How do I test unknown bolts?

Spark test. You will have no problem distinguishing medium carbon from mild steel. Difference between grades 5 and 8 will be harder. Maybe a file or test? The late, great guru of anvilfire.com cautions against depending too much on the extra strength provided by grade 8 over grade 5. I bought a box of mystery drill bits. Some were simple carbon steel. They were impossible to keep sharp. Ended up retiring all of them (that doesn't mean throwing them away).
 
ericc said:
Spark test. You will have no problem distinguishing medium carbon from mild steel. Difference between grades 5 and 8 will be harder. Maybe a file or test? The late, great guru of anvilfire.com cautions against depending too much on the extra strength provided by grade 8 over grade 5. I bought a box of mystery drill bits. Some were simple carbon steel. They were impossible to keep sharp. Ended up retiring all of them (that doesn't mean throwing them away).
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I already know which bolts are grade 2, 5, and 8. I wanted to know if they are up to nominal specs.

The grade 2 made it to almost double the rated torque specs. 35 to 40 ft lbs. I'm not dissatisfied. They are rated for 20 ft lbs lubricated.
 
Bolt torque failure isn't a good indication of bolt strength. Torque specs for fasteners are intended to provide guidance as to the maximum amount of torque that can safely be used without damaging the fastener threads.
 
RJSakowski said:
Bolt torque failure isn't a good indication of bolt strength. Torque specs for fasteners are intended to provide guidance as to the maximum amount of torque that can safely be used without damaging the fastener threads.
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It did show a clear difference between grades.
 
RJSakowski said:
Bolt torque failure isn't a good indication of bolt strength. Torque specs for fasteners are intended to provide guidance as to the maximum amount of torque that can safely be used without damaging the fastener threads.
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Torque to failure may not be a 'good' indication of bolt tensile strength but surely both values would always increase together? Would be really interested to know where that relationship does not apply to the world of common fasteners.

Cheers
Clint
 
If you find any marked A325 Those are structural torque spec is above 300lbs. keep them for the real serious jobs. From a farmer your not likely to come across any of them.
 
Lots of good info here. But let me add a few important ones.

Testing to failure is a great way to test the final strength of a bolt. But then the bolt has been ruined.

Testing a sample of Bolts to failure is not a valid test unless all the Bolts and their histories are identical. Yours are not.

Therefore testing to failure isn't very useful.

You can certainly test to recommended torque for that size and grade. This won't have any affect on a good bolt. However, this does not in any way evaluate the strength of the bolt. Recommended torque is the torque required to hold a bolt in the fastened state under load. It is not intended to permanently deform the threads. Design torque maintains the stresses well inside the elastic range - well below the point where they deform or stretch permanently.


My advice is to use them as is. If they take their design torque they will likely be just fine. The only exception would be bolts that have started to fail in fatigue. But there is no way to test for that in a home shop. For mission critical applications, go get new bolts.
 
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