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Unread 08-08-2015, 03:43 PM   #1
GlenL
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Default Stripped the spark plug threads!

First off... I used the 3/8" bending beam torque wrench and always have. I'll take them to 25 Nm as that's where the sealing rings stop yielding. At 20Nm it keeps moving.

Stripped out a spark plug on my Euro S with the rare and expensive heads. Not replacing that and it seemed like a simple enough job to use a rethread kit to fix...but which kit?

The Heli-Coil kit looks good. It doesn't have the typical Heli-coil coil replacement but has a threaded tube that the other kits have. I like the self-aligning reamer/tap. The sleeve's are "straight" with knurling at the top that get's expanded using a driver. My concern is the sealing approach with high-temp RTV.

There's a kit from NAPA that seems good enough. The reamer/tap isn't self-aligning, though. The sleeve has a lip around the top to seal with the head but it doesn't get inset into the surface. The sealing ring for the plug won't add to the sealing between the head and sleeve. The kits comes with some special sealant and the instructions say the engine is "ready for service" immediately after installation. RTV take 24 hours. I don't know what's in the tube.

The best-looking kit is the Time-sert. Of course, it's over $200 and the others are sub-$40. This has the self-aligning reamer/tap. The sleeves have a lip at the top and the kit includes a tool to ream out a groove in the head surface so it's inset. It'd take some skill to get that just right but it'd seal to the block and then let the sealing ring on the plug seal it all up.

Time-sert looks like "the best" but also the most money and not readily available. The Heli-Coil and NAPA kits are available at nearby parts stores.

Well, it could have gone better.

I got the Heli-Coil kit and set to it. I tucked a few paper towel pieces into the cylinder to help catch chunks and fluids and then tapped the hole. The reamer/tap is nice as it starts with a 14mm tap to guide/pull the reamer section and then there's a 15mm (?) tap section. Still had to push on it to get it straight but the tapping sent well.

Cleaning out the hole was a challenge. I thought to use the shop vac to suction them out all out. I got the crank aligned on the exhaust stroke to let air in and sucked out the shards. The paper towel suck in the hole and I pulled those out with pliers. Still, when I went at it with compressed air and a long-nosed blower attachment there were many chunks flying out. Finally got it clean and could proceed with the sleeve.

The sleeve has these knurls at the top and I was concerned about test-fitting it as that might be the final fit. Using an old plug as a tool, it went in part-way well so I covered it with the high-temp red threadlocker (Permatex) and ran it in. I had smoothed the threadlocker off with a bit of paper towel so it covered the threads and there wasn't a lot of obvious excess.

After running it in tightly, I could see the last tiny bit of sleeve was proud of the surface. Maybe 0.2mm or so. Easy! I'll tighten it a bit harder. No progress. So maybe it's supposed to be like that and the driver thing with the kit rolls it down. Hit it strongly but no movement. After all that and the threadlocker setting it wasn't coming out nor did I try. The hole looked tapped right and the tap was deep but maybe it needed a few more turns. The answer, as it so often is, was more tools.

I got a few small grinding bits at the hardware store. Not Dremel as those looked to be too light. One cone, one ball and one barrel that has a flat top. The seat is about 3/4" across so need small tools. Using the ball on the drill quickly took most of the exposed lip off. Then using the end of the barrel made it all flush. Looked and felt good and didn't eat into the aluminum head much.

So it's ready? No!

When I went for the spark plug it had a hard time getting down into the sleeve much. Worked slick on the bench. I got the 14mm thread chaser out and it refused to even start. That left the 14mm tap as the option. It was a bit of a challenge to start but it found the threads well and cleaned them out. Some resistance along the way so the sleeve had become distorted. Probably from using the driver thing on the exposed lip.

Now the plug went in fine and snugged up tight. Done. Car fired up and has run well. (after swapping WUR but that's another story.)

Lessons: Prepare to blow out the cylinder. A compressor and long-nosed blower tool are required. Make sure the hole is tapped fully.
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