Original FE plugs where 18 mm. tapered seat. BT and Edel. heads use 3/4 reach 14mm. plugs. Make a stop by taking and old plug and in a vise, punch out the insulator. Then, tap the shell so it will accept a bolt. You now have a stop. Make sure when using stop that you are on the compression stroke as if you are on the exhaust/intake, you can bend up the valves. Hope this helps. Good luck.
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