By 1937, all Harley-Davidson's flathead engines were equipped with dry-sump oil recirculation systems similar to the one introduced in the "Knucklehead" OHV engine. The revised 74 cubic inches (1,210 cc) V and VL models were renamed U and UL, the 80 cubic inches (1,300 cc) VH and VLH to be renamed UH and ULH. The 74" U & UL flathead models were produced up to 1948, effectively having been replaced by the 74 cubic inches (1,210 cc) "Knucklehead"s that were introduced in 1941 as the F and the FL.
- Mitchel, D. (1997). Harley-Davidson Chronicle - An American Original. Publications International Limited. pp. 94-95. ISBN 0-7853-2514-X.