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) Harley-Davidson V and Harley-Davidson VL models were renamed Harley-Davidson U and Harley-Davidson UL, the 80 cubic inches (1,300 cc) Harley-Davidson VH and Harley-Davidson VLH to be renamed Harley-Davidson UH and Harley-Davidson ULH. The 80 cubic inches (1,300 cc) flathead UH and ULH models were discontinued after 1941
- Mitchel, D. (1997). Harley-Davidson Chronicle - An American Original. Publications International Limited. pp. 94-95. ISBN 0-7853-2514-X.