OS #8 plugs are way too short for .21 engines., look for something like the RB#6,#5 standard plug.. racers edge & McCoy also have standard plugs..
No not really, maybe for some but not all. Some engines suit short plugs & some suit long plugs. The Alpha RTR engine I had suited short plugs like the O.S #8 etc, long plugs like the O.S LC3 & 4 are to long for some engines & protrude into the burn room a small amount. (Yes, I tested it with a short & long plug on the Alpha rtr engine). Also, the short plugs in an engine like the O.S TM21 are to short, the long O.S LC3 & 4 plugs were made for them & sit level with the burn room. (Tested that too).
Best way to check is take the cooling head & burn room off the engine & try some different plugs to see how they fit, they shouldn't protrude past the threads into the burn room, they should sit level so the fuel flows evenly around the burn room.