Verified answer

The "r" in the designation means it is a "Resistor" spark plug. Some ignition systems are designed to operate w/ this type of plug. The resistor is most often made of ceramic and has around 5K ohms resistance. Resistor plugs are used to reduce "white noise" in the ignition system. This reduction of noise is needed for most engines that have an electronic control module for the ignition. Your lawn mower has a magneto type ignition system. Magnetos generate their own electricity therefore a battery is not needed. The control module on most of the newer lawnmowers and other yard equipment is a solid-state electronic device. These electronic controls replace the older style breaker-points which needed to be replaced from time-to-time. Replacing a resistor type plug w/ a non-resistor type plug can cause your ignition to malfunction or fail. Also note that not all spark plug manufacturers put a "R" in the plug designation number. So always use the specified plug type in any engine. All auto parts suppliers have a conversion table to convert a plug's designation from one plug manufacturer to another plug manufacturer. So it's OK to use a different brand than the Champion brand. But it is not OK to change plug types. As another Answerer pointed out, each plug is designed to operate in a specific heat range. Plus each plug will have a different "reach". The reach is how far the plug sticks into the cylinder. There is also the tip "reach" which is how far the firing electrode protrudes from the plug. Resistor type plugs were first used in vehicles to reduce interference in the audio equipment. Now they are used primarily to reduce noise interference in electronic control modules because modern audio equipment has their own noise reduction circuits.

Give your lawnmower a real treat. Toss the Champion RJ17LM for the NGK same heat range replacement - BR4LM.

the difference is in the ,r,its a resistor plug and the other one isn't,they both will work good in it,one will cause interference small objects such as radios and the other one wont,and one will last longer,the ,r, in any plug means Its a resistor plug,good luck,

I totally agree with d it is the heat range of the plug

you wouldn't see the diff with the naked eye...but the diff is in the "heat" of the spark and how efficient it burns all the fuel in the cylinder...too weak a spark and you will build up gunk on the vavles...good luck

or the r could simply represent a resisted type of plug. this is done to minimize the rf interference to the local radio and tv reception.