[Vegvisir]

If you already know this skip on by, for everyone else I just thought I'd share. So I work in an auto plant. We have periodic classes to "retrain" on many topics. An important one is torque. On themany fasteners that have NHTSA required torque recorded values we have to be extra careful. I have seen many times on these forums people saying to use WD40 on CAC boots. This is fine for removal as long as you thoroughly clean and dry them before re-torqueing them. Here is why, bolts have an assigned amount of friction to get to correct clamping force. when you use things like silicon (WD40), oils, wax, or even spit (yes we get this in automotive), your torque value is inccredibly off because the lack of bolt thread friction. I am attaching a photo of what only slight amounts of wax or even spit can do to a bolt. When we go through the class we train in, we see bolt torque triple over what the value is on the wrench after it is torqued to what even our sophisticated computer controled torque wrenchs hit desired torque. When we put a pressure pad between them to get actual clamp force we see the real story of what happens. I'll attach some pictures of what bolts look like that have "hit" torque but clamp force is overly high after using a friction modifier.

 

[Stt]

Very interesting information! I'm just a backyard mechanic and I didn't know about lubricant effect on torque until you explained.

 

[The Evil Twin]

Yeah man. The most common error in my world is where folks don't reduce the torque when they apply thread locker to a bolt that doesn't need it. Lock tite + spec torque = broken bolt. My favorite service call back in the day.

 

[1994fordf350]

This is one reason arp requires thier special liquid for torquing thier studs and bolts, with thier lube you get exactly what they want out of the stud.

I've seen this many times as well as happened to myself and not following arp instructions to a T.

 

[TooManyToys]

Not "exactly"; there is still some fastener to fastener variation even with the ARP lube. They show that in some of their graphs. But, lubricant or not, an angular tightening sequence is more accurate.

Vegvisir has it right; 85% of the torque values measure force against friction, varying on the lubrication. It varies on the quality of the threads too, and more. The clamping load/tension also changes every time you do another tightening sequel due to thread polishing. The same torque now tensions the bolt to a higher value.

Many times with new bolts, you can stay under the yield value of the bolt with some lubricant, strength dependent. But that also depends on how many times the receiving threads have been tightened or polished. But that's a judgment call that many don't make well.

Ford's black phosphate screws and bolts already have oil on the surface, unlike the bare steel bolts shown in the image. It's not a wet oil, but if you clean the screw/bolt with acetone, it will change to grey without the oil. Unless they spec'd a different finish for that fastener such as black zinc or "cold blackened."

The other situation is re-using bolts with the blue thread locker in place. Those re-use torques are way far off.