plus 1 spark plug? need a fix

[merlin]

So i drilled and tapped out my broken L20b spark plug. About 40% of the threads are still good. options? Weld and retap? JB Weld? go up one spark plug size and retap?

[datzenmike]

I like the idea of JB weld. It's easiest and you can always drill it out larger if it fails.

[MikeRL411]

Parts stores sell spark plug repair kits for [especially] aluminum heads precisely to fix this problem. Not Helicoils, they might unscrew when you next change plugs. The kit comes with a drill bit, tap, and a cylinder with a stop flange. Drill the old thread out, tap for insert, apply sealant [locktite or equivalent sometimes, not universal to all kits], put the insert on a spark plug with some grease on the plug threads and torque into place. Have a beer or two while the sealant cures and you are done. Alternate would be to use, I believe, "thread serts" unsure of the brand name. drill, screw into place and hit the hardened steel anti-rotation pins down the sides of the threaded insert to keep in place and prevent torque out.

[Dguy210]

How many threads are actually left in the hole? Generally, although this is not exactly true for mismatched materials, the amount of load applied to the threads decreases per thread, giving the 6 threads rule for 98% of the load. So 34% of the total load is on the first thread, 55% on the first 2 threads, 70% on the first 3 threads 80% on the first 4 threads (etc..). I believe there are 8-9 threads on an L-head spark plug hole, so you should have 3-4 good threads left, right? Meaning the JB weld should work fine, you are just reducing your engineering factor of safety..

 

http://www.gizmology.net/nutsbolts.htm

[merlin]

yah its likely closer to 3 good threads... i'll go bug the Partstards at the local shoppe about a collar.

[HRH]

Do a heli-coil. Properly installed you won't have any issues. I had one in the L20 race motor for three years with 11:1 or so compression. Never spit it out.

[merlin]

ordered an insert kit, should be here on friday.

[Pacific coast Datsun]

JB wont work....tried it. Go with a helicoil.

[merlin]

getting the local mechanic to do the overbore, retap and epoxy sleeve method... he's doing one on another car today that is much worse than mine.