Realized I was over-torquing spark plugs for years after a cylinder head crack

I was working on a Continental IO-470 last Tuesday and one plug felt weird going in, so I looked up the spec. Turns out I was cranking them down way past 30 ft-lbs like some kind of gorilla, been doing it since trade school in Phoenix. Previous mechanic on this engine had cracked the head from overtightening and I almost did the same. Ever had a gut-check moment like that where you realized your old habit was actually wrong, what tipped you off?

3 comments

3 Comments

gray8751d ago

Old habit or not, vibration plays a bigger role than most guys give it credit for. On a radial engine I worked on, a loose plug backed out mid-flight and the cylinder head temperature spiked before we caught it. Overtorquing squeezes the crush washer flat and robs it of its ability to dampen harmonics, so the plug seats hard but the cylinder takes all the shake instead of the gasket. Next time you see a cylinder with fretting marks around the plug hole, that's a dead giveaway torque was off, tight or loose.

the_anthony2d agoTop Commenter

You ever get that feeling where you replay every job from the last five years in your head? That happened to me on a Cessna 172 last fall, cylinder was getting hot on runup and I smelled something off. Turns out I had been setting plugs with the same wrist torque I use for lug nuts. Took a digital torque wrench and a factory manual to finally snap me out of it.

gibson.morgan2d ago

Hold up, I gotta disagree with @the_anthony here. If you've been torquing plugs that tight for five years without a failure, maybe your wrist is more calibrated than a factory manual. A digital torque wrench isn't going to fix the root problem, which is probably a bad plug or a fuel issue on that runup.