[mjf454]

Are there different grades of metric bolts? I need longer spindle bolts, and I want the right strength bolt.

[35th_Anniversary...]

So should I be afraid that home depot (hillman brand) grade 8 bolts hold in my lap belts?

Thing I would be worried about from Home depot is source of said bolts.

Chinese steel = crap

Also, if you can find them, "rolled" threads are much preferred over your normal cut threads. Grade X is just specifying the ultimate strangth of the bolt.

Rolled threads actually strengthen the threaded region where cut threads will make it weaker since you have sharp points at the bottom. All our threads are rolled at work, we have a thread rolling machine. Needless to say when you have almost 300,000 pounds of force reversing on a piston rod 5 or 5.5 times a second and you're compressing hydrogen you don't want cut threads.

Other things to look at is to ensure there is enough stretch on the bolts. Proper torque and lubricant (anti-seize has a lower nut coefficient than mineral oil and we have two seperate torque tables at work that we provide to our clients) are important to get the right pre-stress in the bolt. There are many boltings that have literally no stretch. You should have at least 2.5 times the diameter free length of the bolt to stretch (not engaged in the tapped hole). This is most important on bolting that sees cyclic loading. The idea is to always have the bolt in tension rather to have it cycle between tension & compression.

Tightening a weaker grade bolt to a higher grade bolt torque specification can yield (damage) the bolt.

Randy, did your bolt break at the first thread?