Nasty grinding noise

[chilly_75]

My GT is making a low-pitched grinding sound when speed gets up to about 25 mph or higher. I had someone run it through the gears when it was up on a lift, and I was able to listen closely to narrow down where it was coming from. It seems to be loudest around the front of the rear differential. Looking at an exploded diagram, my best bet is that the front pinion bearing is the culpret.
Two questions:
1- Does anyone feel strongly that this may sound like something else?
2- Should this kind of work be left to a real mechanic?

I'm usually a do-it-yourselfer, but I'm nowhere near a mechanic, and it seems like this assembly would be a tricky one requiring special tools.

Appreciate any help and opinions!

[First opel 1981]

Don't count it out. It's pretty close to the front of the rear differential.
I've never done a front differential bearing. Scares me to even think about it. I don't scare easy.

[grslightng02]

Seems like an obvious question but I'll ask. Have you removed the differential cover and drained the fluid out and cleaned it out with some brake kleen?

The fluid in my differential had not been changed in years (I'm sure because of previous owners) and it was very dark and dirty. Since changing the fluid and differential seal it sounds much smoother when I drive it.

[jeff denton]

Like a lab technician looking at your blood for a disease, study the gear oil. It will tell you if the rear is coming apart or not. Are there little tiny specks of metal in the oil? A magnet dipped in will help show this.
Need an experienced, educated guess? The pinion bearings are going. Both GT diffs I've been into had this problem.
There are threads about the overhaul of the diff. My advice is to have a real mechanic do it. Forget the manual, it will have you buying special tools and doing too much math. It's a simple differential. A mechanic can figure it out.
However, it will be pointed out over and over in this thread that you really want to start over with a later rear end. The '70 (early) style has problems in its design. Plus it's a lot simpler (and cheaper, that's important, right?) to swap the rear end assembly than to overhaul the early diff. You will be horrified at what the parts will cost!
Been there. Thanks again, Rallye73 for the later rear end, and thanks again Stan for all the parts to build it!

[GTJIM]

I trust that you have stopped driving it!

The GT diffs do have a habit of the pinion bearing spacer collapsing and then the pinion nut loosens so that the pinion moves back and forth - grinding the diff gears away.

If caught soon enough the pinion nut can sometimes be re-tightened and the diff will last a little longer.

Best start looking for a 1971-73 GT diff in good condition if the one you have has been driven for any length of time with that horrible grinding noise!

[chilly_75]

I had the diff oil drained and replaced earlier this fall. I checked for flecks with the magnet trick, and it appears very clean. I found no flecks in a couple attempts.
While under there, I noticed the torque tube donut has a lot of dry rot, so it needs to be replaced for sure. I think I'll take it in and have the pinion bearings, bearing spacer, and possibly the u-joint replaced. Any guesses on which is going to be in the worst shape when it's cracked open?

[kwilford]

Another possibility is the central joint bearing, which is a "sealed" bearing sitting inside a shell case, which sits inside the torque tube donut. It is quite common for dirt and such to get inside the bearing shell case, which then causes the bearing to make a grinding noise. And since it is directly connected to the rear drive shaft, the noise can sometimes be mistaken for a piled-up differential pinion bearing.

HTH

[chilly_75]

Originally Posted by GTJIM Best start looking for a 1971-73 GT diff in good condition if the one you have has been driven for any length of time with that horrible grinding noise!

I think it's time I begin searching for a later rear axle. Anyone have any leads on one?