Knucklehead Harley Davidson Engine

The Harley Davidson Knucklehead V-Twin engine was officially
referred to as the 61 OHV when it first debuted in 1937 as a
replacement for the Flathead engine, but over the years, engine
enthusiasts have referred to this engine as Kh, a Howie, or a Marks. 
“Knucklehead” was the most common name because of the trademark rounded
knobs on its valve covers, and it was the first overhead valve V-twin
engine made by Harley Davidson. 

Photograph courtesy of the Harley-Davidson Motor Company Archives. Copyright Harley Davidson.

This third generation engine was used
in Harley Davidson motorcycles until 1947, when it was supplanted by
the Panhead engine. There are two original versions of this engine, a
60 cubic inch engine with 40 horsepower that was referred to as the EL
and a 74 cubic inch engine with 45 horsepower that first debuted in
1941 as the FL.

The Harley Davidson V-Twin  Knucklehead has a lot of the same
components and design features that it inherited from its predecessors,
as a 2-cylinder, 2-valve per cylinder, push-rod operated engine. It was
an improvement over the Flathead engine mainly because of the use of an
oil lubrication system that re-circulated the oil, rather than just
leaked onto the ground as with the Flathead engine design.