TIL a lot of people are mixing up the torque specs for AN3 and AN4 bolts

I mean, I saw it twice last week at the hangar, guys going by feel or using the same setting for both. An AN3 bolt at 3/8 inch needs about 25 inch-pounds, but an AN4 at the same size jumps to 50. I had to pull out the manual for a Cessna 172 job to show a new guy the chart. Anyone else run into this mix up on smaller general aviation stuff?

4 comments

4 Comments

davis.david1mo ago

Man, that's a scary one to mix up. I keep a printed torque chart right on my toolbox lid for exactly this reason. It's too easy to grab the wrong wrench setting when you're in a hurry, especially with those smaller bolts. I always make the new guys read the spec out loud from the manual before they tighten anything critical. Seen a stripped out fitting before?

ruby_henderson361mo ago

Oh man, making the new guys read the spec out loud is a great idea. I should probably do that myself, because I've definitely been the guy in a hurry. I once mixed up inch-pounds and foot-pounds on a cylinder head plug. The sound it made still haunts me. Let's just say I had to get very friendly with a helicoil kit that afternoon.

viola_allen9d ago

My old shop had a laminated chart on the torque wrench box, but the plastic got so greasy you couldn't read it. I just double check the manual every time now, it's saved me a few times.

victor_robinson1mo agoMost Upvoted

Yeah, the printed chart on the toolbox is smart. I do the same thing. I laminated a sheet with the common AN specs and just stuck it to my torque wrench case. You can't trust memory on this stuff. Saw a guy over-torque an AN3 on a carb heat box once. Cracked the flange. That was a bad day.