What torque do spark plugs strip out?

Should be ok. Torque spec with clean/dry threads is 15 foot/pounds btw (see attachment from Repair Manual). Probably best to stay with clean/dry, but if you lube, say with anti-seize, maybe reduce torque by 25%, say to 12~13.

The Repair Manual is a dogs breakfast btw: the attachment is a compilation from two section of the book. The main spark plug section makes no mention of spark plug torque. For that you need to go to valve cover removal instruction,

 

I've had some experiences that left me thinking threads tapped into aluminum holes have a limited lifespan, period. I once was reattaching an old transmission to an engine and the tapped holes in the aluminum bell housing stripped clean out at well below the original specified torque. Had take it back out and drill and heli-coil the holes. Happily, that's an established repair technology and works well, leaving a very lasting repair.

Since then I've been much more cautious tightening anything into old tapped aluminum holes. I'll start with the specified torque as a goal, but if I don't like how it feels on the way there, I'll stop earlier.

I've never had to heli-coil or time-sert a spark plug hole (he said, knocking a nearby wooden object), but they do sell kits for doing it, so it isn't a death sentence if the worst should happen.

... that's assuming there's enough aluminum meat around the hole to support enlarging the hole and adding the insert. The New Car Features manual said they went to the extra-skinny 12mm spark plug hole to make room for the four valves around it to be bigger, so it could be tight.

 

What is the range of your torque wrench? Torque wrenches should only be used from 20-100% of their scale. You may have been outside of your torque wrench’s accurate range.

The newer Honda, Toyota and Nissan vehicles use extended length 12x1.25 spark plugs. Timesert has a kit for this. My machine shop just used this kit (supplied by me) to install an insert into a Honda J35 head.

 

Yeah 1/4” drive wrench is most appropriate, lower range.

 

1/4” drive isn’t practical since most spark plug sockets are 3/8” drive.

I use this torque wrench: Precision Instruments C2FR600H TORQUE WRENCH, SPLIT-BEAM, 3/8 INCH, 100 - 600 IN LBS | Tool Discounter

 

If you got deep pockets.

That's 10x the cost of the Harbor Freight version.

But you get what you pay for.

 

Yeah 3/8” is in the ballpark too; in fact that’s the smallest I had when I had a look at ours a few years back. For 1/4” drive you can use an adapter; I have to do that more often than not.

Then too, you need to convert to inch pounds, no big deal.

 

Dang, I clicked on @The Critic ‘s link...

 

Too rich even for your blood?

 

Have a 1/2, 3/8 and 1/4, collected over the years. Always seem to be just under $30. The 1/2 I got about 35 years back. I did some recent testing with a digital pull scale: all seem ballpark, at least in mid-range.

 

I have the same ones, just with less years on them.

Haven't done a calibration in awhile, as we haven't needed them.

But probably soon they'll be required, so I'll need to verify.