Some engines 85cc 125cc 150cc 250cc need to use more stronger/robust type spark plugs.
If the engine has higher internal pressures, excessive crankcase stuffing, too high compression ratios, incorrect crankshaft balance factor, Detonation, engine frequency type vibrations/pulses can harden and weaken the earth electrode and it will FALL OFF causing expensive damage.
Some modern engines are already made high performance from the factory and pressures are really high and they need better spark plugs. Some 85cc and most larger 2 strokes will need these plugs. part number NGK R6918B-9 is one of the best choices. $60.00 each. Replaces BR9EIX or BR9EG or BR9EV or BR9ES etc. ( 9 ) is the heat range ,so you can buy 8,9,10 heat ranges normally.
Be careful, if the bike comes from the factory with a R6918 type plug,(like the current model Suzuki RM125) dont put a cheaper plug in as you will reduce performance and damage the engine.