From 1988, the specification for the NSR250R changed to a newer "short" resistor-type spark plug, for improved front wheel clearance due to the revised chassis geometry.
For the MC16 model, use NGK B9ECS or B9ES spark plugs. [Recommended alternative: ND W27ES-C]
For MC18 onwards, use NGK BR9ECM. [Recommended alternative: ND W27EMR-C]
Gap [all types]: 0.7~0.8mm
When using the HRC head conversion on MC21 and MC28 models, use the HRC RS250R “shorty” #10 spark plug 31930-ND5-003 [NGK R5300A-10] or #10.5 31940-ND5-003 and special HRC RS250R cap 30700-ND5-751.
NSR-WORLD.COM recommends the use of a #9.5 plug [R5300A-95] for street use.
Before fitting new spark plug cap, trim 5mm from the end of the HT lead.