That time I was torquing spark plugs wrong for way too long

I've been turning wrenches on small planes for about 12 years now, mostly piston engines. About 6 months ago I was doing a routine top end on a Lycoming at my shop in Spokane. I was putting in new spark plugs and just doing the usual, torque wrench set to 30 foot pounds, giving each one a good pull. Then the engine lead on number three felt weird when I was threading it back in, like it was a little loose. I pulled the plug back out and saw the crush washer was totally flattened out, like pancaked. That's when it clicked that I had been overtorquing those plugs for years, probably since I started. The spec is 30 foot pounds but only with a light coating of anti-seize, and I had been putting them in dry without thinking about it. Realized the dry torque is way different than wet, and all those plugs I thought were fine were actually getting cranked too tight. Has anyone else had a brain fart like that where you follow a spec but miss the fine print for years without noticing?

2 comments

2 Comments

rileyprice3d ago

Is it really that big of a difference though? I mean, I get that anti-seize changes things, but 30 foot pounds dry is probably still fine for most plug threads in an aluminum head. Idk, maybe I've been lucky, but I've never had a plug hole strip or a plug break from being a little over on the torque.

cole_murphy3d agoMost Upvoted

Stripped one in a Ford once, learned my lesson real quick.