Harley Davidson Panhead. Service manual (1948-1957 Rigid) - page 6
SHOP DATA
EXHAUST VALVE ROTATING MECHANISM
Starting with engine No. 52FL3910 a different type exhaust valve went into production. The new valve in open position is free to rotate, and there is less possibility of valve head warping and burning. Engines will give a longer period of service without valve reseating.
Valve rotating mechanism functions as follows:
Valve stem cap,
item I, has slight clearance between end of valve stem and seats against
valve key, item 3. When rocker arm applies pressure against valve stem
cap, valve key is moved away from valve stem shoulder and valve i s free
to be rotated by the escaping exhaust gases.
