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If you somehow strip the threads in a spark plug hole like I did, don't panic.
Find an M10x1 thread chaser, coat it with grease to limit any shavings dropping into the cylinder, and see if you can repair the threads. Be careful not to run it in too far, if it falls into the cylinder you might say bad words. Also, make sure to clean the threads afterward with a rag soaked in Brake Kleen or something to get the grease out as much as possible.
I got lucky, and was able to torque the new plug to 110 in lbs with no issues.
I would like to add some information regards a better alternative of thread chaser I used after some minor thread damage caused by a New Champion Spark Plug, I had a box of 8 and after checking the threads using a die nut 3 were bad & tight in places and the one that caused the damage very bad, as far as I am aware they were Genuine purchased from a reputable dealer!
To the point, I didn't have the right size chaser in my tool box M10 x 1.0mm so looked to purchase one, I'd only ever had the standard type. I had never heard of the Back Thread Spark Plug chasers (Back-Tap).
It is basically an expandable chaser, that you can run or put through the thread until you get to a good part of the thread, normally the bottom of the thread, then expand the chaser and run it back, has several advantages by doing this when the head is still on the bike.
I successfully used it on the Front Right side M8 plug in situe, very tricky and would have been very hard to use a standard chaser to ensure it went in parallel to the thread and I must say it worked very successfully with some careful and methodical use.
There are a lot of videos & explanations of how they work when searched I will not put any links here for that reason, I used a Neilson one as I was very limited as to what I could get quickly.
I would however say a longer version would have made the task easier on the right side (center) plug holes due to them being deeply recessed as I had to hold the chaser with a Crows Foot to adjust the knurled wheel with pliers when it was in the head, then could put a socket and ratchet on to back it out, a bit tricky but worked with care full use, a longer tool would make the need for the crows foot and pliers unnecessary, I believe the longer versiosn are available, often sold as a Diesel 'Glow Plug' (Pre-heater) tool in the right M10 x 1.0mm size.
Last edited by SJC69; Jan 24, 2026 at 05:11 AM.
Reason: Spelling correction
Spark plug manufacturers say not to use anti seize, because modern spark plugs have a sacrificial coating on the threads for that purpose.
Harley Davidson service manuals now say clean dry threads.
What do they know?
They don't say it's bad only that it's not needed and that you can overtorque the plug.. Now NGK does use a special coating to keep them from seizing. Champion has it's own anti-seize..
Never use anti seize on spark plugs, can cause an intermittent misfire that can be difficult to track down. Learned this many years ago in a driveability class at the GM Training Center.
Never use anti seize on spark plugs, can cause an intermittent misfire that can be difficult to track down. Learned this many years ago in a driveability class at the GM Training Center.
