You get unpredictable results once in a while, even on the most routine procedures.
I had a very similar situation happen with my busa when I was installing the exhaust. The busa has bolts that fasten the headers to the manifold (instead of nuts like the 14). I don't remember noticing anything unusual when I removed the header bolts but when I installed the aftermarket headers, one bolt stopped after a couple turns like your spark plug. WTF? I just removed those bolts without a problem, how could there be anything wrong? I even matched the bolts I removed to the same hole they came out of. Upon closer examination, the head of the bolt that was stopping was not parallel to the tip of the bolt. It wasn't very obvious, it was only a few degrees off and there were no metal flakes that came out on the bolt. I bought a new bolt and just like the original, it only went in a couple turns and stopped.
Someone on hayabusa.org suggested a thread restoring tap of which none were immediately available to me. He then suggested using an ordinary tap and threading it in just past the sticky point. It probably was only a small section of the threads that were bent and I might not need to thread the tap in deep enough to really do any major cutting. So, as recommended, I oiled an ordinary tap and threaded it in making sure the tap was very straight. Then I put the handle on tap and turned it a quarter turn or so, maybe a half turn. There wasn't much if any metal removed, the top threads are wider than the deeper ones. The new bolt went in nicely.
I doubt very much you had a bent spark plug in there but the top threads are wider and looser like my situation and that's what allowed the old plug to come out without damaging the top threads. If the plug didn't stick the whole way out, it's probably just the threads closer to the top that got roughed up. I have a feeling you might need to chase the the threads the whole depth though. The only likely cause was a chunk of rust on the old plug that broke away and left a bur. Who knows at what point that was on the spark plug threads? If the plug came out easily, probably only a short distance of thread near the top of the hole and a slight bur was turned up in the counterclockwise direction. That would be best case scenario. Top or bottom, it could only be a slight deviation from normal smooth threads right now. The chaser tap will fix that. You already avoided a much worse problem by not forcing a new plug through marginally rough threads. I would try chasing just the top few threads after the tap goes in and hits the rough spot. Then take it out and try the plug. If the plug now stops after three turns, thread the tap in deeper and then try the plug again.
I have a feeling the chaser will fit tighter to the threads than the spark plug. Maybe get a nice quality M10 nut and try that on both the tap and the spark plug to get an idea of how they each should feel in smooth threads. I would make an effort to not tap beyond the bottom thread. If you grease the tap up good, it's doubtful anything will fall out of the flutes in the tap but no need to tempt fate. Count the threads of the spark plug and that's how many turns you need to run the tap in at max. Also, you want to avoid hitting the piston and I believe Hub has told me that combustion chamber up top of the cylinder does not have much depth. If you are close to TDC on that cylinder, the piston dome is right there waiting.
I think this will work out great Greenie. Just be careful like you have been. No rush. Slow methodical work is the way to get this done right.
* Last updated by: Rook on 5/16/2020 @ 3:30 AM *
'08 MIDNIGHT SAPPHIRE BLUE Now Deceased
