Connecting rod bolts

[1972ho]

Can a 7/16 arp rod bolt just back off the threads are did I forget to tighten it are will vibration cause one to back off.
Thanks

[dannobee]

Did you use a torque wrench or bolt stretch method when tightening? If you used a torque wrench it could be that they weren't tightened enough to "stretch." That could cause them to back off.

[FC-Pilot]

Did you use a torque wrench or bolt stretch method when tightening? If you used a torque wrench it could be that they weren't tightened enough to "stretch." That could cause them to back off.

Had that happen. I did not “cycle them” the way the instructions said and did not have a stretch gauge at the time. Now I always cycle them AND measure stretch while cycling. I like to make sure that the intended torque produces the intended stretch.

Paul

[Kevin Johnson]

... will vibration cause one to back off.
Thanks

If it is not torqued properly or at all (forgotten), yes, vibration can cause it to back out.

It is extremely easy to be distracted when assembling an engine. My guess is, that is why Porsche stopped giving tours years ago that allowed you to see technicians working on them. Many shops use a daub of paint or a special marker to indicate when a particular step has been completed.

Here is a video of an inclined-plane fastener (threaded) loosening during vibration:

https://www.boltscience.com/pages/junkertestvideo.htm

https://www.boltscience.com/pages/vibloose.htm


The next question that will likely arise is whether connecting rods experience transverse loading; here is a thesis that mentions this:

https://www.steel.org/~/media/Files/Aut ... d_thesis.p

4) The stress multiaxiality is high (the transverse component is 30% of the axial
component), especially at the critical region of the crank end transition.

The stress is present -- whether movement of a cap with respect to a beam will occur during operation in a particular case is empirical.

[1972ho]

The engine did experience some vibration but I figure it was from a bad motor/transmission mount and I had the innovator west damper rebuild along with the mounts and it went away,but it may have been something else.

[modok]

Rod bolts do not vibrate loose.

Short bolts in sheer force do,
but rod bolts are not short, and they are not in sheer.

[mag2555]

The bolt must be stretched beyond where any load on the rod will ever try to stretch it, then it will not back out, but could just break .
If you use rod bolts with the locating dimples so they can be set by stretch and you do not do so then any failure is on your shoulders!

[Kevin Johnson]

Rod bolts do not vibrate loose.

Short bolts in sheer force do,
but rod bolts are not short, and they are not in sheer.

There was a time that many connecting rod bolt fastening systems included lock washers of one sort or another. They fell out of favor because they would loosen during service.

Safety wiring or the use of split pins with castellated nuts was also tried.

Here is one example of the hits returned for [castellated safety wiring connecting rod bolts]:

https://books.google.com/books?id=S9YIA ... ts&f=false

These systems were not abandoned because they were working flawlessly.