Helicoil Repair KitDear Ole Mechanic;

My kid was trying to do some maintenance on his little school car by changing the spark plugs and wires.  I told him to let it cool down before he started on it, but you know how impatient kids are today.  When he got done and fired it up, it immediately blew two spark plugs out of their holes.  He came to me with them and asked why they wouldn’t stay in.  To make a long story short, when he removed the old plugs, the aluminum threads in the head stuck to the spark plug threads and stripped the holes.  There was just enough ragged aluminum to sort of hold the plugs as they were put in, but not enough to hold them when the engine started.

The car is old enough that I really don't want to sink a bunch of money into buying and replacing the head.  Is there a cheaper way to patch it up?  Why did the threads strip out when he pulled the plugs?  How can I keep this from happening again?  Thanks

No Big Repair Bill


Dear Bill;

There are a couple of things that could have caused the threads to strip out--one of which could have been prevented and one that could have only been prevented when the old plugs were installed.

You actually hit on the first one when you told your son to let the engine cool down before working on it.  When aluminum is hot and expands, the threads expand and really grab a hold of the steel spark-plug threads.  Also, the aluminum is not as strong when it is hot as it is when it is cold.  If you try to remove the spark plugs when the head is hot--or even warm--you stand a very good chance of removing the aluminum threads, too.  Let an aluminum head get cold to the touch before you try to remove the plugs--or any bolt in the head, for that matter.

The other possible reason for the threads to be stripped is the corrosion that forms between two dissimilar metals--like an aluminum head and steel spark plugs.  If the person that installed the spark plugs did not use an anti-seize compound between the steel and aluminum threads, then you may strip out the threads no matter how cold the head gets.  It is just more likely to occur when it is hot.  An actual anti-seize compound is not needed, as a light coat of grease will usually prevent corrosion between the two dissimilar metals.

Now for the big question: “Is there a way to repair the spark plug threads without replacing the head?”  Yes, there is!  It is called a HeliCoil® thread repair kit.  The correct kit will include an oversized thread tap and a stainless steel insert that is actually threaded into the oversized threads and allows the spark plug to thread into the stainless steel.  Most auto parts stores either have these kits in stock or can order one kit for you.

Care must be taken to make sure that the new threads are cut into the head at the same angle as the old ones.  If you get them in crooked, then it may be possible for moving engine parts, like the valves or the piston, to hit the spark plug--causing more damage.  Also, as you are cutting new threads, chips of metal are produced and these must not be allowed to get inside the engine.  The easiest way to prevent that is to coat the HeliCoil® tap with grease.  The chips stick to the grease and don’t fall into the engine.  Also, there is a tang that is used to install the stainless steel threads that must be removed.  Again, make sure that the tang is not allowed to fall inside the engine.

That is the good news.  Even if you have your favorite mechanic install the HeliCoil® thread insert, it will be much cheaper than buying and installing another head--either new or used.

Herr Professor Nuzanbolts