Figured out I was tightening spark plugs way too tight for years

I was working on a Cessna 172 last week and snapped a spark plug clean off in the cylinder head. Pulled out the manual and realized the torque spec was way lower than what I had been doing by feel. I had been cranking them down like lug nuts for 6 years before that. Anyone else have a habit they had to unlearn after seeing a repair manual?

3 comments

3 Comments

the_riley1d ago

Had been doing by feel" is exactly it. I did the same thing on my old truck for years - hand tightened oil drain plugs and spark plugs until they felt "snug plus a quarter turn." Turns out that was way past spec, especially on aluminum heads. I stripped a spark plug hole on a Toyota 4Runner back in 2014. Had to helicoil it, which was a pain. Now I use a torque wrench for anything that goes into aluminum, and I actually look up the number instead of guessing. It feels weird at first because it's looser than you expect, but nothing breaks anymore.

william_craig71d ago

@the_riley torque wrenches are overrated. Been going by feel for 20 years, never stripped a thing.

mitchell.avery1d ago

I mean, I get where you're coming from, but I read this one article a while back about how torque specs aren't just about not stripping things. It talked about thermal expansion and gasket crush. Like, with aluminum heads especially, the metal expands way different than steel. So if you guess by feel, you might be way off on the clamping force when the engine gets hot. Idk, that article made me rethink it because I always thought it was just about not snapping bolts. But it's apparently also about the joint holding together properly under heat. Maybe it's just me, but after reading that I started looking up specs for critical stuff like head bolts and transmission pans.