spark plugs

I have a 1987 cadillac fleetwood brougham (RWD), I have 7 spark plugs and wires done but there is that one I cannot get to screw in. Ive tried putting a tap to clean and refine the screw heads in the hole but
it seemed not to work. I need help. What should I do
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Ouch, where do you think that the stuff you dislodged went? First take a small magnet and try to clean up the debris. Perhaps put the old plug back in and take it to a repair shop before you damage things further.
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Fred wrote:

If the old plug came out without incident, the new one should have been started by hand for a couple turns before any tools were applied. You can have the plug in a socket when starting it by hand this way. You don't say what the original situation was.
When you attempted to clean the threads with a tap or cleaner, you don't explain what you mean by it "seemed not to work." Did the tap not screw in? Did it screw in and then the plug still would not screw in?
At this point I'm back full circle to trying to screw it in by hand. Do not force anything. Maybe you need to have another hand give it a try.
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This is taking on ominous overtones... If you run a thread chaser in the hole the plug should go on in UNLESS you have put the thread chaser in crooked and finished removing the threads. Now, sometimes if you don't have a plug oriented just exactly right, the threads wont catch.
So try again, gently, with the new plug. Sometimes the angle of the plug with respect to the bore is critical...This is a job for a lover, not a fighter.
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and if all else fails, you may have to pull the head and install a helicoil kit on that plug port.
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Sounds like he should pull the head and clean the debris from the tap.

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If you get to where you try putting the plug in again, try putting vacuum hose on the top of the plug and use that to turn the plug in and start the threads. It's gentle and won't allow you to strip the threads, plus you can feel things line up fairly well using that method.
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