There's no such thing as a Grade 10 bolt. With regard to SAE hardware standards, there are Grade 2, Grade 3, Grade 5 and Grade 8 bolts.
Grade 2 and Grade 3 are commonly known as "mild steel" bolts and have only moderate strength, being made of simple low or medium carbon steel.
Grade 5 are commonly called "high tensile" and are made of medium carbon steel, quenched and tempered. They have a minimum tensile strength of 120,000psi.
Grade 8 fasteners are the strongest of the SAE ratings, being made of heat-treated low alloy steel and meeting a minimum of 150,000psi tensile strength.
Grade 5 fasteners can be recognised by the 3 dashes on the head of the fastener. Grade 8 fasteners are recognised by the 6 dashes on their heads.
All automotive seatbelt hardware I have ever seen (that's quite a bit) utilises Grade 8 fasteners.
Some manufacturers rate their hardware higher than Grade 8 - but this is not recognised by the SAE.
Caterpillar rate their crawler tractor track shoe bolts at Grade 9 and 180,000psi tensile strength, but this is a Caterpillar rating, not SAE.
Caterpillar quite often refuse to adopt SAE ratings and produce their own standards.
Unbrako produce alloy steel socket head (Allen head) capscrews that are rated at a consistent 190,000psi tensile strength, when the usual standard for U.S.-made socket head capscrews is 170,000-180,000psi tensile strength.
There is no SAE Grade for the high strength alloy steel capscrews.
Metric hardware is rated as Classes, not Grades. Metric Classes are Class 8.8, CLass 10.9 and Class 12.9.
These "classes" are approximately aligned in tensile strength with the SAE Grade 5, Grade 8, and Grade 9.
Aircraft AN bolts are rated at 125,000psi tensile strength, thus making them only slightly stronger than SAE Grade 5 hardware.
25mm webbing cam lock straps are only rated to about 200kg load capacity. A car seatbelt tongue is rated to hold 4000lbs (approx 1815kgs) for 30 seconds.
I do not know the strength rating requirement for an aircraft seatbelt, but I would be surprised if it was little different to automotive seatbelt strength ratings.
What the OP really needs to do is install some approved tie-down brackets for load restraint, mounted in manufacturer approved areas in the aircraft frame.
I don't understand why he thinks a few hundred dollars is a steep cost to protect yourself from cargo getting airborne and hitting you, in the event of rough flying conditions - or even worse, a sudden stop in the event of an emergency.
Last edited by onetrack; 31st May 2016 at 12:48.