There are some rotors out there for sale that are too tall (center area too high) and some caps with the center electrode tower area too long. If you use them together they will rub and make a red powder/dust that will cause a short in the cap and an engine miss. If you use them one at a time with the correct counter part (just replace the rotor and not the cap) you are ok. Key to do here is to compare to your old ones before you replace them.
Also, once you have replaced them just watch the cap as someone cranks the motor over. If they are hitting (the rotor and cap) the cap will wiggle some as the engine spins over. A good tell-tale sign of trouble to come! Good luck………..tom