How do you replace main bearing on 350?

Answer 1

Unfortunately, there is no simple way to replace main bearings.

The main bearings support the crankshaft as the engine is running. Technically, the crankshaft is never supposed to be in contact with the bearing. As the engine runs it drives the oil pump which moves a relatively small amount of oil and is supposed to maintain about 30 to 60 PSI of oil pressure. The crankshaft "floats" on a film of pressurized oil and the crankshaft and bearings SHOULD never wear out if the oil pressure is maintained and if the oil is changed according to schedule.

If/when the main bearing wears out the gap between the bearing and the crankshaft becomes great enough that a relatively large volume of pressurized oil leaks past the bearing and the pump will be unable to maintain the necessary pressure.

So...

To replace the main bearings you need to remove the crankshaft, and typically that means to pull the engine. While it is sometimes possible to pull the crankshaft while the engine is in place, usually the problems end up being far worse than just pulling the engine, and the potential for damaging other components is quite high.

After the crankshaft is removed, examine the bearing surfaces. You'll find that worn bearings will be a different color than good bearings. As the babbitt surface wears through (it's only a couple thousandths of an inch thick) you get down to the copper (typically) surface.

Any worn bearings, rod or main, should be replaced, and if you're replacing any rod bearings, replace them all; same with mains... but don't be in too big of a hurry just yet. Next, take the crankshaft to a machine shop and have them use a micrometer on each "journal" to make sure they are all "round" and within specifications. Journals that are worn can either be too small or worn out of round. In either case you need to make sure the surfaces are within specifications before re-assembling the engine.

While the engine is apart, make sure you check the head for worn valves, and check the cylinders for wear. There's no point in reassembling a worn engine.

Now for a little unsolicited advice: if the main bearings are worn you can often get many thousands of additional miles out of an engine by just switching to a heavier grade (thicker) oil. If you've been using 5W15, change to 10W30 or even 20W50. That single step can get you out of an expensive rebuild and keep the engine running for a long time.