Don't know what it is about knuckleheads, but they've always been a motor that I've been drawn to. I was playing around on the net and came across this beauty. A Drake watercooled v-twin knucklehead. Not something you see everyday!
Earlyriders article on Harley-Davidson Drake knucklehead engines
In the 1930s Drake Engineering began producing special Harley-Davidson based motors for racing that were used in quarter-mile midget cars. The cases were early HD 61 OHV, usually beefed with extra welding and fitted with steel strocker flywheels. The trans was an in-or-out gearbox  - no clutch - made from Ford "A" parts that bolted to the engine sprocket boss. Since the cases were spun around and mounted in the chassis crosswise, this was the back of the engine.
In place of the stock knuck to end, one piece water jacketed cylinders and heads with mounts for dual 1 1/4" carbs [cast in Iron by Drake] accepted standrad HD valve gear. With a 4 5/8" stroke and a monster 3 1/2" bore, displacement worked out to 89 cubes with a 96-inch version appearing later.
Most of the internals started out as standard HD, but the power that the engine produced made stronger replacements necessary. Years before the factory did it, the cases were bored out for Timken type bearings, and mainshaft sizes grew to 1 1/2" and eventually 2". Early stock rods bnt like pretzels and racers used Cartwright forgings or 4130billet units made by Barker. Cams were either Lightning Knuckle or Schaeller regrinds. The standard generator was replaced with an Edison-Splitdorf mag and later, with Bosh or Wico magnetosrunning in a special front drive cam cover by Elder.
Since the powerplant was just for racing, a number of Los Angeles area shops made special conversions for themselves. Pordugiel and Smiley produced some with steel crankcases [whitled out of a solid billet!] and one of the Hoak brothers' motors had alloy valve covers like later Panheads!
The engines were used in midgt car racers in the late 1930s and in the post WWII era. Their low speed torque made them competitive with the Offy's [Four cylinder engines Offenhauser engines also used in midgets] on short or slippery tracks. However, on longer circuits, they would overheat and lose power. Because the cooling system had no water pump and depend on thermal currents for circulation hrough a font mounted mounted radiator. Another major problem was vibration. The "shakers" or "popers, as they were called, would work a car to death, splitting float bowls and slowly shedding parts all over the track unless revswere limited to about 4000. A few Drake engines were use by bike drag racers in the 1950s, but most of the 400-odd produced have probably gone to the scrap yard now. The production of the engine parts passed out of Drake through several owners, ending up in 1952 with Edgar Elder, who still has the orginal patterns.
Taken from an article in Earlyriders, Published by Easyriders in the late 70's or early 80's