Find a heavy duty truck parts store, they should have what you need. On u-bolts on a class 7 or 8 truck, we always used hi-nuts with a hardened flat washer between the nut and mounting plate. Most any heavy truck parts should be able to help. Most parts counter people have no clue how different a large truck is from a car or truck. When I was working, at certain stores there were people I wouldn't talk to because they though they knew it all. I don't know every thing but 99% of the time I knew what was needed, and most of them had no idea what I needed. I hated to have to break in a new counter person. Heavy truck specialists can usually offer suggestions as to what will work.
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