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Breaking Bolts

Mark Jackson

Active Member
Patron
I'm having a problem breaking bolts. RV-8, installing AN3-5A bolts holding the F-843-R-1 longeron to the WD-803-R-1 Engine mount bracket. I've used three different torque wrenches (CDI, Snap-on, and cheapo) in two different holes. Two different lots of bolts to make sure they weren't defective. Torque is set to 100 in-lbs.

Have checked everything many times.

Any ideas?
 

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100" lbs. Is way too much torque for an AN3 bolt. It should be 20-25 inch lbs.
 
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There is a torque chart in section 5 of the build manual. Print it and hang it on the wall of your shop. Follow it. ;-)

It says 20-25 inch pounds on those bolts.
 
As the others said, but additionally, add the running toque of the nut's locking element, which you'll have to measure.

Dave
 
Brain Fart

I mis-read the torque settings. I was reading AN5 instead of AN3. It was an AN3-5 bolt. Duh.

Threw out all the over-torqued bolts.
 
It is certainly over torqueing but I am not sure if these should break.

Do they break after some use or just as your torqueing them?
 
Replacing the overtorqued bolts is a must. Overtorquing can/will damage bolts and has especially catastrophic potential for a bolt anchoring anything besides a gusset plate.

As a side note, make sure you're not lubricating the bolts or nuts with anything before you install them (unless specified). The torque is a function of the compressive force and the friction between the nut and bolt, so adding lubricant increases the stress in the bolt given the same tightening torque.
 
Mark
No worries! We've all had our ADHD builder moments. On the positive side, I've always wondered where an AN3 would break; now I know. :)
 
I mis-read the torque settings. I was reading AN5 instead of AN3. It was an AN3-5 bolt. Duh.

Threw out all the over-torqued bolts.

Mark. No offense intended, but if you’re already to the point of installing engine mount weldments to longerons and not pretty automatic about knowing these bolt torques, even without looking at the charts, you might consider as mentioned above going through entire airplane and replacing/retorquing bolts. Some matter more than others, to be sure, and the failure of some like the longeron to engine mount could be catastrophic. The problem is that already installed bolts may look just fine but could be very near to their failure point without you knowing it.
Again, posted in the spirit of helpfulness.
 
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