The question was asked: "I hear a lot about running an oil line to the center main, and it sounds like it could be a good idea."
Most of the guys commenting replied to your question before you revealed that you had just installed a Burlington crankshaft and insert bearings. I think they were framing their answers with the thought of Babbitt bearings and the standard oil delivery grooves present with Babbitt bearing material.
Most said it has no benefit because the oil needs to be delivered to the top of the bearing, away from the area of the bearing under extreme pressures. When using insert bearings, I do not agree with their comments and the following is why:
Typically, inserts have a radial groove in their design or installation. (See picture of AER insert main bearings and the Federal Mogul bearings placed in position as typically installed.) In both bearings sets, the only delivery of oil to the bearing surface is from the radial gap or groove. Note that there are no wells at the bearing parting lines to spread the oil across the width of the bearing. With the radial groove, I think/know the oil delivery will be greater to the bearing under pressure from the pump rather than gravity from the valve chamber. Resistance in the oil passage creates some line pressure as many have discovered when installing an oil pressure gauge. Supplying oil to the dynamic areas of a bearing, center main, where it can be drawn into the bearing at 3-10 pounds from the pump is better than about .5 pounds from the well. Without the radial groove, top oiling from the valve chamber is better for all the reasons George and Mike state above.
The problem, as I see it, with an inserted engine, the oil should be as clean as possible. Delivering unfiltered oil directly to the bearing without passing through the valve chamber will result in dirtier oil in the bearing. The valve chamber with its three wells, two dams creating 3 wells, actually acts as a settling area for heavy particles in the oil. If you study the floor of the wells, you will discover that the inlet holes for the oil to the mains are slightly raised. The heavy particles, metal, would need to rise up from the floor to get into the feed holes. Rarely will this happen.
With inserts, I think after market oil filters are best but unfiltered also works well if you keep your oil clean. If you change your oil often, or add an oil filter, the pressure feed to the center main will help prolong the life of your bearings. All I have stated is just my opinion and I strongly believe I am correct but...I also believe in the content of Red Green's "Men's Prayer": I'm a man, but I can change, If I have to, I guess. So to, can my opinions
Good Day!
ps: I have an inserted engine with Federal Mogul inserts in my Phaeton. It has a valve cover mounted oil filter that filters about 95% of the oil each time it is pumped to the valve chamber. We have over 80,000 miles on this engine. I have never had the pan off this engine since it was rebuilt. If I make it to 100,000 miles, and I think I will, I will open it up just to see what's going on inside. It has oil delivered to the center main, with the same system you describe, at 8-10 psi when running at road speed from a tap near the oil pump...so technically at that point, it is unfiltered oil. The filter system creates the pressure in the delivery system to the center main from the internal restriction to flow from the filter element. When the car is started, the oil is delivered immediately to the center main. I change my oil at intervals of between 2,000 and 2,500 miles. If you are willing to invest in an oil filter (see photo below) for your inserted engine, I think the pressure to the center main will be a good investment of your efforts. You already have the center main kit...JMO.
