Tractor Repair: 8n oil pressure, low oil pressure, main bearings

Question
I have an 8n that starts off with 25 lbs of pressure, but as soon as it warms up, it drops to zero.  We have changed the main bearings and the oil pump, but still does it. too high of a throttle, and you can hear the bearings rattle.   any ideas as to why this could happen?

Answer
Hello,

  Could be alot of things, but hard to say what without some more information.  Not sure why the main bearings were changed and not the rod bearings, what condition the crankshaft is in, or if the bearing clearances were checked when the new bearings were installed.  If there was lots of wear on the main bearings when they were replaced, the crankshaft is probably worn too and might need to be ground to the next undersize.  Rod journals can be checked with a micrometer.  The bearing clearances should always be checked with plastigage to make sure they are within spec. before final assembly.  If you can hear bearings rattle, there is WAY too much clearance.  I have seen lots of bearings and bearing caps installed incorrectly, wrong size bearings installed, and bearings with the wrong size stamped on the box they came in.  There is also an oil pressure relief valve on the front of the engine that could cause low oil pressure, but if the rattling noise is actually the bearings and not something else, you will never have good oil pressure no matter how good the oil pump and relief valve are.