Main problem with improper bore alignment is a broken crank shaft. Is this a large marine engine, or a car engine?

If it is big like a marine engine, then you really need to do a bore check on it. A broken crankshaft might cost a million dollars and a year to get a new one.

If this is a car engine, maybe not so critical.

Thanks Steve. Its for engines powering earthmoving equipments in mining industry. Can you tell me the progressive sequences that can lead to Crankshaft breakage. Can misalignment of Main Bearing Bore will lead to Main Bearing seizure also due to rotation of Main Bearing in its bore.

As a cast iron engine works and goes through many heat cycles, if can move and shift. If the holes that are bored in the engine block, combined with the main bearing caps do not line up exactly in a straight line, it will be impossible for the crankshaft to spin freely, because it was manufactured with all of the main bearings in a straight line.

If the first hole for the main bearings is not in line by .005", the crankshaft will have to bend by that dimension constantly every revolution. How long before the added pressure causes that bearing to wear out, or for a crack to develop between the first and second main bearing. Now remember that a six cylinder will most likely have seven main bearings. Any and all of these bearing bores can be misaligned.

The correct way to check for main bearing bore alignment was to have an accurate straight bar that is laid on the main bearing saddles. Then the space between the bore and the straight bar is measured. If the space is larger than what the manufacturer recommends, line boring is required.

This process involves removing a small amount from the main bearing cap mating surface. Then the main bearing hole is re-bored in a straight line until the holes are correct size again, and in a straight line. A word of caution excessive line boring can change the distance between the crank and cam shaft. The machine shop that does the work should be aware of this though.

If you want to check for the main bore alignment yourself, you can lay the crankshaft into the inverted engine block with the new bearings in place. Measure the effort to turn the crankshaft. Now install the bearing caps in place and torque them to specs. Again measure the effort required to turn it now. If the effort gets noticeably harder, the block, needs line boring. You should also inspect the old main bearings. Any unusual wear patterns are another sign that line boring is needed.

Cast body gets very dimentionaly stable after a number of heating cycles but there is a small chance a distortion happening on new blocks because of casting problems or serious overheating, althowgh most engine manufacturers do this stressing before cuting it. The main bearing metals visual condition will give you answer wether is essential or not for your case but it's a chance I wouldn't take. The cost is small comparing to total. And a good reason to discard if distortion noticed and spare yourself money thrown off the window