i got my A body 8.75" axle this past weekend, somebody removed the original perches sometime prior to me finding and buying it. i would assume he was probably going to relocate the springs and never did. i had a set of new perches from mopar performance to fix this problem.
an old timer i deal with locally told me that the perches are set at the same angle as the flats on the top of the tubes end flanges. i looked at the 7.25" axle tube that i was keeping as a chinese blueprint from my 74 dart. it has the same flat on the top of the flanges lining up perpindicular with the perches.
i set my 7.25" axle upside down on saw horses letting one of these flat flanges rest on the saw horse, the other flange i let hang off the other saw horse so i could use my inclinometer app on my phone turns out the flats on the end flanges are at the same angle as the flats on the perches.
knowing this i set my 8.75" axle housing on the saw horses the same way, got my 43" center to center for the perches, double checked perches to end flanges dimensions, double checked the the angles of the new unwelded perches in relation to the flange flats to make sure they were at the same angle, tack welded , double checked dimensions and angles, then welded em up tight.
I'm not sure of the pinion angle, but i verified what the old greybeard mopar guy i deal with told me by checking the housing on my old 7.25" so apparantly this is how the factory set the perches for the pinion angle.
from what he says this is set when the rears are built at the factory the tubes are jigged up and the ends are installed with the flats at a certain angle compared to the mating flange on the housing for the center chunk determined by the jig then the rear is put in another jig that locates the perches at the correct width, and with their flats perpendicular to the flats on the flanges.
this sounds weird until you look at yours if its a unmodified axle. mine was unmodified except somebody whacked the perches off to relocate them then never did. using my inclinometer app on my phone, i found the angle the flange flat was at while the axle housing was sitting on my sawhorses, then put my perches at exactly the same angle the axle end flange flats are sitting at.
i am sure there are other angles you can set them at depending on your buildup, and what you are going to use the car for. i have never modified axle perches from stock angle, and have never had any problems in the past. also since my current project car is going to sit pretty close to stock height in the back i'm sure i wont encounter any driveline problems, as a matter of fact my old greybeard mopar friend says he welds em in at this angle using the axle end flange flats as a guide all the time, and he has never had any problems.
also the stock width from flange to flange is supposed to be 52&5/8" width, but this axle housing was 52&7/8" a full 1/4" wider than what the chart shows. thats only 1/8" wider on each side. according to my old greybeard friend there are slight variations since this stuff was mass produced by hand (no computers or automation). he has seen this with mopar 8.75" axles, as well as ford and chevy stuff.
Im not sure if this is the right way to go about it, but it seemed to work for me. Btw the inclinometer app on my phone works great for this.
hope this helps someone out looking to find the "stock angle" to set the perches at.
matt
