spark plug helicoil repair ?'s

[Yamaha_Rules69]

Hi, I have a nice original 1984 Yamaha Tri moto 200 that I am working on, and was looking for a little input on an idea I had. I got this 3 wheeler from the original owner. He used it for ice fishing, and thats it. About 15 years ago, the pullstart rope broke, and it had been sitting in a shed until I got it. I started working on it, and new right off the bat that the motor was stuck. I went to take the plug out, and it snaped off in the head! I really didnt put excessive force on it, it must have just been really rusted in there. I took the motor out, got the head off, and banged the piston free with a sledge . The bottom end looks really nice, and clean inside. At least it had oil in it, and the oil even looked fresh. The head had a lot of corosion and rust, so I blasted out the inside, and plan on having the valve seats re-cut, and new valves and seals installed, I have an in spec jug, and new piston and rings.
Now for my question - I was wondering if I could drill out and tap the threads that are in there now - for a 12mm plug, to a 14mm plug (install helicoil for 14mm plug). The numbers on the plugs are slightly different - NGK 12mm plug (factory size) is somthing like D8EA and the 14mm I would be using would be a B8ES (you see a lot in 2 strokes). I think they would be the same heat range, and the thread reach is the same, so would I have any issues going this route? I have the helicoil kit for the 14mm - special tap for the helicoil OD and drill bit, seater tool, and everythig to do it correctly. Has anyone done this with good results? Thanks for any thoughts.

[fabiodriven]

I wish I could tell you I've done that before, but I haven't. I must say though, that is a very interesting question. I really can't see any reason why it wouldn't work.

[Vootie]

Heli coils are an acceptable repair for spark plug threads, but I can't help you with the heat range/reach comparision between the two plugs.

[dksix]

I think 14mm plugs are 1.25 thread pitch. You should be able to just drill and tap the hole, no heli-coil needed. My chart shows 14x1.25 threads should be drilled with a 12.8mm drill.

[Dirtcrasher]

Whats a 3 stroke??

If it's in the heat range and the threads aren't longer there is no reason you can't.

Many times thumper plugs come very close to the valves, but probably not that close...........

[Yamaha_Rules69]

[QUOTE=Dirtcrasher;932259]Whats a 3 stroke??
Oops, that was a typo. My finger hit the 3 instead of 2. lol . I'll give this idea a try if I cant find a decent head soon, and let you guys know how it turns out. Thanks!