NGK CMR6H Spark Plug

The NGK CMR6H spark plug is a compact 10 mm resistor plug used in many small handheld engines, including trimmers, blowers, pole saws and small chainsaws. Its small size suits the tight cylinder heads of lightweight engines.

What the CMR6H code means

The cmr6h is part of NGK's compact plug family. The C means a 10 mm thread with a 16 mm hex. M marks the compact design. R means it has a resistor. The 6 is the heat range, and H indicates a 1/2 in (12.7 mm) thread reach.

Compact plugs like the ngk cmr6h are made for small engines where there is not much room in the cylinder head. They are common on trimmers, blowers, edgers, pole saws and small chainsaws. Some small four-stroke handheld engines use them as well.

Finding the right plug

Always check your operator's manual or the plug you are replacing. Thread, reach and heat range all need to match. The CMR6H and the CMR7H look identical but have different heat ranges. In NGK's system a higher number is a colder plug, so they are not interchangeable unless your manual lists both.

Searches for cmr6h cross reference usually come from people trying to match a plug from another brand. Cross-reference charts are a starting point, but heat ranges do not always line up perfectly between brands. When in doubt, use the plug the engine maker specifies.

If you run several small machines, label which ones use which plug. Crews often run trimmers, blowers and pole saws from different makers, and the plugs look alike. A small chart taped inside the parts bin saves mistakes.

Installing a compact plug

Small plugs strip threads easily, and many small engines have aluminum heads. Thread the cmr6h spark plug in by hand first. If it does not turn smoothly, back it out and start again. Snug it with the correct 5/8 in plug wrench, following the torque guidance in your manual, and do not over-tighten.

Check the gap with a wire gauge against your manual before installing. Make sure the plug boot snaps fully onto the terminal. A loose boot causes misfires that feel like a fuel problem. Always replace a plug with the engine cool and the plug wire or boot removed, and never test for spark by holding the plug near the open spark plug hole. Fuel vapor can ignite.

Signs it is time for a new plug

Hard starting, rough idle, misfires and loss of power are all signs to check the plug. Look at the tip: heavy carbon, oil fouling or a worn electrode all mean it is time to replace it. Because small engines are sensitive, a fresh plug often makes a bigger difference than you would expect.

Plugs are cheap, so keep a few on hand. If your engine calls for the colder version, see the NGK CMR7H, or browse more power equipment parts.

Keep spares in their packaging so the gap stays set and the electrode is protected until you need it.