Sorry if you know some of this already, but I'll go for all of the basics just in case you don't.
The C-bushings that are clamped around the axle tube by the radius arm caps were rubber from the factory with zero offset. Using them as "0-degree" then, the aftermarket molded polyurethane replacements can generally be had in 2°, 4° and 7° offsets. There are 4.5, 6 and some 7.5 degree offsets too, but not as common.
The offset is a tool that the aftermarket developed to adjust the "caster" value with regard to the front-end alignment. Caster is the amount of tilt away from vertical of a straight line between the upper and lower steering pivots. In our case, the ball-joints.
A rearward tilt at the top is "Positive Caster" and is what we try to achieve with our Broncos to improve (or even introduce!) straighter and more stable driving and some return-to-center if possible.
If you imagine the suspension traveling in an arc up and down, pivoting on the rear mounts of the radius arms, you can see that as the suspension moves, Caster changes. More to the positive side when the tires move up, more to the negative side when they move down.
When you lift the suspension then, you reduce positive caster and the offset bushings try to bring some of it back.
As we've found in the intervening years, a good number (majority maybe?) of EB's ,before about '75 or so, didn't even have the called-for caster right from the factory. Lifting those trucks just made a bad thing even worse.
For that reason, even though "the book" calls for about a 4 degree bushing with a 3" lift, I've just been telling anyone with a pre-'75 EB to just jump right to 7 and hope that it's enough.
The best way to know what you have and what you will end up with is to have the caster reading taken before the vehicle is disassembled. Which in most cases is not feasible it seems, just like in your case.
Factory specification over the years varied from between about 3.25°+ to 4.75°+ caster. Those with power steering can get away with higher numbers (more positive caster makes it harder to steer too) and those with bigger tires like to have even more than normal. Sometimes you're lucky to just get close to the minimum with an EB, so it's nice to know what you will need ahead of time.
You CAN have too much positive caster, but it takes quite a bit to get there on a Bronco.
On a side note, there are ways, other than C-bushings, to correct caster, but the bushings are the easiest and most direct and least expensive (usually) for most people. And, there is a point of diminishing returns with our rigs. The reason for the 7 degree limit on bushings is that even that amount is putting some front driveshaft u-joints near their limit. Since as you tilt the steering yokes back, you tilt the pinion down. At some point then, if you have to add more positive caster, the only method left that can work to both factor's benefit is to cut off the steering yokes (also called "outer C's") and rotate them back independent of the axle tubes. That way the pinion yoke can get moved back up inline with the driveshaft and the steering yokes can be rotated to whatever caster reading you want or need.
Sorry for the long-winded dissertation, but I thought you might not know any of that yet, and this would be a good thing for anyone else to read who's just now getting into these aspects of 4wd truck modifications.
As with most things we talk about here, making a change to even one little aspect of a vehicle leads to several other things that you have to co-modify so they can continue to work together safely.
Paul
