The inside of a spark plug is less than exciting—so stay out of there.

Dear MXA,
My local shop mechanic suggested that I run a hotter plug in my well-used 2006 Suzuki RM125. I nodded, but after he walked away I realized that I didn’t know what he was talking about. I currently have an NGK B9ES plug. Is that hot?

The heat range of a spark plug is indicated by the number in the plug’s identification sequence. On NGK spark plugs, the lower the number the hotter the plug (higher numbers are colder). Thus, your NGK BR9ES is colder than a BR8ES. There are spark plug brands where the numbering system is reversed and the lower numbers are cold and higher numbers are hot, but not that many motocross racers use these plug brands (Champion, Autolite and Bosch). It should be noted that the terms “hot” and “cold” do not refer to the temperature of the plug’s actual spark but the speed at which the plug transfers heat from the combustion chamber to the engine head.

Autolite plugs have the opposite heat ranges of an NGH plug.

When a spark plug is referred to as a “cold plug,” it is one that transfers heat rapidly from the firing tip into the engine’s head. The rapid heat transfer keeps the firing tip cooler. A “hot plug” has a much slower rate of heat transfer, which keeps the firing tip hotter. The difference from one full heat range to the next is the ability to remove 158-degree to 212-degree Fahrenheit from the combustion chamber. The primary factors that affect a spark plug’s heat range are the internal length of the core nose, the alloy compositions in the electrode and the amount of porcelain insulation.

We don’t know what your mechanic saw in your engine, but most likely it was carbon build-up, soot or combustion deposits on the plug or the combustion chamber. It should be noted that the 2006 RM125 came with a very expensive NGK R6918B-8 spark plug. Although it was a better engineered plug, it didn’t necessarily make the engine run any better. It was really spec’ed on the RM125 because there were issues with the electrode strap breaking off on the standard-issue BR9ES plugs. In 2006, MXA ran an NGK BR9EVX plug. We had no issues with the BR9EVX’s electrode strap, but we changed the plug at regular intervals.


ask the mxpertsAutoliteBoschBR8esbr9eschampionNGKNGK R6918B-8 spark plugspark plugs