yes, 8mm works. You just have to "shave" a little off the outer rubber to get it to go into the hole. No biggie though.
i replaced my plug and not much difference at all. the spark plug wire i changed and it made a huge difference
Where did you get the "spark plug wire"? Do you mean the single distributor cable that connects to the kill switch? Just asking because I bought a second hand engine with no spark plug wire?
Found a spark plug wire for a royal enfield on ebay, think it'll fit? type in ROYAL ENFIELD SPARK PLUG WIRE ASSEMBLY £3.95 plus £1.99 p&p should do the job dontcha think?