thanks for the input guys. i did some more research and found that the washers are actually crush washers that collapse to make a tight seal so stacking them is not a good idea. i was just desperate to get another plug when the one on my lightning went bad because i wanted to run the engine dry since i was about to put it away. ill make sure i have extras on hand from now on. btw heres info from the traxxas forums that may be useful to people:

Crush gasket
New plugs include a copper washer to achieve a proper seal. They are designed to be installed with the concave side facing down, like an upside-down bowl. When the plug is tightened, the soft material flattens slightly and provides a strong seal. Never install the washer with the convex side down; it will deform and leak. It is also important to re-install a used washer the same direction as before; although it may look compressed, the side with the imprinted ring should always face up.