RC51 Spark Plugs (all are Iridium plugs) From Rogue site
FR9BI-11 (00-01 NGK)
IK27C11 (00-01 NIPPON DENSO)
IFR9H11 (02-03 NGK) 31919-MEB-671
IK27PRZ11 (02-03 NIPPON DENSO) 31919-MEB-672
From Honda Parts Catalog listed in a on online- discount parts Cycle-Parts.com (part number 19 on Front Head)
19 SPARK PLUG (IK24C11) (DENSO) (OPT) 1 $39.56
19 SPARK PLUG (FR8BI-11) (NGK) (OPT) 1 $11.63
19 SPARK PLUG (IK27C11) (DENSO) 1 $25.69
19 SPARK PLUG (FR9BI-11) (NGK) 1 $11.63
19 SPARK PLUG (IFR8H11) (NGK) (OPT) $13.69
19 SPARK PLUG (VK24PRZ11) (DENSO) (OPT) $12.14
19 SPARK PLUG (IFR9H11) $17.80
19 SPARK PLUG (IK27PRZ11) $10.23
The 24 is a hotter plug and corresponds to an NGK 8 and your manual is listing this as a possible winter riding plug, which translates to only use it if you are fouling plugs in the winter. The IFR is so rare that when you plug it in Google only RC51 and CR450xs come up!
NGK - ND (Nippon Denso, not Denzo) Heat Range
7 - 22
8 - 24
9 - 27
The 11 at the end is for the gap, they are pregapped to 1.1mm (1.05-1.15) and should not be manually gapped or may be damaged.
I wondered what's the difference from IK to VK and they are both Iridium but different lines (Iridium & Iridium Tough, there is also I.Plus and I.Racing in ND plugs) From ND's site:
IK16. IK20. IK22. IK24. IK27. IK31. IK34. IK16G. IK20G. IK22G. IK16L. IK20L
･ Mainly used for cars
. ISO type. ･ 14mm dia. X L19mm length type. ･ IK22 and above are for tuned engines, with a spark gap of 0.8mm. ･ IK16G, IK20G, and IK22G have a solid, stainless steel gasket (mainly used with Honda engines). ･ IRIDIUM TOUGH
VK16, VK20, and VK22 are also on sale at the same time. ･ IK□L is extended type (spark position 5mm)
VK16. VK20. VK22. VK20Y. VK16G. VK20G. VK22G
･ Mainly for automobiles
and trucks ･ ISO-compatible small plug ･ Mounting base: 14 mm dia. X L19 mm ･ VK16G, VK20G, and VK22G have stainless steel gaskets. ･ VK20Y has a gap of 0.8 and is mainly for use with turbo engine vehicles
If you are in the US go to Advance or AutoZone and get your plugs at around $9-11, you may also find equivalents to Honda oil filters in Purolator numbers as well as other brands at fraction of cost.
Lifted from the internet someplace !