The BSA M20 is a legendary British motorcycle, originally produced during the 1930s and widely used during World War II. Known for its durability and simplicity, the M20 remains a favorite among classic motorcycle collectors and enthusiasts.
But if you own one, you’ll know that keeping it in good running condition requires attention to detail , especially when it comes to the spark plug. The spark plug is a small but essential component that directly affects your bike’s performance, fuel efficiency, and reliability.
In this guide, we’ll explore everything you need to know about the BSA M20 spark plug , from correct specifications and types to maintenance tips and FAQs.
Understanding the BSA M20 Engine
Before diving into spark plugs, it’s important to understand the BSA M20’s engine.
- Type: 499cc side-valve, single-cylinder, four-stroke engine
- Compression Ratio: Low (around 5:1)
- Power Output: 13–14 horsepower
- Design Purpose: Reliability over speed, especially during wartime service
Because of its relatively low compression, the BSA M20 requires a specific spark plug type that suits older, cooler-running engines.
BSA M20 Spark Plug Specifications
Choosing the right spark plug is crucial for proper combustion and smooth engine running.
Recommended Spark Plug Types
- NGK: B6HS or equivalent
- Champion: L86C
- Bosch: W7AC
Spark Plug Gap
- Standard Gap: 0.018–0.020 inches (0.45–0.50 mm)
Heat Range
Since the M20 has a low compression engine, a medium-heat range spark plug is usually recommended. Using the wrong heat range could lead to:
- Fouling (if too cold)
- Overheating and pre-ignition (if too hot)
How to Install a Spark Plug in a BSA M20
Installing a spark plug in a classic motorcycle like the BSA M20 requires care.
Step-by-Step Guide
- Remove the Old Plug – Use the correct spark plug wrench to avoid damaging the cylinder head.
- Check the Threads – Clean and inspect for wear. Damaged threads may require a thread repair insert.
- Set the Gap – Adjust the electrode gap to 0.018–0.020 inches using a feeler gauge.
- Hand Tighten First – Always screw in the plug by hand to prevent cross-threading.
- Final Tightening – Use a torque wrench if possible. For most plugs, around 18–22 ft-lbs of torque is sufficient.
Maintenance Tips for BSA M20 Spark Plug
Classic motorcycles require regular maintenance, and the spark plug is no exception.
Cleaning the Plug
- Use a wire brush to remove carbon buildup.
- Avoid sandblasting, as it can damage the insulator.
Checking Condition
- Light Brown/Tan Color: Normal
- Black & Sooty: Running too rich or plug too cold
- White/Blistered: Engine overheating or plug too hot
Replacement Interval
- Replace every 3,000–5,000 miles, or sooner if fouling occurs.
Common Spark Plug Problems in BSA M20
Even with proper maintenance, issues can occur.
1. Plug Fouling
Caused by running rich fuel mixtures or short journeys. Solution: Adjust carburetor and ensure proper plug heat range.
2. Misfiring
Can result from worn plugs, incorrect gap, or faulty ignition coil.
3. Hard Starting
Old plugs with weak sparks often make kickstarting difficult.
Where to Buy BSA M20 Spark Plugs
Finding genuine or compatible spark plugs is easier today thanks to online stores.
- Classic Motorcycle Parts Suppliers – Specialist websites stock correct NGK, Champion, or Bosch plugs.
- eBay & Forums – Enthusiast communities often sell spares.
- Local Auto Shops – Many NGK or Champion plugs are widely available.
When buying, always confirm compatibility with BSA M20 specifications.
Final Thoughts
The BSA M20 spark plug may be a small component, but it plays a vital role in keeping your classic motorcycle running smoothly. By choosing the correct plug type, setting the right gap, and maintaining it regularly, you’ll ensure reliable starts, smoother rides, and longer engine life.
For vintage motorcycle lovers, paying attention to these details is what keeps history alive on the road.
FAQs
1. What is the correct spark plug for a BSA M20?
The recommended spark plugs are NGK B6HS, Champion L86C, or Bosch W7AC.
2. What should the spark plug gap be on a BSA M20?
The correct spark plug gap is 0.018–0.020 inches (0.45–0.50 mm).
3. How often should I replace the spark plug in a BSA M20?
It’s best to replace the spark plug every 3,000–5,000 miles, or sooner if fouling or misfiring occurs.
