Understanding Aluminum Heads and Spark Plugs

Aluminum Heads and Spark Plugs

Spark Plug seizure in an aluminum cylinder head is usually caused by improper installation and too much torque.

Aluminum alloys are softer than cast iron. Different plug shell and head materials cause these metals to expand and contract at different rates.

When the engine is hot the threads of the plug base distort somewhat. When they cool down again, the head and plug thread tolerances are back to their normal dimensions. You should not remove or install spark plugs when the heads are hot!

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, small particles begin to have a snowball effect (bead up) 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 spark 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.

Plug Tightening Tips

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