???? BSA Spark Plug Guide (1950–1985)

Thread Size: Most BSA models use 14mm spark plugs.
Reach (Thread Length): Typically 1/2″ (12.7mm) or 3/4″ (19mm), depending on the head.
Heat Range: Use a medium to slightly hot plug unless racing or under heavy load.
Plug Gap: Usually .020″ to .025″ (0.5mm–0.6mm) for magneto ignition, or up to .028″ (0.7mm) for coil ignition.
Plug Type: Non-resistor plugs for magneto systems; resistor plugs for electronic ignition upgrades.

Model	Engine (cc)	Common Years	Standard Plug (NGK)	Champion Equivalent	Gap
BSA Bantam D1–D7	125–175cc	1948–1971	B6HS	L82C	0.020–0.025″
C11, C12	250cc OHV	1950–1958	B6HS	L82C	0.020–0.025″
C15, B25	250cc OHV	1959–1973	B7ES	N3	0.025–0.028″
B31	350cc OHV	1945–1959	B6ES	N4	0.020–0.025″
B33	500cc OHV	1947–1960	B6ES	N4	0.020–0.025″
A7	500cc Twin	1946–1962	B7ES	N3	0.020–0.025″
A10 (Golden Flash, Super Rocket)	650cc Twin	1950–1963	B7ES	N3	0.020–0.025″
A50, A65	500–650cc Twin	1962–1972	B7ES	N3	0.025–0.028″
Rocket Gold Star	650cc Twin	1962–1963	B7ES	N3	0.020–0.025″
Gold Star (DBD34)	350/500cc OHV	1950s–60s	B8ES	N3G	0.020–0.025″
B44 Victor	441cc OHV	1965–1970	B7ES	N3	0.025–0.028″
B50	500cc OHV	1971–1973	B7ES	N3	0.025–0.028″
A70 Lightning	750cc Twin	1971	B7ES	N3	0.025–0.028″
BSA-Triumph (post-1973, including TR5T)	500–750cc	1973–1985	B7ES / B8ES	N3 / N3G	0.025–0.028″

Cold climate or slow-speed riding: Use a hotter plug (e.g., B6ES).
High-speed/race use or hot weather: Use a colder plug (e.g., B8ES).
For electronic ignition: Use resistor-type plugs (e.g., NGK BR7ES) and matching resistor caps.

Inspect Plug Color:
- Tan/gray = good.
- Black = rich or oil fouled.
- White = lean mixture or overheating.
Use Anti-Seize (lightly) on plug threads for aluminum heads.
Check Gap Regularly: Older ignition systems can wear quickly, increasing the gap.
Torque Correctly: About 18–22 ft-lbs on aluminum heads; hand-tight plus 1/4 turn if unsure.
Upgrade Option: Iridium plugs (e.g., NGK B7EIX) offer longer life and better spark, but check compatibility with ignition.

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