|The MGA With An Attitude
Inspecting a ROLLING ELEMENT BEARING -- UT-107
Ah, this is one of my favorite lessons, figuring out what parts you can save so you can keep some of your money. If you're going to inspect a ball bearing or a roller bearing, it helps to have it out of assembly and in your hand, but it's not absolutely necessary. The most important thing is to have it clean and dry so you can do a visual check of the balls or rollers and the raceways. So wash out the grease and oil with some solvent. Mineral spirits is good for this, or almost any other solvent, as long as there's no plastic parts involved. Give the bearing a good scrubbing with a stiff brush if necessary, shake it out and give it a blow dry with compressed air. Capital rule here. Do not ever spin a ball bearing with compressed air. Internal clearance between the balls and races is minimal, so if you should accidentally get a hair or a grain of sand inside it may lock up instantly. And if you happen to have it over your finger when that happens, you may spend the next couple of minutes looking for your lost finger.
Switch on a nice bright light, or take the part out into direct sunlight, and get your reading glasses if you need them. Hold the bearing at an angle so you can see inside the raceways. Turn the races slowly while looking at the balls and running tracks of the ball grooves, or the rollers and conical tracks of a tapered roller bearing. All raceways and rolling elements inside of a ball or roller bearing should have a nice bright mirror finish. If you can see any roughness or pitting or any marks at all on these parts, it's good for the nearest trash bin. But just because a bearing looks good to the naked eye doesn't mean it is. Hold the bearing with the outer race in one and the inner race with your other hand. Get more creative if you don't have two hands. Apply a moderate thrust load with your fingers, rotate the bearing race about half a turn at a time twisting it back and forth while you hold it near your ear for a listen. If it sounds like gravel rolling across cement, or it feels like it has sand in it, the trash can is still right there. If it looks, sounds and feels good, then put a few drops of oil in it and try it again. With a touch of oil in the bearing it should be perfectly quiet when turned with a little load on it. If you get that far and everything is smooth as glass, then it's probably good to oil up and put back in service. But there are a couple of exceptions.
It is possible for a ball or roller bearing to be clean and smooth and quiet and still be badly worn. For a ball bearing you should hold the inner race secure and try to wiggle the outer race sideways at one edge. Keeping in mind that most ball bearings will have some small internal clearance, the tiniest amount of free play is not criminal. But if you can think about how thick a couple pieces of paper are, and the outer race will wobble sideways more than that, then it's probably trash. A similar test for a tapered roller bearing would be to hold the parts together with a little thrust load, hold one race stationary, and try to rock the other race sideways a bit. Here you should get absolutely no relative motion, so if it allows any rocking motion at all there is something desperately wrong with it.
There is a dimensional check you can do with a tapered roller bearing if you happen to have a new one in hand along with the old one. With tapered roller bearings the thrust load side inner race will usually extend out beyond the side of the outer race. You can use a caliper to measure the overall width of the bearing assembly, in which case the old bearing should be the same width as the new one within a few thousandths of an inch. If the old bearing is narrower overall than the new one, that's a good indication that at least one of the roller races has been worn down that much, and again it's likely to be trash even if it does pass muster otherwise. This is particularly important for the front wheel bearings in a MGA Twin Cam or Deluxe, or any MGB. If the tapered roller bearing is worn to the point of being too narrow, then you might take all of the shims out of the hub and the bearings might still be loose in assembly. When you can no longer shim the bearings to the specified end float requirement, then they definitely have to be replaced.
There's one more test I like to do with ball or roller bearings, perhaps more out of curiosity than necessity. This is to listen to it with a mechanic's stethoscope while it's turning under load. A mechanics stethoscope is a fairly cheap tool, currently listed for $5.99-USD in the J.C.Whitney catalog (Sept 03), and it's nice to have around for a variety of other listening tests. Setting up on a solid flat working surface, lay the bearing on its side and support the outer race only with three steel hex nuts or flat washers around the edges. On top place a fitting that will rest on the inner race only. For a tapered roller bearing almost anything flat may work. For a flush sided ball bearing, maybe place a big hex nut or a wrench socket on the inner bearing race. Then you need to center and balance something with a little weight on top of that, like maybe a paint can in one quart to one gallon size. Then you carefully spin the assembly on your Rube Goldberg turntable, rotating it slowly, one turn per second should suffice nicely. While it's turning you can listen to the sound it makes with the stethoscope placed anywhere against the outer bearing race.
The stethoscope is quite sensitive, so at least expect to hear a little hissing sound, and all is well. If you hear a little cyclical clicking noise like a tiny horse galloping along, then you're probably noticing a flat spot or two on the rollers (commonly called square rollers). This condition can be caused by hammering too hard on the inner race while removing the bearing from the housing. If you hear a rumbling noise or even just little louder hissing noise, especially if it's somewhat irregular, then you might be tuning in on the earliest stages of the beginning of the end for that bearing. For a comparison you can test a new bearing too, and compare the old to the new. A new bearing should be very quiet. If the old bearing is significantly less quiet than the new one, then you make your judgment call to decide if the old one is bad enough to toss out. For ball bearings you can flip the bearing over and test it again with the thrust load in the opposite direction, which will be checking a different set of load bearing surfaces in the raceways.
That's about it. If it looks feels and sounds like a good bearing, it probably is. Then if you keep it oiled up nice and keep the dirt out, it may well last indefinitely. As far as I know, when my MGA had over 250,000 miles on the ODO it was still running all of the original factory issue wheel bearings. I changed the front wheel bearings about that time just because some SCCA tech inspector wasn't convinced that the front wheels of an MGA were supposed to have a small bit of wobble. The new bearings did eliminate most of the wheel wiggle at first, but within a few weeks and about 1000 miles the new bearings had exactly the same amount of wiggle as the old ones had before the change, and that amount of clearance hasn't changed in another 80,000+ miles. And at 334,000 miles the car is still running nicely on the original issue rear wheel bearings. Now I almost wish I hadn't changed the front ones. Sometimes I'm curious to know just how far some of these original parts can run. Heck, the car is already 45 years old, so I suppose a lot of these parts will outlive me.