Stripping plugs

[Ratbat]

What are the chances of stripping the threads on a cylinder head with a Spark Plug? I performed routine maintenance last week and the next day my Type 1 was running extra rough. I pulled number 4 and the threads on the Spark plug were boogered up. I attempted to look in the hole with a mirror and I think I might have seen some stripped threads in the hole. I'm getting another lug to check and see if it will also get mangled. I would assume that the plug is a bit less of a hard metal than the heads. Am I correct?

[Marc]

Plug = steel
Head = aluminum

Other than forced cross-threading, the most common cause of spark plug failure is a crack in the head. Typically they originate at the valve seat(s) and propagate over and up the sparkplug hole. The threads then shift out of line at the crack, and removing the plug tears metal loose. Replacing the head is the only real repair, but there are several ways that the threads can be restored well enough to continue driving the car. The easy-sleaziest fix is with a steel insert, installed after hogging the threads out to a larger size. These'll do in an emergency, but are the worst possible choice because the insert invariably seizes to the plug and comes out with it next time, wearing the oversized threads (which weren't very well-formed to begin with). When the insert no longer tightens, the only last-ditch fix remaining is to tap the hole out to 18mm and use a thick 14 to 18mm adapter, because the first insert was already larger than either of the two "good" repairs, the Time-Sert and the Helicoil.
Both the Time-Sert and Helicoil require a greater investment in the tools needed to install them; some places will rent you a kit though. IMO it's not a great idea to attempt either with the engine in the car if you've never installed one into a head on the bench, but the Time-Sert has fewer pitfalls than the Helicoil; the worst that should happen is you'll get it in a little crooked but it should still function.



http://vwparts.aircooled.net/Time-Sert- ... l-4412.htm

It's a relatively small investment to buy a plug-thread "chaser" that you can operate with a deep socket, you may want to try one of those first to see if you can clean up what's left of the threads well enough to hold a plug. But be verrrry careful to get it started straight or you'll just wipe them out.

[Ratbat]

Thanks Marc. As always a great and quick answer. Unfortunately I do not have the proper size jack for engine pulling. That one is on the "get soon list". I got the new plug in and the car ran fine until I took it for a test run. Very low power and a clacking from the right side valves. Then I noticed quite a bit of oil leaking from #2 cylinder push rod tube. This requires a few nights at the mechanics. One day I will have all the tools I need and all the time to use them. Until then I have two of the best VW mechanics in Oregon within 5 miles of my garage. Gotta keep the locals employed.

[shepherd]

If you install a too long plug, the threads that hang into the combustion area will get built up carbon on them. Then when you go the remove them, the carbon can strip out the soft aluminum threads as you wind it back out! Make sure you get the correct threads for your heads! Its also unlikely that the steel plug is stripped, it's probably the head, and if so there's not an easy fix. Successfully installing any sort of insert with the engine in the car sounds like a nightmare to me. Maybe in a bus, maybe.

[Marc]

...Successfully installing any sort of insert with the engine in the car sounds like a nightmare to me. Maybe in a bus, maybe.

Done it more times than I care to think about, both with TimeSerts and Helicoils. #2 is the hardest in a Bug, with #4 a close second. As I said above, it's not something I'd recommend you undertake if you've never even done one on the bench - not being able to see what you're doing increases the degree of difficulty immensely. The Helicoil mustn't be too long (a ½" reach will end up going into the chamber by the time it's the required minimum of ½-thread below flush at the top so must be cut about 1¼ turns short) and it's a nerve-wracking moment when you reach in to break off the installation tang - turn off your phone ....With the TimeSert there's nothing to fall into the chamber, so the hardest part is getting the counterbore depth correct. Again, don't use a ½" insert - the 7/16" length is needed. Another advantage of the TimeSert is that if you do get the threads slightly crooked the spot-facing tool will create a flat pad for the sparkplug gasket - the Helicoil is far less forgiving.
Using a greased tap collects most of the chips, but I always fire the engine up on three for a few seconds to sweep out any that fall in before installing the insert or Helicoil.