Tried a new torque wrench adapter on a Cessna 172's prop bolts and got a surprise

I was doing a 100 hour inspection on a 172 at our small field in Bend last Tuesday. The old torque wrench adapter for the prop hub bolts was pretty beat up, so I grabbed a new one from the box, a simple 3/8 drive to 1/2 inch square. Torqued the first bolt to the book spec, 450 inch-pounds, and the click felt soft. Did the second one, same thing, just a mushy click. I stopped, checked the adapter with a known-good wrench on a test fixture, and it was reading 50 inch-pounds low. The new adapter had enough flex to throw off the calibration. I learned to always check a new adapter, even a simple one, before putting it on a critical component. Has anyone else run into a simple tool causing a big calibration error like that?

2 comments

2 Comments

mia59224d ago

That's a good catch on the adapter flex. Just a small point, but inch-pounds on a prop bolt seems off. The book spec for a 172 is usually in foot-pounds, like 50 or 60 ft-lbs depending on the model. Maybe it was a typo in your post. Using inch-pounds there would be way under-torqued. Either way, your main point stands about checking new adapters. A soft click is always a red flag.

thea14324d ago

Actually, the inch-pound thing might NOT be a typo. Some newer torque wrenches for small fasteners are in inch-pounds, and you have to convert. If the manual said inch-pounds, you should follow that. Calling it a clear red flag is jumping the gun. Maybe the adapter just needs a break-in period, or the click is fine but sounds different. We can't assume every soft click means danger right away.