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. Always try to use a ECM type spark plug. Meaning NGK BR9ECM or BR9ECMIX IX is just a fine wire iridium type . The IX lasts longer if it stays clean, less energy is used to fire the spark from the coil. The std type plug BR9ECM can be better for slower riders, as it keeps firing even if dirty.
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. Detonation is normally the cause of sparkplug earth electrodes breaking off, high compression presures and internation high frequency vibrations.
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.