This is a brief summary of the Champion Spark Plug instruction for connecting the spark plug to the connector:
Terminal sleeves should be handled only with clean, dry hands. Before installation, wipe off the connector with a clean, lint-free cloth moistened in methylethylketone, acetone, wood alcohol, naptha or clean unleaded gasoline. Make certain that the inside of the spark plug shielding barrel is clean and dry. Then, without touching the connector or spring with the fingers, insert the assembly in a straight line with the spark plug. Screw the connector nut into place finger tight — then tighten an additional 1/8 turn with the proper wrench. Damaged threads or cracked shielding barrels may result if the connector nuts are tightened excessively. Avoid excessive side load while tightening.
By comparing the two spark plugs in this picture, you can see the results of overtorquing at the distorted end of the left spark plug. Since this distortion may make the spark plug unusable, great care should be taken to tighten the connector in accordance with the manufacturer’s instructions.