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.
It is, in fact, a straightforward job, but it does come with a bit of a pucker factor: Cutting new threads presents the very real risk of introducing metal swarf into the cylinder. You’ll have to decide for yourself if you’re comfortable with that risk, but experience tells us that if you take your time and use plenty of grease on the cutting tool to collect any metal shavings, cutting new spark plug threads can be done with minimal stress. As with any project, the key to success is patience.
Before you begin, remove the old plugs and then roll the engine over until the piston for the offending plug is at the top of its stroke at top dead center so the valves will be closed. Depending on how close the piston comes to the plug hole, you may want to spin the engine a bit more so the piston is farther down in the cylinder. This will ensure you don’t hit the top of the piston with the cutting tool. Finally, it’s a good idea to change the oil when you’re done, as any cleaner sprayed into the cylinder will wash down the cylinder wall.
How to Rethread a Spark Plug: Photo Guide
This is what we started with on our 1968 BSA Firebird Scrambler 650. Although you can still see the threads, close inspection shows them to be damaged and very worn out, allowing the plugs to loosen up and wobble, making the problem worse.
Required tools are few: a 3/4-inch wrench, spray parts cleaner, grease, rags, a hammer, and an insert kit like the one above with (from left) a rethreader, a thread insert and an insert staking tool.
After ensuring the cylinder is at top dead center so the valves are closed, gently stuff a grease-soaked rag into the cylinder cavity. Use a single piece, and avoid packing it in place.
Next, liberally coat the rethreading tool with grease and thread it into the plug hole as far as it will go just using hand pressure.
Applying downward pressure in line with the spark plug hole, turn the rethreader with the 3/4-inch wrench. The original threads will help guide the tool. Keep turning to engage the tool’s cutting edge and over-sized thread cutter.
After you’ve cut through the old threads and have just started to cut new threads, remove the rethreader. Check your work and clean it off.
This is what our spark plug hole looked like after our first, tentative cut. The old threads have been chased and the tool is cutting them out as it works down, subsequently cutting a new, larger set of threads for the insert that will replace the bad threads.
This is what our first plug hole looked like after our second run with the rethreading tool. We had stopped just short of fully rethreading the plug hole to check our work.
Here’s what you’ll see on the rethreading tool after really cutting into the head. Between cuts, clean and then liberally reapply grease to the threading tool. Reinstall it in the plug hole and continue cutting new threads. Work the cutter down slowly, easing off when it tightens up, working it down until it bottoms out. Remove the cutter and clean off the swarf.
Once the new threads are cut, use a pick and gently remove the grease-soaked rag you stuffed into the cylinder, cleaning off any swarf as you go.
With a flashlight, visually inspect the plug hole and cylinder for any signs of swarf. Use picks, rags or a vacuum to clean up any remaining metal. Clean the new threads with spray parts cleaner.
Next, it’s time to prepare the new spark plug thread insert, which is threaded on its outside to match the new over-sized threads you’ve just cut.
After ensuring the new insert is clean and dry, thread it onto the rethreading tool until it just seats. Apply red (permanent) thread-locking compound to the outside threads of the insert, enough to just cover the threads.
Now, start the rethreading tool with the new insert into the freshly cut threads. Slowly turn the tool until the insert cleanly bites the new threads.
Thread the insert into the spark plug hole until it seats. Use the 3/4-inch wrench if necessary. When it’s just seated, tighten it just a bit more. Remove the tool.
Although the insert was installed with “permanent” locking compound, it can still come loose as the locking compound can lose grip in high-heat situations. The supplied staking tool is used to lock the insert in place.
To stake the insert, place the staking tool into the insert, stepped end first. Making sure it’s firmly and evenly seated, give it a quick, hard rap with a hammer to stake the insert in place.
And that’s it! Assuming you’ve taken your time, removed all the swarf and squarely installed the insert, it’s time to reinstall the spark plug, change the oil and go riding.
