MotoService Log
BSA M20 / M21 (side valve)
Example photo: Joost J. Bakker, CC BY 2.0, Wikimedia Commons

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BSA · Side-valve four-stroke single, dry sump, separate gearbox oil, chain drive

M20 / M21 (side valve)

Library entry covers 1937-1963

36 factory 62 guideline

Published source

B.S.A. 500 c.c. S.V. Solo Motorcycle (M20) Maintenance Manual and Instruction Book, Book No. 100/BC2C (Air Publication 1657B Vol. 1), transcribed .doc

Factory owner's handbook. Covers WD M20 (wartime War Department issue); M21 referenced in data.

98 items for a 1941 model, grouped by interval the way a factory chart is laid out.

How this bike is built (what the schedule is based on)
Engine cycle
Four-stroke
Engine lubrication
Dry sump (separate oil tank)
Oil filtration
Mesh screens / strainers only
Valve gear
Side valve (flathead)
Ignition
Magneto (incl. magdyno, flywheel magneto)
Charging
DC generator with regulator
Battery
Wet cell (flooded lead-acid, needs topping up)
Fuel system
Single carburetor
Cooling
Air / oil cooled
Primary drive
Chain in oil-bath case with its own oil
Gearbox oil
Separate gearbox oil
Clutch control
Hand lever, cable
Final drive
Chain
Front suspension
Girder fork
Rear suspension
Rigid (hardtail)
Front brake
Drum (or band / stirrup)
Rear brake
Drum (or band / belt rim)
Wheels
Spoked, tubed
Sidecar
No sidecar
Maintenance schedule

1941 BSA M20 / M21 (side valve)

On the road: every 250 mi

1 item
  • Check engine oil level
    Pre-ride / on the road
    Factory

    Manual: Engine oil tank: replenish. Factory: Lubrication chart: replenish tank daily and at 250 miles.

On the road: every 300 mi

3 items
  • Check lights, horn, brake light switches
    Pre-ride / on the road
    Guideline
  • Check tire pressures
    Pre-ride / on the road
    Guideline
  • Lubricate drive chain
    Pre-ride / on the road
    Guideline

    Lube warm, ideally at the end of the riding day so it soaks in overnight. Sooner after rain.

On the road: every 500 mi

2 items
  • Grease girder / springer fork links
    Pre-ride / on the road
    Guideline

    Grease gun on every nipple until clean grease shows. Touring days add up fast.

  • Walk-around: check nuts and bolts for vibration loosening
    Pre-ride / on the road
    Guideline

    Old machines shake things loose: exhaust nuts, mudguard stays, footrests, tank mounts, battery strap.

On the road: every 600 mi

1 item
  • Check chain slack and rear wheel alignment
    Pre-ride / on the road
    Guideline

On the road: every 1,000 mi

1 item
  • Check road kit: plugs, points file, master link, tube patches, wire, tape
    Pre-ride / on the road
    Guideline

Every 250 mi

26 items
  • Speedometer drive
    Engine
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Throttle handle bar grip
    Engine
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil Can

  • Contact breaker cam
    Ignition and electrics
    Factory

    Factory: As printed: 'Slight smear' under 250 Miles (Inclusive); lubricant CG-1 (AL); Hand smear

  • Contact breaker tappet
    Ignition and electrics
    Factory

    Factory: As printed: 'One drop' under 250 Miles (Inclusive); lubricant OE30; Oil Can

  • Generator (commutator end)
    Ignition and electrics
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil Can

  • Check gearbox oil level
    Primary and gearbox
    Factory

    Manual: Gearbox: replenish. Factory: Lubrication chart: replenish gearbox at 250-mile interval.

  • Clutch bowden control wire
    Primary and gearbox
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil can

  • Clutch handle bar grip
    Primary and gearbox
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil can

  • Clutch push rod
    Primary and gearbox
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE-50; Oil can

  • Primary chain case: Replenish
    Final drive
    Factory

    Factory: As printed: 'Replenish' under Daily and under 250 Miles (Inclusive); lubricant OE50; Reservoir

  • Rear chain: Few drops
    Final drive
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil can

  • Front fork (center)
    Front end
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Front fork (top)
    Front end
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Steering Head (bottom)
    Front end
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Steering Head (top)
    Front end
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Steering Stem (bottom)
    Front end
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 2 nipple

  • Steering Stem (top)
    Front end
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Saddle nose pivot
    Rear suspension and frame
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Brake cam (front)
    Brakes
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Brake cam (rear)
    Brakes
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Brake pedal
    Brakes
    Factory

    Factory: As printed: 'Grease Gun' under 250 Miles (Inclusive); lubricant CG-1 (AL); 1 nipple

  • Foot brake linkage
    Brakes
    Factory

    Factory: As printed: 'Few Drops' under 250 Miles (Inclusive); lubricant OE-30; Oil Can

  • Advance Retard Cable etc
    Controls and cables
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil Can

  • Bowden control wire
    Controls and cables
    Factory

    Factory: As printed: 'Few Drops' under 250 Miles (Inclusive); lubricant OE-30; Oil Can

  • Carburetor control cables
    Controls and cables
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil Can

  • Speedometer cable
    Controls and cables
    Factory

    Factory: As printed: 'Few drops' under 250 Miles (Inclusive); lubricant OE30; Oil can

Every 1,000 mi or 1 month

1 item
  • Check battery electrolyte, top up with distilled water
    Ignition and electrics
    Guideline

    Wet-cell batteries on old machines need regular topping up, more in hot weather.

Every 1,000 mi or 6 months

7 items
  • Inspect, clean and gap spark plugs
    Engine
    Guideline
  • Lubricate manual advance / retard control and cable
    Ignition and electrics
    Guideline
  • Adjust front drum brake
    Brakes
    Guideline
  • Adjust rear drum brake and rod / cable linkage
    Brakes
    Guideline
  • Check spoke tension and wheel true
    Wheels and tires
    Guideline
  • Inspect tire tread, sidewalls and date codes
    Wheels and tires
    Guideline
  • Lubricate control cables and lever pivots
    Controls and cables
    Guideline

Every 1,000 mi or 12 months

1 item
  • Grease saddle pivot, check saddle springs / seat post
    Rear suspension and frame
    Guideline

Every 1,000 mi

2 items
  • Change engine oil
    Engine
    Factory

    Manual: Engine oil tank: drain & refill. Factory: Chart says 1,000 miles; Section 3 text says drain tank and crankcase every 2,000 miles.

  • Change primary chaincase oil
    Primary and gearbox
    Factory

    Manual: Primary chaincase: drain & refill. Factory: Lubrication chart figure.

Every 1,500 mi or 6 months

1 item
  • Clean / inspect air filter
    Fuel and air
    Guideline

    Mesh or oiled foam: clean and re-oil. Paper: blow out or replace. Some very early machines have no filter at all.

Every 1,500 mi or 12 months

2 items
  • Check and adjust valve clearances
    Engine
    Guideline
  • Adjust clutch free play, lubricate cable and lever pivot
    Primary and gearbox
    Guideline

Every 2,000 mi or 12 months

2 items
  • Check / adjust primary chain
    Primary and gearbox
    Guideline
  • Check girder fork spindle play, adjust friction / hydraulic damper
    Front end
    Guideline

Every 2,000 mi

3 items
  • Clean oil strainers / mesh screens
    Engine
    Factory

    Manual: Oil filters: wash in gas. Factory: Lubrication chart figure.

  • Clean and gap contact breaker points, oil cam felt
    Ignition and electrics
    Factory

    Manual: Magneto cam wick: few drops thin machine oil. Factory: About every 2,000 miles.

  • Clean drive chain thoroughly (remove and soak, re-lube)
    Final drive
    Factory

    Manual: Rear chain: wash in paraffin and soak in oil. Factory: Lubrication chart figure.

Every 3,000 mi or 12 months

14 items
  • Check engine mounting bolts and plates
    Engine
    Guideline
  • Check throttle cable(s) free play and return
    Fuel and air
    Guideline
  • Clean fuel tap filter / sediment bowl
    Fuel and air
    Guideline
  • Check / set ignition timing
    Ignition and electrics
    Guideline
  • Check generator drive belt or chain
    Ignition and electrics
    Guideline
  • Oil / check generator bearings (where an oiler is fitted)
    Ignition and electrics
    Guideline
  • Adjust clutch pushrod / clutch adjuster
    Primary and gearbox
    Guideline
  • Check chain guard, chain oiler (if fitted), rear chain case
    Final drive
    Guideline
  • Inspect chain and sprockets for wear (measure stretch)
    Final drive
    Guideline
  • Check steering damper (friction or hydraulic)
    Front end
    Guideline
  • Check steering head bearing adjustment
    Front end
    Guideline
  • Check footrests, pegs, rear brake and gear lever pivots
    Rear suspension and frame
    Guideline
  • Grease and check center / side stand and springs
    Rear suspension and frame
    Guideline
  • Check wheel bearings for play
    Wheels and tires
    Guideline

Every 4,000 mi or 12 months

2 items
  • Replace spark plugs
    Engine
    Guideline
  • Lubricate brake cam spindles, check linings and drums
    Brakes
    Guideline

Every 5,000 mi or 2 years

1 item
  • Check cylinder head bolt torque
    Engine
    Guideline

    Air-cooled iron and alloy engines can lose head bolt tension; follow your manual's sequence and whether it is done hot or cold.

Every 5,000 mi

1 item
  • Repack / replace wheel bearings
    Wheels and tires
    Factory

    Manual: Front and rear wheel hubs: re-pack. Factory: Lubrication chart figure, both hubs.

Every 6,000 mi or 12 months

5 items
  • Clean carburetor float bowl(s), check float level and jets
    Fuel and air
    Guideline

    Ethanol fuel varnishes old carbs quickly in storage.

  • Check charging voltage and regulator / cut-out
    Ignition and electrics
    Guideline
  • Inspect generator brushes and commutator
    Ignition and electrics
    Guideline
  • Inspect girder fork links, springs and spindles for wear or cracks
    Front end
    Guideline
  • Lubricate speedometer / tachometer cables, check drive
    Controls and cables
    Guideline

    A dry speedo cable whips the needle and eventually kills the instrument.

Every 6,000 mi or 2 years

3 items
  • Check engine breather and oil return
    Engine
    Guideline
  • Lubricate and check automatic advance unit
    Ignition and electrics
    Guideline
  • Magneto: clean slip ring and pickup brush, check HT lead
    Ignition and electrics
    Guideline

Every 6,000 mi

1 item
  • Change gearbox oil
    Primary and gearbox
    Factory

    Manual: Gearbox: drain & refill. Factory: Lubrication chart figure.

Every 8,000 mi or 3 years

1 item
  • Decarbonize head, grind / lap valves (top-end decoke)
    Engine
    Guideline

    Period practice. Leaded-era valve seats may need hardened seats for unleaded fuel.

Every 10,000 mi or 2 years

1 item
  • Compression test (or leak-down)
    Engine
    Guideline

Every 10,000 mi or 3 years

1 item
  • Repack / replace steering head bearings
    Front end
    Guideline

    Many old machines use loose balls in cups and cones. Taper roller conversions are common.

Every 10,000 mi

1 item
  • Replace chain and sprockets (wear based)
    Final drive
    Guideline

Every 12,000 mi or 3 years

1 item
  • Replace points and condenser
    Ignition and electrics
    Guideline

Every 15,000 mi or 5 years

1 item
  • Inspect clutch plates and springs
    Primary and gearbox
    Guideline

Every 15,000 mi

1 item
  • Replace brake linings / skim drums (wear based)
    Brakes
    Guideline

Every 25,000 mi or 10 years

1 item
  • Magneto overhaul / re-magnetize, replace capacitor
    Ignition and electrics
    Guideline

    Weak spark when hot is the classic symptom. Old capacitors fail with age, not miles.

Every 6 months

1 item
  • Clean battery terminals, check charge and strap
    Ignition and electrics
    Guideline

Every 12 months

1 item
  • Check speedometer / odometer against a known distance
    Controls and cables
    Guideline

    Old speedos drift with tire size and drive wear. Use phone GPS over a measured 10 miles / km.

Every 2 years

3 items
  • Inspect fuel tank for rust, sealer failure, leaks
    Fuel and air
    Guideline
  • Inspect wiring, bullet connectors and ground points
    Ignition and electrics
    Guideline
  • Inspect frame, welds and lugs for cracks
    Rear suspension and frame
    Guideline

Every 4 years

1 item
  • Replace fuel lines
    Fuel and air
    Guideline

    Old rubber and modern ethanol fuel do not mix well.

Every 6 years

2 items
  • Replace inner tubes and rim tape with tires
    Wheels and tires
    Guideline
  • Replace tires on age (regardless of tread)
    Wheels and tires
    Guideline

No interval set

2 items
  • Air cleaner
    Fuel and air
    Factory

    Factory: As printed: row printed with 'N/A' and no interval or lubricant

  • Generator (drive end)
    Ignition and electrics
    Factory

    Factory: As printed: 'Re pack w shops' under General; no interval printed; lubricant WB-2

Guideline lines: Intervals here are general guidelines for the machine's era and layout, compiled for this app. They are not factory manual figures. Check your workshop manual, and edit any interval in the bike's Schedule tab.

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