Crankshaft bearings is in 5 steps spanning 5 web pages.
Click green arrow at bottom to follow pages in sequence.
    A - Bearing Wear and Oil Pressure - (this page)
    B - Connecting Rod Bearings
    C - Main Bearings
    D - Crankshaft R&R and Regrinding
    E - Bearing Installation

The first sign of bearing wear for the crankshaft may be a reduction of oil pressure. An MG engine in good condition should be able to give 50-60 PSI oil pressure at cruising speed (over 2500 rpm), as much as 80 PSI for later MGB engines, and about 40 PSI at idle with hot oil. General rule of thumb for minimum allowable oil pressure is 10 PSI for each 1000 RPM engine speed up to the relief pressure. This implies it may be okay to run as little as 30-40 PSI at road speed, and 10 PSI at idle. Indeed the engine is not likely to suffer immediate catastrophic failure in that condition, but it is a sign of worn bearings. When I see 40 PSI at highway speed and 20 PSI at idle, I figure it's time to change the crankshaft bearings before it does something nasty (and more expensive).

A knock in a connecting rod bearing sounds like a light tapping noise at idle and a louder clattering noise when you blip the throttle. A main bearing knock sounds more like thumping on the engine block with a heavy hammer when idling, and may actually get quieter at higher speed (with more oil low). Failure of main bearings to the point of hearing a knock is rather rare in these engines, but noisy rod bearings are more common. There are a few other things which may make a light tapping noise at idle speed, such as a worn camshaft thrust plate or a loose rocker arm, but these things generally will not make more noise with increasing speed.

If the engine has developed an audible knock, it most likely will need to have the crankshaft reground. If you have low oil pressure but no audible knock, you may get away with just replacing the bearing shells, which can be done with the engine in the car. But you will not know the condition of the crankshaft for sure until you remove the oil pan and bearing caps for close inspection.

Start by putting the car on jack stands or otherwise lifting it enough to have relatively comfortable access underneath. Then drain the oil and check the article on oil pan removal. When you have the pan off it will be most convenient to remove the oil pump before working on the bearings, and you will want to inspect the oil pump anyway. One way of having low oil pressure without a bearing knock is to have a badly worn oil pump and low oil flow rate. This is one time when a bad oil pump may be good news, and replacing the pump and bearings may restore the oil pressure without removing the crankshaft.