Primary chain covers made of sheet steel from 1936 onwards

Progress in terms of oiling came in 1936 with the oil circulation system of the OHV Twins, which became known as Knucklehead. This circulating lubrication later was also integrated into the side valves, and from 1937 all Harley V Twins had a feed and return pump. The engine ventilation was improved and redesigned, and the amount of lubricating oil for the primary chain could be regulated from then on. The primary chain covers were redesigned accordingly and were fitted with a cork seal all round, the inner and outer primary covers of the large Twins were precisely matched sheet metal stampings. Only the small Twins (750 cc Flatheads) retained the cover without a seal.