Engine clearances and fits BSA WM20
Bush sizes quoted are finished sizes after fitting
Piston skirt clearance .004~ .006  Maximum permissible wear .010
Piston ring gap: .008~.010
Piston ring groove clearance:  Free but not exceeding .005
Valve stem diameter: .348 (Inlet and Exhaust)
Valve guide bore: .3515~ .3525 (Inlet)
Valve guide bore: .3525~.3535 (Exhaust)
Interference fit of guide into cylinder: .001~.002
Tappet stem: .373~.3735 (Inlet and Exhaust)
Tappet guide bore: .3745~.3755 (Inlet and Exhaust)
Tappet clearance: .010 (Inlet)  .012 (Exhaust)
Cam pinion spindle diameter: .623~.6235  (Inlet and Exhaust)
Cam pinion bush bore: .6245~.6255 (Inlet and Exhaust)
Camshaft end float with outrigger plate fitted: .002~.003
Idler pinion spindle diameter: .748~ .7485
Idler pinion spindle bush bore: .7495~.7505
Idler gear end float with outrigger plate fitted: .002~.003
Gudgeon pin diameter: .750
Gudgeon pin bush inside diameter: .7503~ .7506 (upper size recommended )
Gudgeon pin bush outside diameter: .9395~.9405 (upper size recommended.)
Crankshaft main bearing (drive side) spacer length:  1.000~1.010
Mainbearing interference fit into crankcase: .002~.003 (cold)
Timing side outrigger plate bush bore:  .813~.815
Crankshaft pinion gear outside diameter: .8115~.812
Connecting rod side float: .010~.015
Contact breaker gap: .012
Spark plug gap: .018~.020

Ignition advance (fully advanced): 7/16 (.4375) B.T.D.C.

Ian Wright