oh just an FYI b8hs is not a hot plug. 8 series is the coldest you can get from NGK. and a stock CDI is most often not the reason for vibration. cranks are notorious for not being balanced to the rod and piston the motor is later assembled with,, then there is a whole rash of bevel gears not being really centered. all contributes
BS!!!! (though i do agree about the CDI, but wheres the BEVEL gear? you mean helical/spiral right?)
ngk b4hs. as hot as they get (or is that cold? i always mix em up!)
ngk b10hs. didnt see an 11, but i do know theres a b11hIX... iridium. coldest i can see in the B thread range, at H length.
4-10 in the bxxhs series. seems to be a wider range than the 5 to 8 that you suggest.
if you REALLY want a hot plug...
champion sells one, called an "EZ start" (mostly for lawnmowers...). it equates to an NGK of 2! vague memory... j19mIH. dont quote me on that.
i dont recommend it unless you really need to now what excessive pre-ignition feels like.
as i said before, i avoid champion. they have this completely illogical coding system, like they just randomly assigned numbers to any old plug. and they fail prematurely in most engines.
NGK. uses a logical and sensible number system.
NGK! make aero grade plugs.
NGK!!!!! best damn plug in the world.
as for length...as long as its a H, or equivalent, with a thread of 12mm...it will be fine.
or did you get a BPXXHs? the P stands for "projected tip". this can cause issues with piston contact in SOME, not all, engines. if it turns over by hand, its fine.