The MGA With An Attitude
Main Bearings

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
    B - Connecting Rod Bearings
    C - Main Bearings - (this page)
    D - Crankshaft R&R and Regrinding
    E - Bearing Installation

If the rod bearing journals pass the fingertip and micrometer inspection, you still need to check the main bearing journals. Start with the center main bearing cap, as that is the easiest for access. Remove nuts and washers, screw a matching bolt into the female threads on the bearing cap, and pull it off with a slide hammer. If you don't have a slide hammer you might get creative, use a bolt with a large flat washer and a heavy pry bar with a wood block for back up against the crankshaft or engine block. The female threads in the main bearing caps are 1/4 BSP (British Straight Pipe threads). These are 0.518" major diameter and 19 threads per inch. A 1/2-18-UNF bolt will screw in with a little clearance and will work well enough for a puller. Been there, done that, and it works okay.

The lower bearing half shell will usually come out with the bearing cap. The lower half of the crankshaft thrust washers will also come out (and likely fall on the floor if you're not ready to catch them). The upper halves of the thrust washers are not keyed and can be rotated around the crankshaft for removal. The upper bearing half shell can be removed with a little finesse, but be careful not to scratch the ground surface of the crankshaft journal in the process. Using a thin flat screwdriver as a punch, tap on the plain non-tabbed end of the bearing shell to break it loose and get it moving a little. Once the tabbed end of the half shell is exposed you can press the shell against the crankshaft journal, then rotate the crankshaft, bringing the bearing shell around with it, until the bearing shell is on the bottom where it can be picked off. The lower half of the main bearing shell will generally have more wear than the top half. Otherwise inspection of the bearing and the crank journal is similar to the rod bearings, visual inspection, finger tip test, and finally measurement of the diameter on the crankshaft. White metal is good, copper is suspect, and any steel showing may mean the crank needs to be reground.

If the center main journal is bad, out comes the engine to remove the crankshaft for regrinding. If the center journal appears to be serviceable, there is a good chance that the front and rear main journals will be okay as well, and all it needs is new bearing shells. Then it's time to re-install the center main bearing and thrust washers and bearing cap before working on the others. With a three main bearing engine, never remove more than one main bearing at a time, keeping the other two in place to maintain proper location of the crankshaft.

Installation of upper bearing half shell is the reverse of removal. Apply oil to the mating surfaces of the crank journal and bearing shell, put the shell in place on the bottom side of the crank journal, then rotate the crank to move the bearing shell around into the block cradle until it is flush with the block at both ends. Thrust washers must be installed with the white metal bearing side against the crankshaft and the steel backing side against the engine block. The factory workshop manual has a print error here when designating which side has the oil relief slot. The slot is in the soft bearing side and goes against the crankshaft. Finish installation with the bearing lower half shell and main bearing cap. The two anti-rotation tabs on teh half shells go together on the same side of the journal to prevent rotation of the shells in both directions.

Removal of the rear main cap is similar to the center one, except that it has a much taller length of engagement with the engine block, so it will require more pulling. You also have to break the paper gasket loose from the engine rear plate (which is usually no big deal). For the 1500 engine you can unscrew the drain tube from the bearing cap, and the female threads are the same as in the other main caps, good for pulling. When installing the rear cap you can forget the paper gasket and smear a bit of gasket sealer on the back side of the cap. Once the cap is fully seated you can squeeze a new square cork seal into place and trim it near flush with the bottom of the block. A bit more sealer is in order later when installing the sump gasket. Note that the bearing cap must be fully seated and flush with the bottom of the block. A small bit of gasket paper trapped between the cap and block would prevent full seating of the cap, which would enlarge the bearing running clearance leading to premature bearing failure. So keep it clean for assembly.

Removal of the front main cap is similar to the rear one, except that you also need to remove the bottom two bolts from the timing cover, as they screw into the bearing cap from the front. This most likely will require removal of the crank pulley, which may prompt you to remove the radiator. By that time it's also a golden opportunity to R&R the timing cover to replace the front seal and maybe also the timing chain tensioner. Access to the front main bearing cap is also hindered by the frame cross member, so you will inevitably need to use the pry bar technique to remove the front main cap. Re-installation is similar to the rear cap, clean out the paper gasket, use a touch of sealant on the front face, seat the cap completely, press in the square cork seal and trim flush.

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