Briggs & Stratton Magnetos

Loose magnets require high-temperature epoxy (JB Weld) reattaching cleaned surfaces—temporary solution failing under high RPM or heat stress. Original equipment replacement flywheel provides permanent repair maintaining correct magnetic strength and polarity alignment. Remagnetising requires specialised coils and procedure—incorrect polarity reverses magnetic field preventing spark generation. Two-pole flywheels position magnets 180 degrees apart with opposite poles (north-south) facing outward. Four-pole flywheels alternate north-south-north-south configuration producing two pulses per rotation improving timing accuracy. Mixing magnet strengths or incorrect polarity creates inconsistent coil charging preventing reliable spark. Clean mounting surfaces thoroughly removing rust before epoxy application—rust prevents proper adhesion causing premature failure.

Breaker-point systems (pre-1982) use mechanical switch (points) opening at precise timing collapsing primary magnetic field. Points require periodic adjustment (0.020-inch gap), cleaning filed surfaces removing oxidation, and condenser replacement preventing arcing across opening points. Magnetron solid-state systems replace mechanical points with transistor inside coil housing—no moving parts requiring adjustment. Conversion kit 394970 retrofits two-leg armature breaker-point engines to Magnetron eliminating maintenance—incompatible with three-leg armatures. Magnetron systems more reliable, require no periodic adjustment, eliminate condenser failures, and maintain consistent timing throughout engine life. Both systems use identical flywheel magnets and armature positioning—only trigger mechanism differs. Pre-1982 engines benefit from Magnetron conversion reducing maintenance whilst improving reliability.

Flywheel permanent magnets rotate past stationary armature core legs creating changing magnetic flux through laminated iron core. Moving magnetic field induces low voltage (approximately 200 volts) in primary winding through electromagnetic induction discovered by Michael Faraday. Magnetron solid-state transistor or mechanical breaker points collapse primary magnetic field at precise timing. Rapidly collapsing field moves through secondary winding at near light-speed generating 60 times primary voltage—producing 10,000-25,000 volts jumping spark plug gap. Primary winding (thick wire, few turns) connects engine ground whilst secondary (fine wire, thousands turns) connects spark plug. System requires minimum 250-350 RPM flywheel speed generating sufficient magnetic field strength activating Magnetron electronics.

Air gap 0.010-0.014 inches positions armature core optimally within magnetic field generated by rotating flywheel magnets. Gap too wide weakens magnetic flux density entering core reducing induced voltage—engine requires faster cranking speeds producing spark. Gap too narrow risks physical contact between flywheel magnets and armature legs causing component damage. Wide air gaps slightly retard ignition timing as magnetic field takes longer building within coil winding. Briggs & Stratton armatures tolerate wider gaps providing coil healthy and engine spins fast enough—gap up to 0.030 inches produces spark but requires higher RPM. Proper gap ensures maximum magnetic coupling between flywheel magnets and armature core maximising voltage generation efficiency.

Hold screwdriver blade (10-inch, 1/4-inch standard blade) against flywheel magnet with flywheel positioned magnets facing upward. Release screwdriver—magnet should attract holding screwdriver horizontally without dropping indicating sufficient magnetic strength. Weak magnets barely hold lightweight tools or fail attracting screwdriver completely. Test magnet positioned opposite from armature avoiding interference. Screwdriver should snap to magnet from approximately one inch distance with firm pull. Modern single-magnet flywheels (post-1960s) use ceramic magnets with north-south pole configuration. Alnico magnets degrade approximately 3% strength per decade—50-year-old magnets lose 15% magnetism. Impact damage, excessive heat from welding near flywheel, or magnet polarity reversal cause magnetism loss preventing spark generation.