by Danno A lot of the reason is the evolution similarities to the DeDion-Bouton single, one of the early 4-cycle success stories. Also, a separate tank was deemed necessary to give the oil a chance to cool before being pumped to its various points of service. Air-cooled motors are, after all, air- AND oil-cooled and oil in a sump tends to heat up and not be able to slough off the excess temperature. The air-cooled multis of the 70s and 80s made up for this with nearly a gallon of oil in the sump and huge, finned cases for heat sink. British vertical twins, especially the 180 degree firing ones like Dominators are really like big singles and with ancillaries like timing drives, alternators and clutches outboard of the crank ends, a dry sump makes for a more compact motor.
But, why did BSA, Triumph, Norton, etc. elect to place the tank ABOVE the sump? Is there a reason, maybe column height? Why not put the tank at the level, or slightly lower then the sump? If they were concerned about immediate oil pressure upon startup, couldn't there be a small amount left in the bottom of the sump, around the pump? If heat was a major consideration, why didn't the engineers make the tank out of aluminum and put fins on it? There must be valid reasons for putting the tank up high. I am just very curious as to why. It seems to me that, with some design changes to the center stand, there would be room.
