FJ40 rear springs -
swapping stock packs for FJ55 rear packs.
Conversion performed March 2001
NOTE: This installation is in an FJ40 with a spring over and the rear springs already reversed.
When I had done the planning for my SOA setup, one of the things I wanted to do was run longer springs both front and rear. Since Land Cruiser springs are so soft and flex so well, I did some hunting thru the other models to see what their springs were like. What 
I discovered was that the FJ55 rear leaf packs were about 3.5 inches longer than stock FJ40 packs (47 inches eye-to-eye and 23.5 inches pin-to-eye). I also found that their spring pin was centered...and that the long side of a stock FJ40 spring pack was the same length as both ends of a FJ55 spring pack. So, as part of my SOA work, I also reversed the stock FJ40 springs to prepare for finding a set of FJ55 rear packs to install. The first photo shows a test fit of one of the spring packs...note the the tire is centered in the spring pack.
Another advantage to the longer spring pack is better spring geometry. The stock FJ40 spring pack keeps the short end of the leaf at the solid mount and the long end at the shackle mount. Reversing the spring packs changes the geometry of the spring action and as the spring flexes, will tip the pinion down rather than slightly up as it would in the factory setup. The longer rear helps alleviate this geometric change.
Photo #2 shows the reversed rear shackle hanger. I had already relocated it further forward to achieve the optimum 45 degree angle on the shackle, so I simply removed the two nuts/bolts that held it in place and reversed it's direction. This still keeps the shackle at a good angle for a soft ride and optimum flex.
Photos 3 & 4 show the difference between the main leaves and between the spring packs. In the upper photo, the center pins are aligned, so it's obvious where the added length comes into play. In the lower photo, you can see the additional wrap that the FJ55 rear packs have as well as the additional leaves. Stock FJ40 packs generally run 7 leaves....the FJ55 rear packs run 8. This particular set has all the leaves tapered. The set I used in the front has the 3 bottom leaves squared at the ends. The tapered set will flex and ride smoother, which is fine since the rear carries less load.
Photo #5 shows another shot of the test fit spring...notice that it has a pronounced V shape to it...there was so much rust and junk between the packs that it no longer kept it's arc. Fortunately, wire brushing the springs removed all this scale and solved the problem. I used a 4.5 inch grinder with a wire cup to remove the surface scale and rust. You can also see the difference in spring length. (BTW: the bump stops are still in stock location...they contact the edge of the spring plate and are still effective.)
Photo #6 shows the springs packs almost completely wire brushed and cleaned up. Photo #7 shows the springs after a coat of graphite based paint to help keep them slippery and flexing well. Unfortunately, this paint doesn't survive Wisconsin weather too well, but it sure looks great when first applied! I then asembled the springs using grade 8 bolts and nuts.