Let's Talk Spark Plug Inserts

So I'm working on a tune-up. I pull the plugs to check for trouble and see if they're the right heat range. Cylinders 1-3, no problems, and they look good except for a mild bit of soot from the rich idle, which I expected. So does #4, except that when I pulled the plug, a thread repair insert came out with it. It's stuck on there good, too. I tried heat and penetrating oil but it won't budge. I figured I could pick up a new insert at my local auto parts counter, no such luck. The ones from Sav-A-Thread, Perma-Coil, and OEM brand all have a slightly larger OD and a metric thread pitch. The OD on mine is 5/8" x 18 tpi x 1/2" long.

Has anyone run across this particular insert? Can you point me to the correct brand? None of these folks seem to publish their thread pitch spec, they treat it like it's top secret. I don't want to pull the engine just to redo the insert, at least not until winter, when I'll have time to go through it properly.

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8/31/173:53 PM

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Gordon NicholsMassachusetts 1993 CMC Supporting Member

Well.......The spark plug thread pitch on a VW of, say, 1970 should be 14mm, NOT a 5/8", but then you are measuring the OD of the threadsert, not the plug OD or threads (which would be 14mm). The plug thread pitch is 1.25mm and the reach is 1/2".

If you get a new 14mm threadsert from NAPA (permacoil, Sav-A-Thread or whatever), it should fix it UNLESS removing the plug/threadsert also stripped the head out, making the hole bigger. THAT will need a threadsert adapter to take up the excess space, OR, if that is not an option, a new head.

Threadserts are designed to use a special tool to get them in there and make them (semi)permanent, while pressurizing the combustion chamber with air to force any metal chips or tap windings to blow out of the chamber, not fall in and screw up the cylinder. If you are uncomfortable with this installation process, then find a good mechanic to do it for you. If it were me, I would pull the engine and the head and take it to a competent machine shop to see if they can salvage it.........Maybe not.

I advocate using copper anti-seize on spark plugs on VW engines, because the mis-match of aluminum head and steel spark plug causes corrosion beteen the two and, well.....You've seen the result. Whatever you end up with, find some copper anti-seize compound to prevent this from happening in the future.

8/31/174:46 PM