Not everything, but we know our feet from a pound of... potato's

The only SAE bolt that fits your guidelines is a plain 3/8 Grade 8 bolt.
Available in UNC (16 tpi) and UNF (24 tpi).  Please note that both are SAE spec bolts.
Cross sections for each are .0775 and .0878 sq.in. respectively.
The torque to achieve the same yield strength factor is 47 and 54 ft-lbs respectively.

So if both bolts are tightened equally to 50 ft-lbs, the 3/8-16 will be over torqued and the 3/8-24 will be under torqued.  Adjusting for the difference in cross section gives a 13% drop in applied force.  Varience in torque is 6%.  I believe that gives the 3/8-24 a 1% advantage.

"Just the facts, maam" Sgt. Jack Webb.

yeah, what you said.  

verslagen1 wrote on 06/05/08 at 09:19:50:



Okay,, let me try again,, since I couldnt even begin to grok all that without someone sitting down, teaching for hours & all that, The question I was getting at was, IF one bolt has more threads per inch than another, then if each is torqued to the same torque, doesnt the finer threaded bolt create higher clamping forces?

Now, if I have a bolt thats, as you say, Undertorqued, then is that a problem? Will it tend to get loose? Or, is it just an ineffient use of the bolt & a manufacturer would be better off using a different bolt in an application that shouldnt be clamped so hard?

I thought I could look at the threads & recognize SAE, as opposed to what I dunno, just thot SAE bolts had a finer pitch..duhh,

Clas is open,, teach away..

I'll bite.

The finer threaded fastener CAN/COULD develop more clamping force at the same torque IF the grade of the fastener (mechanical stretch resistance of the fastener alias PSI/point count/hardness/strength) is greater than the coarser threaded fastener to support that extra tension.

The included incline plane of the thread is at a lower angle on a finer thread.

More to the point in the real world:  torqued fasteners should be untorqued, lubricated with appropriate lubes such that the torque applied by the torque wrench gets effectively turned into pure bolt "stretch" which we also refer to as clamping force.

An error is to take a corroded fastener such as a head acorn nut and apply an increase torque with a torque wrench starting from the fully tight and corroded condition.  Nothing is going to move, you just twist the long shanked stud around in a rotational fashion -- does a little damage to the stud's integrity but does nothing to increase clamping force on the head gasket.

First you break the acorn nut free, unscrew it, clean it up, put a little never-seize on the stud threads, put the nut back on then run it up to the specified torque with the torque wrench in one increasing movement.  

Now you got some increased clamping force compared to when you started out.

My analysis was gleamed from here...
http://www.zerofast.com/torque.htm

The requirement for the 75% stress factor is fatigue life.

Story goes, years ago, wheels used to pop off and roll off down the street by themselves cause they were tired of black cars and wanted to roll with color.  Well the owners of the black cars got pissed off and complained that tires were breaking the studs in 2 cause it couldn't be the color of the car.  The car mfg's looked at it and found that the tire would rest all it's weight on one stud to break them one at a time (someone must have told them the story of how to break a bundle of sticks).  So how do you keep the wheel from putting all its load on one stud?  You tightened them all down real tight so that the wheel has no room to move.

Ok, really... there's 2 types of stress, static and dynamic.  Static, there all the time and dynamic, moving, fluctuating, changing all the time.  Static failure in a bolt would be like a head twisting off from torquing it down.  Dynamic failure would be from snapping off while driving down the road.

This might give you a little insight as to why our head cover bolts twist off upon retorque.  The life of a bolt is acummilative.  You torque it down, it goes for 20 years bouncing up and down.  You twist it off without much effort.  Torquing it the 1st time takes a little life.  Bouncing up and down takes more.  Taking it off and putting it on just isn't in the picture. (excuse me I've got to go replace some bolts)  

Sandy Koocanusa wrote on 06/05/08 at 09:51:28:


Huh?  Do you talk like that all the time?  My "Hooked on Phonics" brain can't handle that one