but the inner race should go on or off the spindle with hand force, or at least no more than a light hammer tap. For rear hubs on the MGA and early MGB where the outer race rotates, the outer race should be a tight fit in the hub, and the inner race should be a slip fit on the outside of the rear axle housing. For later MGB where the rear wheel bearing is mounted on the rotating half shaft, the inner bearing race needs to be a tight press fit on the shaft, and the outer bearing race can be a slip fit in the stationary housing. Ball bearings on the mainshaft in the gearbox are mounted on a rotating shaft, so those should all be either a tight press fit on the shaft (or captured with a jam nut) and slip fit in the housing.

In the special case where you might have a ball bearing in an orbital application (non of these in the MG), both inner and outer bearing races would have to be a tight press fit.

The determining factor for requiring a press fit for the bearing race is the direction of the working load. When the load on a bearing race is taken in a single constant direction, the bearing race will remain stationary by virtue of friction on the mounting surface, and a slip fit will suffice. When the load on a bearing race is constantly changing direction, the race will need to be a press fit to keep it from walking or orbiting, which would eventually wear out the mounting surface.

Occasionally you may run across a case where the normally press fit bearing race is loose in a rotating housing. This may be the result of too may cycles of remove and replace using a hand punch, and being too aggressive at rocking the bearing race out of plane in the process. This can take tiny bites to remove slivers of metal from the housing, so after a while the housing may no longer provide a tight fit. For the MG front wheel hubs, and MGA rear hubs, and early MGB rear hubs, when the outer bearing race becomes loose in the hub that hub is usually trash. The process required to rebuild the metal surface and re-machine the bearing mount will be prohibitively expensive, so we usually just toss the part and find another good used one.

In a pinch, when a replacement part is not immediately available, and if the bearing is not too sloppy in the housing, then the bearing may be set back into place using bearing set adhesive. There are various grades of this stuff that can fill a gap up to as much as 0.015" and still provide structural support for the bearing race. See Loctite 620 Retaining Compound. Available from local auto parts stores, Amazon.com, Sears, and sometimes Walmart. These are anaerobic adhesives, meaning that they cure in the absence of air. This may seem like a rather strange concept for a glue, but it's true. The stuff is packaged in a bottle which is left half full of air to preserve the product during storage. When it is applied to a bearing mount the adhesive is trapped between two close fitting surfaces to exclude the air, and the adhesive then sets up very solid and strong. This may require about 20 minutes for initial set, and 24 hours for full cure. For a structural application like a wheel bearing you should not apply any load to the part until the adhesive has had time for full cure.

Ball bearings by their nature will have a certain amount of internal clearance to allow for thermal expansion. It is possible to preload a ball bearing by applying an axial load, but that is seldom done. In fact most automotive applications of ball or roller bearings will allow for a small amount of internal running clearance. All MGA wheel bearings are ball bearings, except the front hubs on the Twin Cam and Deluxe models. MGB uses ball bearings in the rear hubs. MGA Twin Cam and Deluxe and the MGB use tapered roller bearings for the front hubs. The MG rear axle applications use a single large ball bearing in each hub. MGA front hubs (not Twin Cam or Deluxe) use two ball bearings with equal length inner and outer race bearing spacers. These will not require any adjustment at assembly, just put it together and torque up the retaining nut. For MGs using tapered roller bearings in the front hubs, there are also both inner and outer bearing race spacers. These require adjustment with shims to achieve a small controlled amount of internal running clearance (see workshop manual).

Also be aware that the inner race spacer is a structural part of the front bearing spindle assembly. If you were to leave the inner race spacer out and adjust tapered roller bearing clearance with a loose axle nut a cotter pin, then you have reduced the overall strength of the bearing spindle and are at some risk of having a bent spindle next time you happen to hit a large pothole or rough rail crossing. For the ball bearing front hubs the missing spacer is even worse. With the spacer in place both ball bearings will equally share the axial load when you are taking a corner at speed. With the spacer removed one bearing will have to take all of the axial load, effectively doubling the load on that one bearing, which could lead to premature bearing failure.

The assembly of a ball bearing will typically include inner and outer bearing races, a number of load carrying balls, and a cage or spacer element. During assembly of the bearing the inner race is placed off center, and the balls are loaded all into one side of the bearing, not to exceed a space of 180 degrees around the outer bearing race. Then the inner bearing race is relocated to be concentric with the outer race, the balls are repositioned to be equally spaced around the bearing, and the retainer or cage is installed to keep the balls equally spaced. The cage is typically two pieces of stamped steel held together with rivets, although some newer bearings may have a plastic ball retainer which is assembled with rivets or chemical bonding or thermal welding of the plastic. All of these cases represent a partial fill ball bearing where there are spaces between the balls.

It is possible to fabricate a full compliment ball bearing with enough balls to completely fill the circumference, but this requires a different assembly technique. For these there will be a loading slot in the edge of the bearing races where the balls are inserted from the side. Because the side loading slot represents a physical interruption to the otherwise continuous groove of the bearing race, these bearings cannot tolerate much of an axial load. The full compliment ball bearing may be used in applications requiring maximum radial load capacity within the space allowed, but they are not suitable for use as automotive wheel bearings which are subject to considerable side loading during cornering.

Except for the side loading design, most ball bearings will accept a significant amount of axial loading. Looking at a cross section of a ball bearing you would see a ball surrounded by the two semicircular bearing races. The groove in the races will encompass less than 120 degrees of the circumference of the ball on each side (and sometimes much less). You can draw an "X" through the center of the ball, and the points where the "X" intersects the bearing races would be the contact points where the balls can transmit load through the races. Depth of the groove in the raceway determines the angle made by the "X", and therefore determines how much thrust load the bearing can take. A deeper groove allows more thrust capacity. Bearings with a shallow groove are commonly called radial bearings. Bearings with deeper grooves are commonly called angular contact bearings. Sometimes the bearing races are made asymmetrical so the "X" is skewed, and the bearing will be able to take a much higher axial load in one direction. These will inevitably be called thrust bearings. Ball bearings used pretty much anywhere on the MG will be symmetrical and will have as deep a groove as possible in the raceways. These are bilateral or "unbiased" angular contact ball bearings, and they do have the capacity to take quite a high axial load in either direction. These make wonderful wheel bearings, capable of carrying both radial and axial loading.

When you need new ball or roller bearings (or oil seals), you can usually procure these from any bearing supply house. If you have the old parts in your hand you can take them to the customer counter or call on the phone and give the manufacture's name and part number you find stamped on the side of the outer bearing race. The service person should be able to cross reference this information to parts made by several different bearing manufacturers, even if the part you have in hand was made by a company long out of business. Failing that, all they need is three precise dimensions for ID, OD and width, and they can look up a part number according to the size. You may then have your choice of a few different parts at various prices. The MGs were built in a rather conservative manner, meaning that the bolts and bearings were generally somewhat oversize and nowhere near the stress limit. And most any bearing built will be of pretty good quality. As such, it doesn't make much difference which brand name you end up with, so you might as well buy whichever one is cheaper, or whatever might be on the shelf close at hand. If they don't have what you need in stock they can probably get it in a few hours of within 24 hours with the next delivery from a local warehouse. One thing to watch out for would be a bearing with fewer balls in the cage, which may have a lesser load capacity than the one you are replacing.

Whatever is available immediately may have seals or a shield on one side or both sides of the bearing. Sealed bearings are not required anywhere an the MG. Open bearings get either grease and periodic service, or they are bathed in oil in operation. Having seals might only mean that you can't clean and repack the bearing, and that might be about the same as never doing the periodic maintenance. When the sealed bearing gets old, it might just fail and have to be replaced because it couldn't be serviced. Shields are not seals. Shields are intended to keep large particles of trash out of a bearing, but they still allow passage of oil. This allows a shielded bearing to operate well in an oil bath, but it effectively inhibits cleaning and inspection and re-packing with grease. So a shielded bearing may be okay in the rear wheel hub running in gear lube, but not so good in the front hub where it needs to be packed with grease. Shields can usually be removed from a ball bearing just by prying them out with a screwdriver. Seals may be a little tougher to remove. If you find a ball bearing with a plastic ball retainer, you might consider leaving it alone, because some of those have a shield or seal that may be an integral part of the cage, so you can't remove the shield without destroying the cage.