On September 15, 2011, Paul Hanley in Maryland, USA wrote:
"Is it possible to change the mains while the engine is in the car"?

Yes, all crankshaft bearings can be changed from underneath the car without removing the engine. Raise car and support properly for underneath access, and drain the sump. Remove two bottom bolts from the bellhousing, as these are in the way of dropping the rear end of the oil pan. Remove all sump bolts. The five in front are a bit of a challenge above the front suspension cross member but can be done. See OF-106. You may want to remove the oil pump and sump screen for easier access.

Connecting rod bearing replacement is easy enough. Do one at a time, and always keep bearing caps mated with original con-rods. Rotate crankshaft to bring the rod bolts to bottom. Unbend lock tabs, remove bolts, tap the rod cap on the side to break it loose, and pull it off. The lower half shell may come out with the cap or may stick to oil film on the crank journal. Remove the lower half shell as well. Push the con-rod to move the piston upward in the cylinder. You may want to rotate the crankshaft slightly and move the big end to one side for better access. Remove the upper half shell and install a new one being VERY careful not to trap any dirt behind the shell. Crankshaft bearings have clearance for oil film only about 0.001" thin, so even a human hair behind the shell can cause the bearing to seize on the journal when tightened in place. Oil the bearing and/or journal, and pull the piston down to bring the big end to mate on the journal. Install the new half shell in the bearing cap (keep clean), oil it up, and install the cap on the rod (attention to proper orientation). Install lock tab and bolts or nuts, tighten to specified torque. When tightening the bearing cap fasteners, rotate the crankshaft occasionally to assure that the bearing does not bind on the journal. If it does bind you need to take it apart and do over to assure no dirt behind the bearing shells. You need to be able to turn the crankshaft by hand after the bearing cap is tight. Then bend up the lock tabs, and repeat for all connecting rods.

Bolts or nuts are subject to somewhat variable torque when using lock tabs. Torque is not particularly critical for a standard street engine, so the lock tabs work well. For a finely tuned race engine may people will omit the lock tabs for better accuracy of fastener torque, and use thread locking adhesive.

Main bearings are a bit more of a challenge, particularly pulling the bearing caps. For the front main cap you need to remove the two lowest screws in the timing cover, as these go through into the bearing cap. Also the front main cap is above the suspension cross member, so a slide hammer cannot be used. Here you can screw in a bolt with a flat washer and use a pry bar and block of wood against the crankshaft to pull down the bearing cap. Again the lower half shell may come with the bearing cap or may stick to the crankshaft. Remove the half shell as well. To extract the top half shell, tap on the non-tabbed end of the shell to break it loose and start it rotating around the crankshaft journal. Do not scratch the crank journal while doing this. Then pinch the tabbed end of the shell against the journal, and rotate the crankshaft to pull the shell around. When the half shell is at the bottom it may be removed. Installation of the new half shell is essentially the reverse of removal. Clean the shell first, then oil the bearing side of the shell, place it against the crank journal, and rotate the crankshaft backward to rotate the half shell into position in the block.

The center main bearing also has the crankshaft thrust washers. The top half washers can be rotated out and in similar to the half shells, but should be much easier. The face with white metal and slots goes against the rotating flange of the crankshaft, while the flat steel face of the thrust washer goes against the stationary face of the block and cap. The MGA workshop Manual had an error in the instruction here that was supposed to be corrected in a later edition (but maybe was never fixed). The keyed bottom half washers are to be held in place against sides of the bearing cap while it is being installed.

For front and rear main caps, before installation you may smear a touch of gasket sealer on the side that will mate with the the engine end plate. Install the second half shell (clean please) in the bearing cap, add a touch of oil, press the cap into place, and torque up the fasteners. Rotate the crankshaft occasionally while tightening the fasteners to assure that the bearing does not seize on the journal. If it does seize you get to do it all over again. With bearing caps back in place the front and rear caps need installation of the rectangular cork seal. Press this into the slot. When the cork strip is fully seated, use a razor knife to trim it flush with bottom of the block. Later, before installation of the oil pan, smear a touch of sealer over bottom of the cork strip. Intermediate and rear main bearing caps have adequate access to allow use of a slide hammer for pulling the caps. Still do one at a time, and always keep the bearing caps mated in original locations.

With all bearing caps tightened securely, and crankshaft free to turn, reinstall the oil pump with sump screen. Put a squirt of oil inside the pump to assure it will self-prime. If you are the superstitious type you can pack the oil pump with Vaseline (petroleum jelly). Use with the CORRECT GASKET PLEASE. Then it's time to button it up. Install the two bottom screws in the timing cover. Clean and flatten the flange on the oil pan, then glue a new gasket to the pan with a light smear of your favorite sticky sealer (no sealer required on the top side). Put a touch of sealer on the cork strips at front and rear main bearing caps. Place sump in position on the engine, and install all screws loosely. Snug up the screws gently until you see the sealer squeezing out the edge of the gasket all around. I recommend you stop there and wait patiently for several hours to allow the sealer to set up. Then tighten the screws to specified torque. Finish by reinstalling the two lower bellhousing bolts, and it's a good time to install a new oil filter. Install engine oil and crank the engine with spark plugs out until you see oil pressure on the gauge. Then reinstall the spark plugs, top up oil to the high mark on the dipstick, and fire it up.