Originally Posted by earlwb
Think of the Harley V-twin air cooled engines being more in common with a radial engine design than other engine types. It behaves more like a radial engine too. You have to warm them up good and get them nice and hot before you try doing anything hard with them. Otherwise they'll blow a head gasket. During warm up from cold to hot the different parts expand at different rates. The engines actually grow in height a considerable amount too. That uneven expansion and contraction tends to wear out the seals on the intake manifold and the gaskets on the valve covers fairly often. Fortunately for quite a long time now HD uses hydraulic lifters, so you don't have to adjust the valves on them.
They also have a lot in common W/our air cooled R/C engines.
Aluminum crankcase, tall cylinder casting W/external pushrod tubes.