Old timer told me to stop torquing spark plugs to spec in Florida humidity...he was right

I've been an A&P for about 4 years now and I always followed the manual to the letter for spark plug torque values. This guy named Rick who's been turning wrenches since the 80s told me I was gonna strip threads in the Florida humidity if I kept doing that. I told him he was wrong and kept doing it my way. Sure enough I pulled a plug on a Cessna 172 last month and found the threads in the cylinder head starting to gall up. Rick's advice was to back off the torque by about 15% in humid coastal areas because the aluminum expands differently. Now I keep a separate torque setting for planes based in beach cities like Daytona. Has anyone else found regional climate factors messing with standard torque specs?

2 comments

2 Comments

william_taylor14d ago

Did you check the thread pitch and plug type before you started factoring in humidity? On those Lycomings with the short reach plugs, the expansion rate difference between the steel plug and aluminum head gets amplified when you're sitting at 95% humidity day in and day out. Rick might have been onto something but I wonder if he was also accounting for the engine not being fully heat soaked when torqued versus a cold assembly in a dry hangar. Were you doing your torque on a hot engine or a cold one? That could explain why some guys get away with factory specs and others don't, even in the same Florida town.

amy_foster5614d ago

I mean, doing torque on a hot engine changes everything. Seems like that's the real variable here.