How to Repair a Blown Out Spark Plug

The problem, one of my friends was riding his classic motorcycle on the way back home and a spark plug blew right out of the cylinder head. After I received the call I went out to pick him up with my truck. Back at my workshop I looked more closely to the cylinder head and I believe I could repair the thread.

First, there are a number of ways to repair stripped threads. In fact, it may be possible to simply chase the old threads with a tap and clean them up. Or, as I'm currently thinking, I can insert a Helicoil. There are several types of repair inserts, but I personally prefer Helicoils. It's worth trying to fix the damaged thread with a Helicoil. When a spark plug has had a chance to marry a cylinder head for 160,000 kilometers, it's not uncommon for the aluminum threads to come out of the head with the plug. I pull and inspect spark plugs every couple of years and reinstall them with a small dab of antiseize compound, but that's another story.

A proper thread repair should last as long as the life of the motorcycle. This type of repair can be used for almost any threaded fastener, by the way. And that includes cast-iron, steel and aluminum parts. WARNING: Installing a Helicoil or other threaded insert looks simple – but it's not. Any reader out there who wish to attempt it might want to practice a couple of times on scrap parts.
Start by threading the special Helicoil tap into the remaining thread in the cylinder head. This is to ensure the new thread is concentric and parallel with the original. Continue threading the tap in to cut the new, oversize threads. To avoid getting aluminum chips in the cylinder when re-tapping the threads, you should coat the tap with grease. The chips will stick to the grease and come back out with the tap, and clean up any remaining chips.

I've also filled the cylinder (before tapping the hole) with oil-soaked clothesline to catch any chips – but that was in a racing engine with a squish band only a few thousandths of an centimeter deep. Street motorcycle engines with a more normal compression ratio should be fine if you are careful, and blow the chips out the compressed air. Mostly, you don't want any chips to find their way out the exhaust port and wind up in the catalytic converter.

Now you can thread the appropriate-length. There's a raised flat on the mandrel that will catch on the tang in the coil, allowing you to thread the coil into your new threads. The coil is a little big bigger than the thread, which will keep it in place when you're finished. The tang will pull the coil into place from the inner end. Once the coil is in place, remove the mandrel.

Now, use a pair of needle-nose pliers to break the tang off cleanly and easily. Do NOT drop the tang into the cylinder! A few aluminum chips will not damage your engine, but a 1 centimeter long piece of sharp stainless steel wire will tattoo the top of your piston and the combustion chamber before it finally gets out past the exhaust valve (if you're lucky).Tag: Tools Maintenance Thread-Repair Recoil Thread-Insert Repair Technology Helicoils Workshop
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