A thread repair kit was installed on the #7 cylinder of a 2007 3v due to a spark plug blowout. The person that did the repair put in a set of Champion spark plugs because of the problems with the old style Motorcraft plugs breaking. It was time for a tune up so i was going to replace them with the updated Motorcraft SP 515s. Ive got them all replace except the #7. It will not thread on there for some reason. Its the exact same thread as the Champion spark plug, but there is one difference i noticed. The shafts of the Motorcrafs, from the threads to the electrode, are a little larger than the Champions. So I used a 16mm tap to no avail. Im am almost certain it has to do with the larger shaft of the Motorcraft. I have thought of a few repair options but none seem viable. I have 7 new Motorcraft spark plugs and 1 old Champion spark plug in right now. I would love some viable suggestions. Thanks!