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