Hot Rods Bolt grade Question

The pretense that the higher the grade of bolt you use the stronger it will be, a grade 8 being high and strongest. is that really true? As the grade gets higher doesn,t it get more brittle ?? Gene

 

There’s an argument that grade 8’s are more brittle than grade 5’s and that’s why you shouldn’t use them. Well, first you need to understand what the term “brittle” really means. Brittleness in bolts is defined as failure at stresses apparently below the strength of the bolt material with little or no evidence of plastic deformation. Typically, fasteners are not brittle below 180 ksi ultimate tensile strength. Grade 5’s have an ultimate tensile strength of 120 ksi and a grade 8 fastener has an ultimate tensile strength of 150 ksi. This is why brittle is a relative term. Nearly all fasteners are considered ductile and are not Brittle!

 

Argument with who? I mean, who is saying this?

One thing to keep in mind no matter what is stamped on the bolt head, with all the ersatz and counterfeit crap out there these days unless it's strictly necessary to replace a fastener with new we're probably better off using good old genuine Henry steel.

 

It comes up on forums all the time - on here there have been discussions about this Many times over the years. Too many folks have bought into this old wives tale and just spew it forth

 

the more correct way to say is " as metal gets HARDER it gets more brittle."

 

OP do a search..Been beat to death and back with in the last three months or so..

 

How then does constant in use vibration relate to the different bolt grades? I was under the understanding that while a higher number grade of bolt is harder/stronger in tensile strength, it is also more susceptible to fracture failure due to metal crystallization. What is the relationship of toughness and tensile strength in bolts?

 

Do A Search...That very question was answered in depth..

 

Fatigue failure of bolts is usually caused by the bolt coming loose. It is widely believed that vibration causes bolt loosening, but this is rarely the case. By far the most frequent cause of loosening is side sliding of the nut or bolt head relative to the joint, resulting in relative motion occurring in the threads. If this does not occur, then the bolts will not loosen, even if the joint is subjected to severe vibration. There have also been studies done that hypothesize that the gold cad plating on most grade 8 bolts have a differnt coefficient of friction compared to grade 5 and they can end up under torqued because of this, which causes minute loosening which means a loss of clamping fore and eventual failure

 

The street rod build (Engineering) guidelines in this state require cars be built to a minimum safe standard stipulate:-

• Ungraded bolts - Panel fixing floor panel fixing and lightly load brackets;
• Grade 3 bolts - seat bolts;
• Grade 5 bolts - Moderated loaded member; and
  
• Grade 8 bolts - Brake calipers, master and slave cylinder mounts, steering arms and all heavy loaded assemblies.

Other states require engineer inspections before registration. This state has a mandatory 3 x stage inspection process before car registration that ensures compliance with build guidelines.