BMW R 18 Spark Plug Replacement Guide

The BMW R 18 uses a NGK MAR8AI-10DS spark plug. A worn plug causes hard starting, rough idle, and poor fuel economy — and it’s one of the cheapest parts on the bike to replace.

Inspect the plug every service and replace it at the factory interval or sooner if the electrodes look rounded or the insulator is heavily discolored.

BMW R 18 quick service specs

How to replace the spark plug, step by step

1. Let the engine cool completely. Remove the seat/tank panels as needed to reach the plug cap on the cylinder head.
2. Pull the plug cap straight off. Blow any dirt away from the plug well with compressed air so nothing falls into the cylinder.
3. Remove the plug with a spark plug socket. Note how far it was threaded in.
4. Check the gap on the new plug with a feeler gauge and set it to the spec in your owner’s manual.
5. Thread the new plug in by hand first to avoid cross-threading, then torque to spec (or snug plus the turn specified on the plug box).
6. Reinstall the plug cap firmly and refit any bodywork.

Common mistakes to avoid

• Cross-threading the plug — always start it by hand.
• Over-torquing — snug is enough; too much cracks the insulator.
• Dropping the plug — a dropped plug can close the gap or crack the ceramic.

FAQ

What spark plug does the BMW R 18 take?

A NGK MAR8AI-10DS. Stick with the stock heat range unless you have a specific, well-understood reason to change it.

Can I use iridium plugs?

Yes — iridium lasts longer and fires a bit more consistently, but gap it to the same spec as stock.

What does a fouled plug look like?

Wet black deposits mean rich running or oil; dry sooty black means rich mixture or weak spark; white blistered means too hot/lean.

Related guides

• Motorcycle Maintenance Schedule: The Complete Service Guide
• BMW R 18 Engine Oil Change: Oil Grade, Capacity & Interval
• How to Clean and Lubricate the BMW R 18 Drive Chain
• BMW R 18 Chain Slack Adjustment: Correct Spec and Steps