Stripped spark plug threads in forklift

Bob - MI

Well-known Member
One of the Yale propane forklifts wouldn't start yesterday and while doing the mandatory trouble shooting we pulled the
plugs to check for spark at the tips. Seems that a number of years ago whoever changed the plugs put the wrong ones in
with over-length threads. The end threads were exposed in the combustion chamber and they melted a bit as well as got
all carboned up. Aluminum threads were torn out in one of the plug holes so I have to decide on pulling the head off and
doing the repair on the bench or try a thread insert fix while on the engine.

Any suggested repair kits? (14mm thd)
 
Heli-Coil kits are very popular. You can get kits on Amazon. Verify the pitch of your 14mm threads before ordering.
 
A recoil kit will be a place to start.

Grease on the tap will catch most of the chips, then vacuum the cylinder best you can with a piece of tubing duct taped to the shop vac. Even if you miss some the aluminum won't be as destructive as cast iron. But one thing you must retrieve is the break off tab on the end of the coil! Maybe reach in with needle nose and break it off.

The main thing is to keep the drill and tap straight. Normally you would use a drill guide, but this application it would be hard to do. Sometimes it helps to have a second set of eyes on it. A trusted helper who can either drill and tap or coach with the alignment process.
 
I might look into this. I think it is better than heli coil for spark plugs.
http://www.timesert.com/html/sparkplug.html
 
NO need to overthink this.

An errant chip of aluminum or two left in the cylinder won't hurt a darned thing.

Best practice is to position the crankshaft so both valves are closed, then tap for the repair insert followed by inserting the tip of an air blowgun into the hole and (wear eye protection) blowing all the debris out of the cylinder.
 
Here is another one used by people in air cooled aluminum head community.
https://www.kvt-fastening.ch/en/products/brands/keenserts/
 
Use a time-sert for 14mm spark plugs. The Time-sert is an insert that will not allow combustion gases to seep through the repaired threads and has a good surface for the spark plug gasket to seal against. I use heli-coil for a lot of thread repairs but will never use heli-coil for a spare plug thread repair due to poor plug gasket surface and gas leakage through the thread coils.
 

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