Did I ruin my mower?

How thick does the head gasket seem? On a lot of mowers they're reusable, at least to a certain extent, and the bolts holding the head on aren't that tight, and getting them all torqued back down to the same level isn't that critical. Pulling the head, trying to clean the thread up, then refitting it might not be a big deal at all.

cheers

Jules

It will never be "right" again. Even if you get the threads straightened out they're compromised and the spark plug will likely blow out at some point in the near future. Best thing to do is replace the head at this point, and consider it an expensive lesson learned.

Order a new head and gasket and Google proper torque and bolt pattern to tighten them. And, as someone said...clean out the metal shavings!

Yes, you could helicoil it to do it properly. Or replace the head. Or epoxy the plug into the hole and replace the head when the plug wears out ;-)

(I suspect that most mowers die of something long before the plug needs replacing anyway)

cheers

Jules

Pull the head, run the thread chaser from the inside to outside to clean up the threads . If that doesn't work, take it to a machine shop and have them shoot a helicoil in it, Buy a new gasket and put it back together.

Any mower shop will have a head gasket or you can get one online. It sounds like you have all winter to screw with it. You should check the valve adjustments when you get it back together. They probably changed.

Remove the head and have a PROPER Helicoil or other thread insert installed. Don't waste time screwing around trying to "fix" a crossthreaded plug hole with a chaser or a tap.

If it is an L Head motor the valve adjustment will not have changed from removing the head because they are all in the block.

We said he couldn't get a clean shot at the plug hole because of the "valve assembly". That makes it sound like the new Honda inspired/licensed Briggs with OHV.