Pro-level spark plug thread repair made easy

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If you haven’t run into this problem yet, it’s very likely that you will in the near future. There seems to be a rash of spark plug thread failures in a variety of later-model aluminum heads. The most common is the 4.6L Ford head simply because some design genius allowed for only three or four threads of engagement (big surprise that failures occur, huh?). I recently ran into such a failure on my wife’s 2001 PT Cruiser, equipped with the 2.4L DOHC 4-banger. Even though the spark plug threads are about an inch long, without warning, the No. 1 spark plug blew out of its home, taking all of the threads with it.

The repair options: replace the head (obviously a big expense and a hassle), remove the head and repair the threads (also a hassle due to the need to mess with the timing belt, engine mount, exhaust manifold, cooling hoses, etc.) or perform the repair in the car. Being a cheap guy, and too busy to yank the head (at least that’s my excuse), I opted to perform the fix with the head in place. My first bit of research led me to the household name in thread inserts.

Much to my dismay, I found that a kit for this application (since the plug threads are buried deep inside the head) wasn’t readily available (severe backorder), and that the inserts are steel, which I would rather avoid when working on an aluminum head. Steel inserts transfer heat too slowly. The parent aluminum will expand as heat increases, moving the aluminum away from the steel insert. This will eventually cause the insert to loosen. Using aluminum inserts, the heat transfer rate is compatible with the parent aluminum.

A call to Goodson Shop Supplies found the proper fix. They offer a really cool spark plug thread repair kit that allows you to properly restore threads either with the head removed or with the head on the engine. Plus, the inserts are solid, thin-wall, hard-anodized aluminum, making them much more reliable with regard to operation in an alloy cylinder head.

The kit (P/N FT-514LTK) is made by Lock ‘n Stitch for Goodson, so there’s no doubt as to the pro level of quality. Plus, the thread inserts feature Lock ‘n Stitch’s Full-Torque external upward-pitch thread design, which provides an incredibly secure level of clamping.


The repair procedure was actually very straightforward, the only pain involves gaining access. In this particular case, it was necessary to remove the intake manifold and throttle body, coil pack and valve cover. It took longer to remove and reinstall the OE stuff than to perform the repair.

The Goodson kit includes everything you need, including a long (9.017″) reamer, a special tap, a centering bushing that keeps the reamer straight in the hole, a seat shoulder cutter, an insert installation tool, a 1/16″ drill and a drill guide tube, several locking pins, a bottle of tap fluid and a bottle of locking compound. Threaded inserts are available separately, depending on what size and length is required for the specific job. In the case of the PT Cruiser, our inserts feature an internal thread of 14mm x 1.25 with an overall length of 1.00″.

After removal of the valve cover, rotate the crank until the piston is well below TDC to avoid hitting the piston dome with the reamer. I moved the piston down toward the bottom of stroke and then inserted a 3/8″ diameter piece of aluminum rod to make contact with the piston dome. I marked the rod at the top of the head’s plug bore and then marked this same distance onto the reamer shank just as a reference to make sure I didn’t hit the piston.

I applied a coating of grease to the reamer flutes to help capture debris. I installed the kit’s centering bushing onto the reamer shank, which keeps the reamer centered to avoid a crooked path. The 0.493″ diameter reamer nose enters the existing spark plug hole. Using a low-speed electric drill, I then reamed the hole open using the reamer’s main 0.640″ diameter cutting body.

Since a small shoulder at the top of the spark plug hole (this is above the actual plug seat) would prevent full insertion of the insert installation tool, I then installed the kit’s counterbore cutter onto the reamer and kissed the shoulder down for clearance.

Once my cutting was done, I vacuumed the bore with a shop vac, necked down to accommodate a 1/2″ rubber hose. This enabled me to easily remove any aluminum debris from the piston dome area. I verified this with my bore scope.

Next, the hole was cut for threads using the kit’s special tap. The tap was coated with the kit’s tapping fluid and was run-in using a 3/8″ drive ratchet and extension. After tapping, I once again vacuumed the bore area.

After applying a thin coat of anti-seize paste onto the male threads of the kit’s installation tool, a threaded insert was threaded onto the installation tool. Five drops of the kit’s anaerobic locking compound were then applied to the insert’s external threads. The insert was then installed into the newly tapped hole, clamped with 27 lbs./ft. of torque. With the insert still in place, I inserted the small guide tube into the offset hole in the insert tool. I then chucked the kit’s 1/16″ drill bit into an electric drill, set to run at about 2,500 rpm. The guide tube keeps the drill bit straight, preventing flex that could easily snap such a small-diameter bit. The drill bores a 1/16″ hole into both the edge of the insert and the adjacent parent aluminum. After drilling, use compressed air to blow out any chips or use a vacuum to suck chips out of the tube.

With the drill bit removed (but with the guide tube still in place), I applied three drops of locking compound into the guide tube and dropped a locking pin into the guide tube. With the 1/16″ drill bit removed from the drill, I turned the bit upside-down and used the butt end of the bit to tap the pin fully into place. The pin will likely drop fully anyway, but using the bit as a ramrod, you’re assured that the pin fully bottoms out. Note: The locking compound is heat-conductive and will fully cure after exposed to operational temperature but it will still begin to set up quickly, so once the pin is fully inserted, remove the guide tube, and clean it and the drill bit using a solvent. At this point, the insert installation tool is removed.

After applying a thin coat of anti-seize paste to the threads of a new spark plug, the new plug was installed and tightened to 26 lbs./ft., instead of the OE value of 18 lbs./ft. While I was at it, I also replaced the three remaining spark plugs, tightening all to a higher value of 25 lbs./ft. Luckily, the remaining three plugs held this torque. I figured that if the threads failed, I’d simply repair any additional offending plug holes, but I got lucky.

That’s it. The entire procedure took about an hour-and-a-half, with about an hour of that accounted for by removing and reinstalling the valve cover, manifold, etc.
The daily driven PT Cruiser has been back in service for about four months now with no problems at all. The Goodson kit proved to be a real lifesaver for those spark plug thread repairs that need to be handled in a hurry. And although boring and tapping with the head on the engine might seem scary at first, with a little care and common sense, it works out just fine.

It’s important to note that this is a permanent repair and is not a cheapo Band-Aid fix. In fact, the insert provides a vastly superior thread as compared to the OE setup.

Thank you very much. The DOHC 2.4L head in this 2001 PT Cruiser spit the No. 1 spark plug without warning, taking the threads along for the ride. We suspect this resulted from poor thread quality coupled with a relatively low spark plug torque value. It appears that cylinder pressure worked its magic on the threads, finally causing the threads to fail.

Due to the over-chamber design, it’s a long reach from the valve cover down to the plug threads. Removal of the valve cover allows better access.

Even with the cover removed (which requires removal of the intake manifold and coil pack), the plug threads are really buried down at the bottom of deep plug bores, so long tooling is needed to perform the job.

Goodson to the rescue. The spark plug repair kit (P/N FT-514LTK) includes everything needed to perform the fix. The kit is universal for a variety of heads, requiring only the proper-length inserts for any specific application. Inserts of all popular lengths are readily available.

In order to bore out the existing spark plug hole, the kit includes a long two-step reamer. The small nose indexes into the existing hole, while the larger-diameter cutters will increase hole diameter in preparation for tapping new threads.

The reamer is a healthy 9.017″ long, which means that you won’t struggle to clean out the entire depth of the plug hole. The pilot end diameter is 0.493″ and the final reamer diameter is 0.640″.

A handy centering adapter is slipped onto the reamer shank. This adapter is shouldered, allowing the lower portion to enter the top of the hole, while the adapter’s larger diameter shoulder rests on top of the head. This allows centering alignment of the reamer to prevent the reamer from working at an off-angle.

While not needed in every situation, the kit’s seat boring tool can be affixed to the reamer to cut a new seat to ensure that the thread insert’s installation tool bottoms out, ensuring proper installation depth of the new insert.

Closeup of the reamer centering bushing.

Closeup of the seat cutter.

The inserts required for this job (P/N FFT-514125LA) feature a 14mm x 1.25 internal thread and an overall length of 1.00″. The inserts are made of hard anodized aluminum instead of steel. This makes the inserts compatible with an aluminum head. The external threads feature a unique pitched thread made by Lock ‘n Stitch that creates a strong, no-back-out installation. The inserts include 1/16″ locking pins, as well as added insurance against future insert back-out during any future spark plug service.

The aluminum thread inserts feature a built-in spark plug seat.

Once the damaged plug hole has been reamed out to size, a special Full-Torque cutting tap creates the threads that will accept the insert.

The insert installation tool features a small offset hole that allows drilling into the edge of the insert and its adjacent cylinder head wall material for installation of a locking dowel pin.

A guide tube provided in the kit inserts into the installation tool’s offset hole. This allows use of the kit’s 1/16″ drill without fear of drill bit flexing and breakage.

This closeup shows the drill passing through the guide tube and tool, entering the top of the threaded insert.

The kit’s anaerobic locking compound is applied to the external threads of the insert and to the 1/16″ locking pin.

A special tapping fluid is also thoughtfully provided in the kit.

Even though we applied grease to the reamer to help prevent chips from entering the chamber, after reaming and tapping, we vacuumed any remaining aluminum chips using a shop vacuum, necked down to a 1/2″ rubber hose that we inserted into the cylinder. We inspected the bore using a bore scope, assuring ourselves that all debris had been removed.

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