I'm not sure if I'm seeing an issue with the new leaf spring u-bolts. They are grade 8, 5/8" and about 11" long, ordered from Stengel Bros, who provide all sorts of suspension components. I don't believe this to be a quality issue.
When I installed them, I checked the torque sequence and rating. These are unplated Gr8 that have a torque spec of 170 ft-lbs. I installed them to 155 ft-lbs (I checked torque with 2 different torque wrenches) as I thought this would provide plenty of clamping load. Plated u-bolts have a spec of 120 ft-lbs.
I've only put about 30 miles on the van, but as I've checked the torque 4-5 times now, I've noticed that they typically loosen up by about 10-12 ft-lbs. That's even while sitting in the garage. I've installed u-bolts before but not this length - other shorter u-bolts seemed to reach and hold torque without repeated need to re-torque.
We're heading out Monday on a 200 mile trip and I'm taking my torque wrench with me to check them. But it's bothering me and I keep trying to understand what's going on here. To add to this, I checked the factory manual and it shows a torque spec of 103 ft-lbs

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Spring leafs and spacer are fine so that leaves either the u-bolts still stretching or the spring plate getting distorted. Here's a pic of the spring plate (stock photo but correct part number). It's a robust, thick piece with very good bearing on the axle tube at the ends and in the middle. I can't visually see any deformation - I'll see if I can get a straightedge on it though.
The bolt holes in the spring plate aren't extended out from the main part of the plate, and are very well supported by the way the plate is designed to rest against the axle. The axle tubes themselves look ok and don't seem to be distorted. Right now I'm inclined to see if the torque lands on something consistent, but I can certainly replace all u-bolts and spring plates if needed as none of them is a high dollar item.
Any input is appreciated, thanks.
Edit, I should also mention that I used the deep style nuts (not nylock) and extra thick washers provided with the bolts, threads were oiled, with a very light smear of antiseize on the washers.