I can't be the only one with this problem. The sparkplug hole is machined with a relieved area of few mm's at the combustion chamber side, leaving that area open, and the first few threads of the sparkplug are then left exposed. After awhile, I suspect enough crap has built up on the threads, that they will not come out. I run only race fuel, mostly what is available at the track - Sunoco 260 GT. I put a dab of antisieze - Wurth copper goo, on the plug threads. I have never seen a sparkplug hole machined this way, and can't imagine why they even did this, although I think it may have been a way for them to slightly lower the compression ratio, by increasing the cc volume? Beueller?
Quite common on most modern sportbike engines. The solution is to regularly remove the plugs. I've had to remove the head just to get them out.
Thanks Rick - That's what George Dean in Seattle said. When you say quite common - do you mean the seizing or the exposed threads on the end of the sparkplugs, or both? These plugs were in for not many miles, maybe 6 race weekends. I can't imagine street bikes running pump swill are faring very well after 25k miles.
My plan is to chuck the sparkplugs up in a lathe and remove the first 4 or 5 threads, leaving a smooth area for no crust to get a grip on.
