Old lead mechanic told me to stop torquing spark plugs to spec and I laughed at him

I was working on a Lycoming O-360 last summer over at Miller Aviation in Wichita, and this guy Dave who's been doing this since the 80s told me to just give the spark plugs an extra quarter turn past hand tight instead of using the torque wrench. I thought he was crazy and ignored him. About three weeks later I had a plug come loose on a test run and it scared the heck out of me, so I went back and asked him what he meant. He showed me how the threads can gall up if you follow the book torque exactly on those older cylinder heads. Now I do it his way on engines that have seen some hours and I haven't had a problem since. Has anyone else run into this where the factory spec actually causes more trouble on older parts?

2 comments

2 Comments

andrew_wood7224d ago

Yeah, "eat crow" is right. I had the same thing with a Lycoming a few years back. The factory spec is great for a fresh thread but once those heads have been through a few heat cycles the threads get kinda tight and the book torque can actually gall them up bad. Now I always back the torque off about 10 percent on older jugs and it's been smooth sailing.

martinez.karen24d ago

Oh wow, you learned that the hard way just like I did! My dad who's been wrenching since the 70s told me the same thing about older Chevy small blocks and I had to eat crow too. He was right, those threads just don't cooperate with exact torque values once they've got some miles and heat cycles on them.