Pinion angle confusion.

[bblawless]

Hey everyone, I’ve recently completely a 4” lift on my 79 bj40. I went with a 4” leaf kit from skyjacker, bilstein 5125s and a rear extended shackle.

Love the way the truck sits, but I’m now experiencing vibrations at high speeds, which I’m assuming is due to incorrect pinion angle. I’ve gone ahead and purchase 4 degree shims, but I am confused about which direction to install them. I would think you would install them with the thick side facing towards the rear of the vehicle? Thus tilting the rear diff output back downwards to achieve the desired “parallel”? But I’m finding conflicting information online and need some clarity…would greatly appreciate any input thanks

 

[charliemeyer007]

Have you contacted the manufacturer? What about searching forums, such as the frequently ask?'s I'm no expert but 4 degree shims are usually used to correct castor in the front end, which is different than pinion angle(s).

 

[Mark Jennings]

You are correct, fat side to the rear…..and with extended shackles….you may want 6* shims. A 2.5” lift with shackles is generally a 4* shim. A 4” lift rotates the axle even more.

Get a cheap angle finder at Home Desperate, the pinion should be parallel with the TC output shaft. Measure twice, install once

 

[pb4ugo]

4 degrees sounds like too much for the rear. Get a cheap angle finder as @Mark Jennings mentioned and actually measure the pinion and tcase angles. If you over correct with the wrong shims you can wear out the pinion bearing and seal quickly. I run a 4" skyjacker lift. When I measured my rear pinion and tcase. I found it was about 2 degrees out. My driveline is modified. I have some 2* shims but haven't installed them yet. I did need 6 degree shims for the front to correct the caster. 4 degrees worked but 6 degrees was better.

 

[franklin40]

there not really a “correct” shim that works for every lift. there are so many variables in a lift kit that there no a generic answer. as the guys above have said measure your pinion flange and your transfer case output flange with a cheapish magnet angle finder and the difference will be the amount of shim you need. you want to get the flanges parallel i have a 2 inch lift with slightly longer than stock shackles and i needed 4 degrees. it’s kind of a specific to your truck thing

 

[B y r o n]

The info in this thread should help.