Back in the dark ages (the 1950s) when I started tinkering with engines as a preteen kid hanging around Dad's auto shop, they did a lot of what was known as an "in the frame rebuild". The cylinder head and oil pan got removed, the wear ridge at the top of the cylinders got reamed away, and the cylinders got a quick hone job, the pistons got new rings, the valves were reground, and the Babbit-lined connecting rods got "adjusted" to the proper oil clearance by removing a shim or two, and everything got reassembled. That procedure could be done a time or two before the taper wear in the cylinders required a rebore job and oversize pistons. Sometimes, cylinders actually got bored with the engine still in the frame, and a portable crankshaft grinder could be set up to get the crankshaft round again for undersize rod bearings. More often, a rebore and crankshaft regrind was done after the engine was removed and torn down for a major overhaul. By the time an engine had 50K miles or so, it was not uncommon for it to have received one, or maybe two in frame repairs, and possibly a valve job in between ring and rod bearing jobs. Prior to the advent of "positive crankshaft ventilation" (PCV) systems, engines ate a lot of dirt, and rings and rod bearings wore out quickly from abrasive damage. Blowby combustion gas from worn piston rings also mixed with humidity and formed acids that ate away at bearings, etc. Metallurgy was also not the best, resulting in rapid engine wear. Some of the stovebolt engines we tinker with have probably been "rebuilt" multiple times.
About the only advantage I can see to doing an engine run before an overhaul is the ability to do a crankcase flush- - - - -fill the engine with inexpensive oil like Walmart "house brand" non detergent 30 weight, add 3 or 4 quarts of Diesel fuel so the crankshaft and rods dip down into the overfilled crankcase, and run the engine at a fast idle for 30 minutes or so. The crankshaft and rods will splash the diluted oil around like the agitator in a washing machine, and dislodge years' worth of sludge and dirt. That makes cleanup during a rebuild a lot easier. DO NOT DRIVE THE VEHICLE OR PUT ANY LOAD ON THE ENGINE! Drain the oil while it's still hot.
Jerry
