From Oil Tank 
To Engine
Universal Inline Antidrain
Valve
BCS Part #641-21
(Simply cut oil feed line to engine and install as noted,
arrow pointing to engine. The unit has flats for tightening. 0.4" o.d. spigots
for 5/16" or 3/8" i.d. hoses.)
Is your Norton or BSA single or twin dumping all its oil from the tank into the crankcase when you leave it parked? Make sure you are running thick enough oil (50 wt Summer, 40 wt Fall and early Spring) and are parking it on compression (pistons at top of stroke to block off any conrod oilholes).
If the problem persists, you can replace your gear type oilpump with a new one (expensive and sometimes not easily available), or install one of our Antidrain valves into the feed line from the tank to the oilpump. Based on a design used by Velocette for many, many years, these spring loaded valves open as soon as the engine is started. Make sure your valve is installed in the correct direction and well primed with oil, and your problem is solved.
A direct replacement version is also available for the stock oiltank screen on Norton Fastbacks and Pre-Commando models: 06-0592/A.