Craftsman recommends changing the spark plug on their 6.5 HP mowers every 25 hours or the beginning of each mowing season. They also recommend using the Champion RJ19LM spark plug when replacing the plug. Adjust the gap between the insulator and electrode with a gap tool. The simplest method is to use a disc gap tool with graduated gap measurements. Insert the gap tool between the electrode and insulator to the necessary gap range and bend the electrode up or down to meet the tool.
Place your Craftsman lawn mower on level ground. Grab the spark plug wire boot with your fingers and twist counter-clockwise, then pull the spark plug wire boot off the spark plug. The twisting motion will break any seal created by heat.
Remove the spark plug with a socket wrench. Check and adjust the gap on the new spark plug to .030 with a gap tool. You can purchase a simple round gap tool when you purchase the spark plug.
Thread the new spark plug into the engine crankcase and tighten the spark plug with a socket wrench. Place the spark plug wire boot over the plug and push in with your thumb until the boot locks over the plug.