Years ago, the former head of Harley-Davidson racing, Dick O’Brien, described to me what happens when a Harley engine “wet-sumps.” If for any reason oil at the bottom of the crankcase is not picked up rapidly enough by the scavenge pump, it can be swept around by the whirling crank into the narrow gap between the inside of the close-fitting crankcase and the outside diameter of the crank. The oil does not, as some describe, stick to the crank, but is batted back and forth between it and the case. This very rapid shearing of the oil consumes a lot of power—O’B said he could see it when a bike would wet-sump on the Daytona banking.
This is a very old problem, dating back to the early days of total-loss oiling. If the alert rider of 1914 looked behind and saw no visible smoke, he took that as an indication of too little oil in the engine. A few strokes of the hand pump fixed that. Sluggish performance signaled too much oil—time to ease up.
One of the first changes made to the design of the 1948 AJS 7R 350cc Single was a reduction in its flywheel diameter. At first, this seems motivated by a desire for reduced flywheel inertia, but the actual reason for the change was to cut the power loss arising from oil getting swept around between crank and case.
Another approach to the same problem is to better control the movement of oil within the engine. At Daytona in March, I looked at the front of a Kawasaki ZX-10R engine and saw two cylinder-head oil returns that ran from the head, down through passages in the upper crankcase/cylinder casting, then into the lower case, finally terminating at the gasket surface of the oil sump. This carefully routed oil completely around the crank chamber, eliminating any contact.
In factory Velocette KTT 350cc racing Singles, the oil that lubricated the single overhead cam became a bit of a problem, such that a second scavenge pump was located in the head. It picked up the waste oil and sent it directly back to the remote oil tank—no chance of its getting anywhere near the crank.
One of the first really successful high-rpm dohc Singles was Benelli’s prewar 250. Careful examination of photos of its engine reveals two oil drainback lines, one from each cambox. Why not just let the oil drain back down the train of cam-drive gears? High-speed gears are no place for more than just enough oil. A tale is told of oil in a Harley gearbox boiling during a long factory dyno test. Like rolling bearings, gears need just a bit of oil—not to be awash in it.
Benelli’s famous postwar transverse Fours with their upright cylinders had no fewer than four cylinder-head oil drainbacks. The cam boxes were divided at the center by the cam drive gear tower, so there were four sections to be drained. The front drains went all the way to the bolted-on under-engine oil sump and the rears to the gearbox and primary areas.
If engineers were concerned to keep oil away from a 1950 crank turning 6500 rpm, think of the much greater “oilage losses” that must be avoided in today’s 15,000-rpm 600cc sportbikes or 20,000-rpm MotoGP engines.