Machining of aluminum alloy leaves a rougher, less uniform surface compared to cast iron.  As a result, particles of head material (or foreign matter) may cause a cutting condition of aluminum heads.  Then when force is applied to screw in the plug, the small particle (or particles) begin to have a snowball effect (bead) and damage the threads.  This action is called particle beading.  Any plug that begins to bind during installation is very likely to be cross threading or beading.  When installing a plug and it starts to bind up stop and remove the plug and look at the threads.  If the threads appear distorted or beaded, run a thread chaser through the head boss to restore the threads before installing the spark plugs.

     Over torquing a steel shelled plug into an aluminum alloy head can easily distort the aluminum threads; you should never over torque when installing spark plugs.  Over torquing can also cause plug overheating and pre-ignition will result due to the loss of proper heat transfer through the gasket (or taper) seal.

1. On Gasket seated plugs, never exceed 20 ft/lbs.
2. With socket and wrench all you need is a quarter turn (90 degree) past finger tight.
3. On taper seated plugs, all you need is to tighten 1/16 of a turn after being finger tightened.
4. If threads have been damaged during the plug removal, it will feel as if the gasket on the new plug (or the tapered seat) has seated properly when in fact the plug threads have seized on the way in.
5. If plugs are not replaced at the factory recommended mileages, the polug threads can freeze and when you try to remove plug it will strip out the threads in the head.