1975 KZ400 Spark Splug threads stripped????

Well in the process of slowly going over my new KZ400, I did a compression check to see where I was for a starting point as it does not currently run. Needs a coil & wires, and carbs & tank cleaned.

Low and behold the right side cylinder plug is 90% stripped. I am actually amazed the plug stays in.

I am planning on using on of the Spark plug repair inserts. I am looking at the kits that have the solid insert, not the typical Heilicoil ones. They have a knarled top that you use a tool to flare it a bit and it bites into the cylinder to keep it in place. But I do not know the size I need. Are all spark plugs the same threads?

This is what I am planning on using.
www.amazon.com/Perma-Plug-1-25-Spark-Thr...Repair/dp/B0025PMJHA

Google search KZ400 parts

Go to Partzilla and look up the KZ400 model bike.

Found the OEM parts diagram for cylinder head

Click on the spark plug and found out they are NGK B7ES

Google search NGK B7ES and went to NGK's website

Looked up the specs and found out the threads are 14mm x 1.25

That's the size insert you need.

BTW, you can't do a proper compression test on a cold engine. If the engine has sat for some time you may get abnormally low readings anyway since varnish may be gluing the rings down some. You need some mileage on the engine from a shutdown and a good hot engine, with valves properly adjusted, before you can get a compression test number you can trust.

Thank you very much.

And I realize that a cold compression number is not ideal. But with it still being a long ways away from running, I at least wanted to get a ball park. Added a touch of oil into the plug hole and compression seems reasonable. So I will focus on getting it running and then will do a hot test and see how things look.

freeriders98 wrote: Thank you very much.

And I realize that a cold compression number is not ideal. But with it still being a long ways away from running, I at least wanted to get a ball park. Added a touch of oil into the plug hole and compression seems reasonable. So I will focus on getting it running and then will do a hot test and see how things look.

You need to put a couple hundred miles on the engine if the bike has been in storage for some time before you can be assured the rings are not sticky from storage rot. Always check valve adjust before doing a compression test too.

I personally have never had much luck with that style thread repair. I've had good luck as long as it was a gasketed plug with the Helicoil style repair, and some good luck with the Time Serts, although the Time Serts are a bit expensive to buy and basically only use once.
If you do use the thread repair you have linked get some Green Loctite for the outside threads of the bushing, this is some of the stronger Loctite. Also remember to use a little never seize on the spark plug threads.