Rant: Everyone swears by torque wrenches for spark plugs, but I had one snap off in a Continental engine

I was changing spark plugs on a Cessna 172 at KAPA last Tuesday. Used the proper torque wrench set to 30 ft-lbs like the manual says. Third plug in, it snapped the top right off the ceramic. Had to spend 45 minutes with an extractor kit on the ramp in 90 degree heat. I think the plug was already weak from a previous install. Anyone else had better luck just going by feel on older engines?

2 comments

2 Comments

richard_roberts802d ago

The thing nobody's talking about is how the torque spec assumes a clean, dry thread, and on a Continental that's been through a few heat cycles you're never getting that. The anti-seize or even just oil residue from the last guy changes the friction coefficient completely, so your 30 ft-lbs is actually way more force than the plug was designed for. Had a similar incident on a Lycoming O-320 where the spec said 25 ft-lbs but every third plug would bind up halfway down the hole from carbon buildup in the threads. I stopped using a torque wrench on old engines entirely after that, just go by that solid snug feeling with a little extra quarter turn and never had a problem since. The real issue is the factory spec doesn't account for real world conditions like corrosion or thread damage from previous installations.

paul3301d ago

Wait @richard_roberts80, you seriously just stopped using a torque wrench on aircraft engines?