Spark Plug Heat Range

I recently posted a question to the Triumph Mail List about the various heat ranges for spark plugs. Within an hour I had my answer from a number of folks that I'd like to share with everyone. I also found this cool site  that helps you to read plugs. I didn't know that there are marks left on your plugs to indicate the correctness of your timing.

From Andy Mace:

RN7YC; now Champion 332
RN9YC; now 415
RN11YC4; now 322 (also given for N10Y)
RN12YC; now 404
RN14YC; now 405

Andy referenced where I pulled the table below showing the Champion naming convention for their plugs. The higher the number, the hotter the plug.
The table can be found at (

Jay Welch explained that NGK plugs go like this:

NGK BPR5ES = hot
NGK BPR6ES = stock
NGK BPR7ES = cool
NGK platinum plug = BPR6EVX

He also added that the Triumph 6 cylinder engine uses a 14mm with .750 reach.

Randall Young provided he usual thorough explanation:

For almost any given brand of plug, the letters determine all the plug physical dimensions, while the number is the heat range. Unfortunately they differ between brands as to whether a higher number is hotter or colder. For Champion, higher numbers are hotter. The Champion chart I have lists the following plugs, in order from hottest to coolest :


The trailing 'C' indicates Champions "Copper Plus" line of plugs, which replaced the 'standard' N9Y. The leading 'R' indicates an internal resistor, for EMI suppression (and where I've written R/N it means the plug is available both ways). Randall also shared a good site at NGK about plugs.