How To Repair Stripped Spark Plug Hole

Media Platforms Design Squad

Spark plugs that haven't been inverse for a long fourth dimension can go one with the cylinder caput. Fortunately, it's possible to repair damaged threads instead of scrapping the head. (Photograph by Chris Eckert / Studio D)

Q: Yesterday I was driving home and a spark plug blew right out of the cylinder head. The car was towed to the shop where I'd had a tuneup the solar day earlier -- which included changing the spark plugs. The mechanic said he could repair the threads, only my uncle says the cylinder head is ruined and the mechanic should supplant it at his expense. The automobile has virtually 100,000 miles. Thoughts?

A: There are a number of ways to repair stripped threads. In fact, it may be possible to merely chase the erstwhile threads with a tap and clean them up. Or, as illustrated, you lot can insert a Helicoil. In that location are several types of repair inserts, but we adopt Helicoils. I'd give your mechanic a shot at fixing the threads before I held his feet to the fire. When a spark plug has had a chance to marry a cylinder caput for 100,000 miles, information technology's non uncommon for the aluminum threads to come out of the head with the plug. (I pull and inspect plugs every couple of years and reinstall them with a small dab of antiseize chemical compound, but that's another column.)

A proper thread repair should last every bit long as the life of the auto. This type of repair can be used for almost whatever threaded fastener, by the way. And that includes cast-iron, steel and aluminum parts. Alarm: Installing a Helicoil or other threaded insert looks simple -- but it'southward not. Any readers out in that location who wish to attempt it might desire to practice a couple of times on fleck parts.

Showtime by threading the special Helicoil tap into the remaining threads in the caput. This is to ensure the new threads are concentric and parallel with the originals. Continue threading the tap in to cutting the new, oversize threads. To avert getting aluminum fries in the cylinder when retapping the threads, y'all should coat the tap with grease. The fries will stick to the grease and come back out with the tap. Back out the bit-laden tap, and clean up any remaining chips.

I've besides filled the cylinder (before tapping the hole) with oil-soaked clothesline to grab any fries -- just that was in a racing engine with a squish ring only a few thousandths of an inch deep. Street engines with a more normal compression ratio should exist fine if y'all are careful, and blow the chips out with 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 tin thread the advisable-length curl over the installation mandrel. There'southward a raised apartment on the mandrel that will take hold of on the tang in the coil, allowing yous to thread the coil into your new threads. The coil is a niggling bit bigger than the threads, which will go on it in identify when you're finished. The tang will pull the coil into place from the inner cease. Once the roll is in place, remove the mandrel.

Now, use a pair of needle-nose pliers to suspension the tang off. It'due south prescored to suspension off cleanly and easily. Do NOT drib the tang into the cylinder! A few aluminum chips will not harm your engine, but a i/2-in.-long slice of precipitous stainless steel wire will tattoo the tiptop of your piston and the combustion chamber before it finally gets out by the exhaust valve.