Key to thread descriptions:
NSC: Non Standard Cycle
Cycle: British Standard Cycle Thread 
BSF: British Standard Fine
BSW: British Standard Whitworth 
BSP: British Standard Pipe Thread
UNF: Unified Thread
Cylinder head bolts 3/8 BSF
Cylinder head timing access bolt 1/4 cycle (1/4 BSF alt.)
Cylinder base studs 7/16 whitworth to, 7/16x20 NSC to nut (7/16UNF alt.)
Tappet cover 1/4 whitworth
Valve lifter fitting to cylinder 3/4x20 NSC
Valve lifter spindle nut 1/4 cycle (1/4 BSF alt.)
Cam follower guide to 3/4x20 NSC
Tappet adjusters 5/16 cycle
Timing side outriger plate bolts 5/16 whitworth to 5/16 cycle to cam spindles and int. Gear spindle
Timing cover screws 1/4 whitworth
Magneto access plug 1 25/32x19 non standard thread
Timing cover oil pressure relief fitting 3/8 BSF
Timing side c.shaft mainshaft 1/2 NSC (1/2UNF alt)
Drive side c.shaft mainshaft 3/4x20 NSC
Oilpump drive spindle retaining peg 1/4 cycle (alt 1/4BSF)
Oilpump retaining bolts 1/4 whitworth
Oilpump assembly bolts 3/16 BSF
Sump plate studs 1/4 whitworth to c.cases 1/4 cycle (alt 1/4 BSF to nuts)
Primary chaincases to bolts 5/16 whitworth oil pipe union fittings 3/8 BSF to 1/4 BSP to oil pipe nuts breather to 5/8x20 NSC breather to vent pipe nut 7/16x20 NSC (7/16 UNF alt.) breather fitting to breather body 5/8x20NSC joining studs 5/16 whitworth to 5/16 cycle to nut joining nuts and bolts 5/16 cycle
Carb mounting studs 5/16 cycle both ends
Magneto strap bolt and trunnion 5/16 cycle
Gearbox drain plug 7/16x20 NSC ( 7/16 UNF alt.)
Gearbox Filler plug 7/8 BSP
Gearbox inspection cover plate 3/16 Whitworth
Kickstart return spring securing screw 1/4 cycle (1/4 BSF alt.)
Inner cover securing screws and bolts 1/4 Whitworth
Gearbox outer cover screws 1/4 Whitworth
Gearbox outer cover bolts (from rear of inner cover) 1/4 Whitworth (if studs are fitted 1/4 Whitworth to outer case and 1/4 cycle to nut)
Clutch pushrod adjustment screw 1/4 cycle (1/4 BSF alt.)
Gearbox mainshaft (kickstart ratchet end) 9/16x20 NSC
Gearbox mainshaft (clutch end) 5/8x20 NSC
Gear change control shaft 7/16x20 NSC (7/16 UNF alt.)
Operating plate and pawl arm stop screws 5/16 cycle
Pawl pivot nuts 1/4 cycle (1/4 BSF alt.) 
Gear control link adjuster 5/16 cycle
Gear control quadrant shaft 3/8 cycle
Gearbox sprocket nut and sleeve gear 1 1/4x24 NSC 

Primary Drive

Inner primary chaincase to chainguard stud  1/4  cycle (1/4 BSF alt.)
Inner primary chaincase drain plug  1/4  cycle (1/4 BSF alt.)
Inner primary chaincase sliding plate retaining bolts and nuts 1/4 cycle (1/4 BSF alt.)
Outer primary chaincase inspection plug 1 25/32x19 Non standard thread
Outer primary chaincase oil level plug 3/8 cycle
Outer primary chaincase fixing screws 3/16 cycle
Clutch sleeve extractor thread 1”x24 cycle
Clutch sleeve and clutch spring nut 1 3/16x24 cycle
Clutch mainshaft nut extractor tool thread 5/16 cycle
Clutch pressure plate ball bearing fitting 1/2x20 NSC (1/2 UNF alt.)

Clutch cover fixing screws 2BA

Ian Wright