Spark Plug Rethread

| 10/23/2012 12:10:06 PM

Tags: MC How-To,

Spark Plug Rethread

One of the great things about old bikes is that they're typically pretty simple to work on. Tune-ups are generally fairly simple, with fresh spark plugs cheap insurance against poor running. And depending on your bike — especially if it's an old 2-stroke — you might actually need to replace your spark plugs quite regularly. That brings us to our present little How-To; repairing stripped spark plug threads.

Given the relative ease of replacing spark plugs, many owners are willing to give the job a try. But simple or not, like any project things can go wrong, usually because we get in a hurry. Getting in a rush installing spark plugs can easily lead to cross-threading a plug, resulting in stripped or severely damaged threads in the cylinder head. When that happens, your quick tune-up can suddenly turn into a major job.

If you're lucky and don't cross-thread the plug too far, you might be able to chase the threads with a plug hole rethreader. BikeMaster sells a couple different sized rethreaders; nice to have and only $12. But if you've stripped the threads and they're beyond chasing, the plug hole will either have to be welded up and retapped or repaired with a thread insert. Welding and retapping requires — at the very least — top end disassembly. But depending on your mechanical ability and the ease with which you can access the spark plug holes, you might be able to tackle installing a thread insert yourself, and without having to tear the engine down.

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We know that some people wince at the idea of inserts of any kind, but practical experience has proven them to be a secure, permanent fix for stripped spark plug threads. Importantly, there are engines — mostly newer overhead cam units — where inserts won't work because the plug hole is so close to the valves you can't cut oversized threads. Our subject bike for this How-To was pal Cailinn Drouin's 1968 overhead valve, 650cc twin-cylinder BSA Firebird Scrambler. A nice survivor currently receiving a little freshening up, it had spark plug threads so worn they would barely hold a plug. Given the ease of access to the BSA's spark plugs, we opted to install inserts.

There are a number of different spark plug rethread kits on the market. They mostly cost about the same ($20-$50), and they all entail the same basic process; clean out the old threads, cut new threads, install an insert and, finally, the spark plug.

8/21/2016 12:16:04 PM

I have to say I had the same problem on my Yamaha XJ 600 Diversion (model N, 1997), and I followed all you tricks you described here. And it worked ! I did it nice and slow in about 1h30, and everyting went fine. Thanks fo this article and the pictures, you convinced me to do it myself, and it was a relly nice experience ! Thanks again !