Installing 2005-16 SD rear axle in 1967-79 Ford pickup
Leaf Spring Selection:
A modern rear axle can be used with most any leaf spring. You don't need Super Duty leaf springs with a super duty axle. Here is a summary of what to consider when choosing leaf springs leaf springs:
Lift height and block size. This is most relevant for 2wd applications. Most super duty axle conversions can be done with a 3" lift spring and a reasonable block size. Large block sizes are not ideal and should be avoided when possible.
Lift block locator pins. Lift blocks have locator pins on the bottom and locator pin holes on the top. A SD axle perch has a 5/8" diameter pin hole so you want to use a block with a 5/8 locator pin. Any smaller can lead to axle misalignment. A lot of non-Super Duty leaf springs have a pin that s 9/16 or even 1/2" diameter. Similarly, the block's locator pin hole on he top should be an exact match to your laves. Sometimes the mismatched sizing means you need fabricated lift blocks rather than something off the shelf.
Also modern F150 leaf springs (including F150 Raptor) have two locator pins instead of one. This requires a special fabricated lift block and a plate on top of he leaf that has two holes.
Leaf mounting brackets. When converting to any non-originals spring, you will need different brackets to mount the leaves to your frame. Most modern springs are longer than original 1970s springs. 1970s frames are either 34" wide in the rear (and 34" wide from the front axle area on back) or they are 37.5" wide in the rear and transition from 37.5" to 34" wide just in front of the forward leaf spring mounting brackets. If you have frame that is 37.5" wide in the rear and you go to a longer leaf spring, there is a chance that the forward mounting bracket will not be on a flat and square surface. Brackets we make for 2017 SD leaves are shaped appropriately for the application. This is not an issue for 34" wide rear frames.
Spring width. Most leaf springs are 3" wide and these are easiest. Some 1960s and 70s highboys, F250 crew cab and F350 (except supercabs) applications have 2.25" wide leaves. SD axles have perches for 3" leaves. To use 2.25" leaves, you need perches and plates meant for this narrower leaf spring. All of the applications with 2.25" leaves are going to have a 34" wide rear frame and therefore require cutting and welding of perches regardless.
Rear shocks:
Rear shocks are best selected by length rather than application.
We highly recommend converting to eyelet style upper brackets if your frame has stud style brackets. This will give a lot more shock options.
Assemble your rear leaf spring, axle and shock brackets first. Measure the length a shock would be with the weight of the unloaded vehicle siting on the tires and then the length with the frame sitting on jack stands. Take the average of these two lengths and search for a shock with this extended length, plus or minus a little bit.
We often use Bilstein shocks and this link will bring you to summit racing and let you search for eyelet shocks by length:
https://www.summitracing.com/search/part-type/shocks-and-struts/universal/yes/upper-mount/eyelet?SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=bilstein%20shock
And for all brands, not just Bilstein:
https://www.summitracing.com/search/part-type/shocks-and-struts/universal/yes/upper-mount/eyelet?SortBy=Default&SortOrder=Ascending
Some part numbers we have used with good results are:
2wd applications with 7" of lift: Bilstein 33-185552
Highboys with FR173S (3" lift leaf) and a 1" block: Bilstein 33-230320
Rear shock brackets:
SD applications often have on shock in front of the rear axle and one in the rear. You will want to have both shocks mounted in front of the axle.
Most applications will require weld on shock brackets. Applications with a 37.5" wide rear frame an earlier SD axles wit the bolt on shock brackets can use a different style bolt on bracket to keep this bolt on.
Upper brackets may be necessary if your xisting brackets are for stud style shocks. Converting to eyelet shocks is worthwhile for better selection.
more info coming soon
Pinion Angle:
The below pictures are a 1974 F250 4wd (highboy) converted to 2017+ rear leaves (Skyjacker FR173S) and a 2005 Sterling 10.5 axle. I originally had a more level pinion angle and I had a small vibration at 40+ MPH. Changing to this more upward angle and minimal u joint angle fixed that vibration and this truck is now freeway friendly as pictured.
Online you'll read that the transmission (or transfer case) output angle should be parallel to the rear axle input angle. I've not found this to be true. I've had best results with setting up the rear axle so there is a small angle at the rearward U joint, and the front U joint just winds up however it needs to be.
Driveshafts:
Most original driveshafts are fairly small diameter with 1330 or 1310 joints. SD front driveshafts are often larger diameter tubes with 1350 joints in the front and 1410 in the rear. Always measure and never assume the size to avoid errors.
If it's in the budget, new shafts with 1350 and 1410 joints are a really nice upgrade. You can get new yokes for a np205 transfer case that accepts these sizes you don't need any adapter joints.
You can also reuse oem driveshafts with a combination of adapter joints, lengthening and shortening. Summit racing or a local driveshaft shop are good resources for those adapter joints.
U Bolts:
Super Duty SRW axles have 3 5/8" diameter tubes. Many SD applications have the U bolts pointing rearward, but our preference is to change to upward pointing. This is easily done with round U bolts and plates on top of the leaves.
U bolts can be sourced in different thread diameters. We consider 5/8" the best option and all of our leaf spring plates accept 5/8" U bolts.
U bolt length will be a minimum of 3.625 (tube diameter) + block size + 1.5". You'll likely wind up with longer because of limited lenght options and that's fine. This link will be helpful for ordering:
https://www.summitracing.com/search/part-type/u-bolts-leaf-spring/u-bolt-style/round/u-bolt-i-d-width-in/3-625-in?SortBy=BestKeywordMatch&SortOrder=Ascending&keyword=U+bolts