Originally Posted by
catalunya 172
I'm abit unsure on this subject
i think when you do a fresh engine build you have plugs out and crank sensor disconnected and fill oil filter and oil pump before then crank till you get oil pressure
No. On a fresh build, pre-filling the filter is often an outright bad thing. If the oil pump isn't well primed already (or filled with assembly lube) the oil in the filter will trap air beneath it and often prevent the system priming. You'd be better off fitting a dry filter and then cranking, or using a large syringe to force-prime the system through the oil pressure sender fitting while slowly turning the crank backwards.
but for engines that have done a few miles just fill oil filter and start as normal because there would be oil coating bearings from old oil and there would also be oil in the oil pump Chambers which when you turn over engine will supply the bearings
This is a fundamental error. It simply isn't sufficient to have "a coating" of oil in bearings that are trying to withstand significant combustion forces. All that will happen under those circumstances are that the bearing surfaces will be forced together, literally pushing the oil molecules out of the way. You need consistent oil pressure to maintain the integrity of the bearing.
i also think that just disconnecting the crank sensor is not good enough if people say this must be done as you will still get compression in the cylinders which will put the stress on bearings so to me the plugs would have to be removed
You will still get compression in the cylinders, but the force generated by this is a tiny fraction of that exerted by combustion. As you say removing the plugs is the best case scenario but this doesn't appear to be a pivotal issue.