Engine break-in is not the same as it was when our grandfathers were putting their Triumphs and Harleys together 40 years ago. In the old days, the metallurgy required alloys with lots of nickle or chrome for parts like rings and camshaft components to maintain any reliability. The machining processes were not as good as today and surface finishes were made purposefully rough to allow parts to eventually mate. Piston technology in some engines was to use zero clearance and split skirts to keep the engines quiet. Oil was good enough in those days to compress the skirt and still maintain an oil film, but there were also high spots that would protrude through the film and the engine would need to be run slow at in break in to keep from overheating the parts and causing complete failure.
Today's engines have much harder cylinders and steel aloys, piston rings have surfaces like moly that offer less friction and mate to the cylinders very quickly. Cylinder bores and camshaft tolerences are near perfect, very round & smooth bearings and crankshafts have the same qualities, so the break in period is not what it used to be.
Now days, in a brand new engine, the parts that need the most attention are the cam shaft and lifters. These are the parts with the highest load and require the surfaces to mate, removing microscopic peaks that can damage the lobe or lifter, so lots of lubrication is required. A faster engine speed is required for cam break in to form a better hydraulic oil surface. The faster surface speed of the lobe forms a thicker wedge of oil, allowing the peaks to be worn away. This process only takes a few minutes.
When rebuilding an engine, I think it is important to lubricate the components. The piston and rings should have some lubrication, but this should be very light so that it doesn't burn and form a glaze before the ring has had a chance to mate with the cylinder. I have had good results with Total Seal Quick Seat which is tungsten disulfide and I also put a dab of oil on the piston skirt. This slight amount of lubrication alows the high spots to gradually come off and be floated away with the oil and into the filter. In contrast, no lubrication can cause theses particles to be transfered to a softer surface like the piston and ring, causing unwanted wear to the cylinder as the piston rings are being forced against the cylinder with over 2000 psi.
Engine break-in very short for the latest generation of engines. I warm them up to temperature while going through the transmission gears, making sure every thing is working. I then let the engine cool down and check the oil & coolant levels, also checking for any leaks. After that, I start the engine, let it warm up to temperature and its wide open from there. I heard a story from a freind that worked for Rob Muzzy where one of the mechanics was asking him how he wanted this road race engine broken in. His reply was like your new chainsaw: fill it with gas and oil and start cutting wood.
Who is Ron Hamp?
Some of Ron's Work