i don't use O.S plugs , i run Megatech Q series in my engines. the O.S plugs are a thread shorter then other plugs and should only be used in OS engines.. using OS plugs in a RB engine or similar all you are doing is lowering the compression of the engine , make tuning more complicated and will blow plugs more easily especially when your not running the right plus heat VS nitro content VS weather condition.

lower compression will lower the temp of the fuel being combusted plus with the plug not potruding into the combustion chamber like it should it will give a irregular burn of the fuel mixture.