<VV> Grade 8 Bolts

[jwcorvair at aol.com]

The specs for grade 8 bolts are correct; however, bear in mind that these values are for a fastener in pure tension. Grade 8 bolts do not have good cyclic qualities; that is, they do not flex well. In an application that has some movement in which the bolt is required to bend some, the grade 5 will work better. When I was flying ultralites, we would screw the bits (engines, seats, landing gears) to the air frame tubes with grade 8 fasteners. The wings, however, were screwed together with grade 5 fasteners. Why? Well, because the grade 8 fasteners would snap after a few hours of flying time. The grade 5s lasted far longer.

The choice of fasteners depends on many factors. For all suspension hardware, I use grade 8 (cause I'm too cheap to buy Mil-Spec). For most other applications, I use grade 5. Everyone who works on cars (even for fun) needs a copy of Caroll Smith's "Nuts, Bolts, Fasteners, and Plumbing Handbook."

Regards,
Joe White (62 Sedan, 66 Porvair)
CORSA, RMC


I have not done any comparative testing of bolts, but what I understand 
is that while Grade 8 (and higher) bolts do have a higher tensile 
strength, they are overall more brittle than Grade 5 bolts, and are not 
perhaps the de facto 'best choice' for some applications, especially 
those taking a lot of high shock loads and abrasive wear. 

Absolutely incorrect!  Grade 8's are LESS brittle than Grade 5's based on
the fact that they are alloy steel rather than Low-Carbon steel.  And the
numbers are:

SAE Grade 5:  1/4-1"
  Proof Strength:  85,000 PSI
  Tensile Strength:  120,000 PSI
  Material:  Low-Carbon Steel, Quenched & Tempered

SAE Grade 8:  1/4-1"
  Proof Strength:  120,000 PSI
  Tensile Strength:  150,000 PSI
  Material:  Medium-Carbon Alloy (Mb) Steel, Quenched & Tempered