RIDEX spark plug (686S0003) fires the air-fuel charge with a precisely controlled spark at the correct electrode gap and heat range for your engine's combustion chamber geometry. Plug parameters — Electrode Gap [mm]: 0, Carbon mm, Electrode Gap [mm]: 0, Stainless Steel mm, Thread Size: M 14 x 1, 25, External Thread [mm]: Nylon mm — determine ignition timing accuracy, electrode longevity, and resistance to pre-ignition deposits across the full RPM range. Thread reach, seat type, and hex size are specified to fit ALFA ROMEO, ASTON MARTIN, AUDI, AUSTIN and 63 other makes cylinder heads without re-gapping or thread chasing. Replaces OE ignition references: 002 159 77 03, 003 159 30 03, 003 159 37 03, 003 159 42 03 and 175 more. RIDEX ignition components are manufactured to SAE and DIN specifications and are approved for use in vehicles covered by manufacturer service agreements throughout Europe. Standard copper-core spark plugs last around 20,000–30,000 miles, while iridium or platinum-tipped plugs can last 60,000–100,000 miles — always follow the interval specified in your vehicle's service schedule. Symptoms of worn spark plugs include rough idling, engine misfires (often felt as a stumble under acceleration), increased fuel consumption, and difficulty starting in cold weather. Spark plug replacement is a straightforward DIY job on most engines — you need a spark plug socket, a torque wrench, and a gap gauge; allow 30–60 minutes for a four-cylinder engine, longer for boxer or V-configurations where access is tighter. Always match the heat range and electrode material specified by your engine manufacturer — fitting a plug with the wrong heat range can cause pre-ignition, fouling, or catalytic converter damage. Replacing spark plugs on schedule maintains optimal combustion efficiency, reduces harmful emissions, and prevents unburnt fuel from damaging the catalytic converter — a component that costs many times more to replace. Every part is dispatched with tracked delivery and a 14-day money-back guarantee with hassle-free returns.