Spark Plugs do not fit

Yup… you better call Kenny… top guy.

Simply trying to reinsert that existing plug with the insert will never work. It needs to be properly repaired. It looks like a Helicoil or off brand solid insert. Not to be confused with the Helicoil wire type. The Helicoil solid insert will come out. It doesn't look like a Timesert and they almost never come out. You can replace the failed Helicoil insert with a Big Sert but it will be right up to the valve seat. I did one with no problems but you are taking a bit of risk that the seat could come loose. I highly recommend you remove the head and let a pro sort it out.
https://www.timesert.com/html/bigsert.html

A picture is worth a 1000 words. I am only familar with the wire type Helicoil. But now I see the other "solid" type.
Since you have the threads cut in the head. It seems it would be easy Easier to just clean up the threads in the head and get new inserts I would use Locktite or JB weld secure it in., I guess the way in the video, he taps on it to set the knurl is good. I always watch several video to see if there are better info on other people's videos. Does it look like the knurling has been set into the head?

Fast Eddie said:

Yup… you better call Kenny… top guy.

https://nycnorton.com/

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Thanks. Already emailed him.

Carl H said:

A picture is worth a 1000 words. I am only familar with the wire type Helicoil. But now I see the other "solid" type.
Since you have the threads cut in the head. It seems it would be easy Easier to just clean up the threads in the head and get new inserts I would use Locktite or JB weld secure it in., I guess the way in the video, he taps on it to set the knurl is good. I always watch several video to see if there are better info on other people's videos. Does it look like the knurling has been set into the head?

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Thank you for sourcing out that video. I agree, watching videos a few times before beginning a project is very important. As Dirty Harry once said, "A man's got to know his limitations." Taking the cylinder head off is probably the best bet for a proper repair. Since I live in an apartment in Brooklyn and I usually do repair work out in the street, I may leave this job to the experts. I wouldn't feel confident working in the street, taking the head off, repairing it and putting it back on in just one day. Also, watching the video and seeing the repair being done, if I were to leave the head on the possibility of shards of metal falling into the engine are just too great.

Take a lot of photographs. Bore scope shots as well if you have one.
Take all the nasty dirty parts you have from both cylinders and buy an hour of NYC Norton works time for some analysis.
You won't regret it and you'll make a new friend

After understanding the constraints you are working under, I might back up on my recommendation of sending it to a pro right away. I think I would try a new helicoil insert of the type seen in the video one more time. First look into the sparkplug hole and make sure the exhaust valve is closed and piston is down in the cylinder. Rotate the engine until it is closed if necessary. See if the insert will thread into head without recutting the threads. If you need to clean the threads a bit to screw in the insert, pack the flutes of the tap with grease, that will catch 95% of the cuttings. Compressed air is good to blow out the hole, if you don't have access, tape a small hose into a vacuum hose and suck out any other bits. Any micobits left will blow out the exhaust. Degrease hole with brake fluid. Be sure and use high temp red loctite 272 on the outside of insert(not rtv). Hit the knurling tool with a decent rap. Wait over night before installing plugs. Use new plugs with a bit of antiseize on the plug threads and just tighten them enough to crush the gaskets.
I doubt if you will damage the threads any further doing this and you might get lucky and get it to hold. It's easy for us old guys with workshops and lifts and all the tools and the experience to say just pop the head off. Maybe an hour for us. But most of us were in your shoes at one point.

As far as removing the collar from the plug, couldn't it be "double nutted" off? If you can find two nuts with the same thread as the collar, thread them on and tighten them into each other. Maybe put plug in a vise and turn the lower nut to loosen?

Edfury828 said:

If you can find two nuts

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Metric thread M14 x 1.25 pitch on the ID so likely same metric pitch on the OD but larger diameter, doubt such a nut is in the US.

Edfury828 said:

As far as removing the collar from the plug, couldn't it be "double nutted" off? If you can find two nuts with the same thread as the collar, thread them on and tighten them into each other. Maybe put plug in a vise and turn the lower nut to loosen?

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That's a good call. It may be stretch finding them but definitely worth the time to search.

kommando said:

Metric thread M14 x 1.25 pitch on the ID so likely same metric pitch on the OD but larger diameter, doubt such a nut is in the US.

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Trust me, we have plenty of nuts in the United States. I work with a few.

I believe I would start with a new insert. They are pretty thin and you might distort the old one removing it. Besides you might need the tap.

Triton Thrasher said:

Many are confused by the direction of the air laver (choke).

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Lever!