According to Polaris, the NGK BPR9ES and NGK BPR9EVX spark plugs are acceptable substitutes for the Champion RN57YCC and RN57YC spark plugs now found on Polaris engines. The NGK plugs seem to have better sealing gaskets that reduce seepage anytime the plugs have been removed and reinstalled. The ”EV” is a premium fine-wire electrode version of the “ES”.
Which brings to light another question, what is the difference between the Champion RN57YC and RN57YCC? According to Polaris, the RN57YC is also an acceptable replacement for the more commonly used RN57YCC, and that engine damage will not result from this substitution. The CC plug has a longer copper ground electrode for higher heat conductivity, with more coverage (longer ground electrode) for increased plug life, and reduced RFI (radio frequency interference). The CC plug is the better choice, but you can run the single-C version if needed.
It is also generally accepted to substitute the new NGK BR9ECS spark plug for all BR9ES applications. The ECS plug is now common issue on most if not all Ski-Doo two-strokes. This plug has a conical shape base which can allow for a shorter center electrode and a shorter ground electrode in comparison to the old standard BR9ES. The benefits of this are a reduction in pre-ignition and detonation by dissipating heat more efficiently via the electrode and conical shape base, along with a reduced chance of vibration breakage with the shorter ground electrode. And, they cost about twice what the BR9ES does.
The one substitution that is NOT recommended is to use the NGK BR9EYA in anything other than applications that specify this plug. This spark plug is primarily found in Arctic Cat two-stroke applications.