When Harley Davidson decided to redesign their overhead valve big-twin engine to achieve what became know as the “Panhead”. They had these goals in mind:
- Increase engine cooling efficiency
- Reduce oil leaks
- Reduce valve train noise
Major engine components were redesigned to achieve these goals.
Cylinder heads were redesigned and cast in aluminum with large cooling fins. As opposed to the earlier cast iron heads. Aluminum transfers heat over 4 times as rapidly as cast iron. This increases engine cooling efficiency. Internal oiling of rocker arms coupled with large one piece
valve covers for each cylinder that completely covered both rocker arms eliminated oil leaks.
Engine cases and cylinders
Engine cases and cylinders were redesigned to allow oil to feed internally through the engine case, cylinder heads, and rocker arm assemblies. Returning from the cylinder heads
through cylinders into groves cut into the top of engine case. In later model years the grooves were eliminated and oil drains straight through engine case.
Rocker arm assemblies
Rocker arm assemblies were redesigned. They are now one piece shaft and arms, rotating inside a bearing as opposed to the earlier design with the rocker arms rotating on a stationary shaft.
There is an oil passage drilled through the center of the shaft which is plugged at each end using welch plugs. This becomes a problem very early on as the welch plugs loosen and fall out.
Which caused a loss of oil pressure; this is corrected in late 1948.
Rocker arm assemblies change.
In late 1948 the rocker shaft was changed to correct the problem of loose welch plugs falling
out by no longer drilling the oil passage completely through the shaft and plugging the single
end opening with a a steel drive plug.
Tappet guides and tappets
New aluminum tappet guides and tappets were designed. This design changed in the 1953 model year as a result of problems with the hydraulic lifter design.
Pushrods and hydraulic lifter units
A new pushrod is designed with a hydraulic lifter unit pressed into the top of the pushrod. Hydraulic units eliminated the the clearance between the pushrod and rocker arm required by mechanical lifters to prevent parts from binding due to heat expansion..This dramatically reduced valve train noise and eliminated the need for frequent adjustment of pushrods. This first design was only moderately successful.
Early changes to lifters,tappet guides and tappets
The new design worked well on new units, but after periods of service would succumb to inconsistent oil pressure and dirt clogging the lifters. Thus causing sticking collapsing lifter units, valve lash noise, the need for frequent adjustment cleaning and/or replacement of lifter units. These consistent problems brought about a design change in the 1953 models relocating the lifter units to the tappets. This required a new pushrod design moving the pushrod adjustment screw to the bottom of the pushrod, a new tappet that would accommodate the lifter unit, and tappet block designs that would provide oil to the tappets and lifters. A check valve and oil screen were also added to the engine case in the oil feed line to prevent dirt from entering the lifters. Moving the lifter unit closer to oil feed source (oil pump) provides more consistent oil pressure and solved the problem.
Another minor change. external oil line.
From 1963-65 the oil feed lines to the cylinder heads were relocated using a external line from the engine case to the cylinder heads to deliver cooler oil to the cylinder head by passing it through an outside oil line.
Though this engine design remained basically the same for 17 model years, there were many changes. Not all parts interchange. Interchanging incompatible parts can have catastrophic results. Make sure you have the correct parts for your application.