I have a '96 F150 and the rear springs look a little tired. My truck sits way low when not very much weight is put in the bed (700 or so lbs). So I am looking at new leaf springs. I see on Jeffs Bronco Graveyard website that they offer 5 different stock leaf springs:
F150 4x4 - 1700 lbs/spring
F150 4x4 - 1389 lbs/spring
F150 2x4 - 1660 lbs/spring
F250 - 2450 lbs/spring
F250/F350 - 2977 lbs/spring
So, I have a couple of questions.
1) Do leaf springs "go bad", ie - lose their "springiness"?
2) Will the F250 and F250/F350 springs fit my truck?
3) If I want to increase weight capacity of the bed, is it as simple as installing the 2977 lbs/spring leaf springs (or if they don't fit, just install the heaviest capacity ones that will fit)?
4) Would adding these heavier-duty springs possibly hurt my rear axle/internal parts (like bearings, etc)?
5) Per the label on my door jamb, my GVWR is 6250, rear axle GAWR is 3776. Are those weights set by the factory springs that were installed, or some other criteria? In other words, is the 3776 number set because springs that are rated at 1888 lbs/spring (half of 3776) were what was installed at factory?
Springs don't really change their "springiness" much, but they do sag so they sit lower all of the time. If you put an additional 700 lbs on new springs or old springs the truck would settle down about the same amount, but with old springs it will be starting lower so the squatted height will be lower as well.
I don't know if F-250/F-350 springs will fit your F-150. The biggest issue I could forsee is spring width. If the F-250 springs are wider they won't fit into the F-150 brackets. Not sure if they are, just what I'd be looking for. New hangers on the frame, new U-bolts and plates, and new shackels for an F-250 would address that (IF it's necessary).
If the F-250 springs aren't a direct drop-in a local spring shop could probably make you up a set of heavier springs that would fit if you could tell them what you wanted.
The heavier springs won't hurt anything on your truck (other than the ride). But if because of the springs you carry heavier loads, that could cause additional wear and/or breakage if you end up overloading other components.
4x4 F150 will have 3" wide springs, same as the F250 & F350. My truck was in the same condition, I put factory rated springs in for replacements and it was much much firmer. The old springs were shot, sagging and soft. 1000lbs put it on the bump stops and felt overloaded, now 1000lbs gives about 3" sag and doesn't feel overloaded. The F250 & F350 springs would probably lift the back of the truck a bunch too.
The axle itself and the wheels/tires would be some of the first limiting components other than the springs, next may be the frame.