TedSander is correct in stating if the cap is not all the way down on the plug, it will create RF noise sufficient to overpower most 2.4 gHz receivers. If not fully seated, there will be two sparks, one across the gap from the cap and top end of the sparkplug and another spark at the electrode. No, you will not hurt the ignition system by grounding the cap to the engine. It is designed to work that way. The high tension lead carries the charge to the spark plug center post and the cap (touching the spark plug hex wrenching portion) and braided outer sheathing completes the circuit back to the ignition module. You will most likely notice there is no separate ground wire back to the ignition module, although some older modules used a plain rubber high tension lead and had a braided cable that had to be clamped onto the spark plug's body bolt. The quickest way to fry the ignition module is to energize the module and turn the prop without a sparkplug fully seated in the cap.