Rear Diff Pinion Angle on a C3

[Rick B.]

Recently I had my rear diff rebuilt due to a healthy amount of torque my engine was delivering. While I was at it I was going to change out the driveshaft, replace the U-joints and put in new rubber cushions on the leaf spring and front diff carrier. I also decided to replace my old Delco shocks.
Prior to ordering the new driveshaft, I set the car back down so the weight was on it and immediately noted that the rear end of the car sat a couple inches higher than before. Now I wasn't surprised because I had previously rebuilt the leaf spring and it sat about 1.5 inches higher than it should based upon dimensions in the AIM. However now it sat even higher (almost 3 inches). Now thinking about it logically I figured it will settle a bit and given the gas tank is empty and a few components are not bolted on/in the car at present I was not overly concerned.
However, in taking measurements for the new driveshaft I noted that the pinion angle off the diff is -2.8 degrees (pointed down towards the front of the car). The transmission yoke is sitting at a respectable -2.1 and is well within reason of what is expected, but I am puzzled by the downward angle of my diff pinion.
Everything I have read suggests the two need to be opposite and cancel one another out along a parallel line to avoid vibration. Admittedly I didn't take note to see what the pinion angle was before I took the rear out and rebuilt it. Looking at the tight tunnel clearances I suppose it is possible it was pointing up but that is a significant change from a negative to a positive angle at the rear end position - especially considering my numbers.
I've looked at everything to try and figure out what might have caused this, what I could've done wrong, what I missed, but I can see nothing wrong and have even brought in others for a different perspective. So far it has stumped me.
Do these IRS cars typically come with a 3 degree upward angle on the diff pinion or am I applying incorrect logic/info to this situation?
Can someone provide some feedback on this issue please?

Thank you in advance!
Rick

[Edward J.]

Re: Rear Diff Pinion Angle on a C3

Rick, I don't think that anything could change, If you replaced the leaf spring cushions and snubber bushings this may have corrected everything and is back to where it should be.The old bushings do flatten out over the years, The snubber bushings will change the driveline angle since there are new.

The car will settle after driving it a few hundred miles , I would say a inch or two anyway. I would also drive the car and see how it feels before making any judgements on the angles.

[Rick B.]

Re: Rear Diff Pinion Angle on a C3

Ed the problem is that the cushions I changed out really were in quite good shape and didn't actually need to be changed as they had not compressed much at all. I just tend to be anal about such things and wanted to do it now since it made more sense. The Bilstein shocks I put in did indeed increase the static height although conventional logic and wisdom would not support that necessarily - but various searches I have conducted shows others found a similar result. While I am okay with the increased ride height temporarily, I am concerned about whether these angles will cause an unacceptable situation due to stress on u-joints, etc.
I can certainly drive it a bit to see if it does come down, but I am still curious as to what a "normal" pinion angle would be in a similar car.
Thanks.

[Edward J.]

Re: Rear Diff Pinion Angle on a C3

GM did make a tool to check the angle of the drivshafts When I was a GM tech I did use it a few times to check driveshaft angles for driveline vibrations. If the angle is off you will know when road testing, get it up to highway speeds and see how it feels.

If you are judging the pinion angle by sight and not a gage,this isn't acurate.

The had the bilstein shocks in my 72 and it may cause the suspension to lift only slightly from the gas charge. don't forget the rear leaf spring supports the weight of the car not the shocks.

[Rick B.]

Re: Rear Diff Pinion Angle on a C3

You're spot on, but I did use several gauges that were accurate to within 0.1 of a degree and got very close if not identical results.

[Paul D.]

Re: Rear Diff Pinion Angle on a C3

Rick,
I can't speak to what the correct angle is (degrees up or down), but I would suggest you reference the AIM or a sevice manual and make sure that the pinion mount cushions are installed in the correct order and properly seated in the mounting bracket. They can be installed wrong and it will affect pinion angle and make bothersome clunking noises. Also, I wouldn't be too concerned about ride height until after the car is all back together and has been around the block a time or two. It will settle substantially. Chip.

[Rick B.]

Re: Rear Diff Pinion Angle on a C3

Thanks, however that was done several times and all was confirmed as properly installed. Also, just to illustrate just how much of an angle -2.8 is, I removed the rubber cushion entirely and allowed the mounting bracket to come up and make contact with its' metal counterpart on the frame. With absolutely no cushion installed and metal-to-metal contact it only brought the pinion angle back up to -2.1 degrees !
This is why I am hoping that the actual normal angle of the diff pinion is not in the 3 degree range otherwise I have to start looking at whether the Muskegon diff cover was machined incorrectly at the factory - although it appears identical to the stock one I had on originally.