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

9 factory 60 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.

Open the source

69 items for a 1955 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

1955 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

1 item

Check gearbox oil level

Primary and gearbox

Factory

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

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

6 items

Inspect, clean and gap spark plugs

Engine

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

1 item

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 2,500 mi or 12 months

1 item

Check and adjust valve clearances

Engine

Guideline

Every 3,000 mi or 12 months

13 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
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 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 12,000 mi or 4 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 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

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.