Honestly, are we over-torquing spark plugs on Lycomings?

Tbh I keep seeing guys at my hangar in Atlanta cranking down spark plugs way past spec on these Lycoming engines. I know the book says 300-360 inch-pounds for the massive plugs, but I've had two separate instances where I followed the lower end and got zero issues, while a buddy went tighter and cracked a plug boss. Then you got the old-timers who swear they've run them hand-tight for years with no blowouts, which seems crazy to me. Does anyone have a hard failure story from going too loose or too tight? I'm trying to figure out if the torque range is actually that critical or if we're overthinking it.

3 comments

3 Comments

ray5621d ago

Funny you mention that @kimr91, I had a buddy who torqued his plugs to spec but forgot he had a dab of anti-seize on there and basically created a torque multiplier. Cracked the boss just like your story. Guess it's not just about the number but what's on the threads too.

kimr911d ago

Man I read a thing from an old A&P who said the real issue isn't the torque number itself but what the plug threads look like. He said if the threads are galled or dirty, any torque reading is garbage and you'll crack something. I had a buddy strip a plug boss on his O-360 because he didn't chase the threads first. He followed the spec exactly but the threads were full of carbon. So maybe the torque range is fine but it's really about prep. The old timers getting away with hand tight probably have clean threads. Plus they probably don't fly as hard as we do.

zarag171d ago

Real question is how much vibration that plug sees in service though, that changes everything about thread loading over time lol.