New rear bearing bad after 1k miles

Pat S · 02-19-2006, 04:25 PM

I replaced my rear wheel bearings about a thousand miles ago. (74 911). Actually I took the arms to a reputable porsche shop and had them do it for me since I don't have a press.
Today I decided to "tech" my car to make sure it is ready for autocross and spirited driving this spring and to my surprise one of the rear wheels has slight movement. Grasping from 12 & 6 or 3 & 9 I can get a little play, enough to make a slight click sound. The other side is nice and tight. Neither have any rumble and sound good when spun.

Just to rule everything out, I pulled the rotor and axle and with just the hub in place I was still able to get the click, so I know it's the bearing.

My question is, what could possibly make them go bad so quickly?

The only thing I can come up with is improper installation or defective bearings...

Anyone have any other ideas before I replace them again?

Thanks,
Pat

Steve Marshall · 02-19-2006, 05:30 PM

I've heard the aluminum trailing arms can have the bearing housing become slightly enlarged or out of round, causing the outer race to wobble. Considering the age of your car, this could be the problem. I've never actually had this occur to me. I also understand that the answer is to shim the bearing. Surely some Pelican has had this happen and can elaborate.

patkeefe · 02-19-2006, 06:02 PM

Quote:

Originally posted by Pat S

The only thing I can come up with is improper installation or defective bearings...

Thanks,
Pat

I agree with your opinion. If you had a bad trailing arm, the reputable shop would have brought it to your attention.

Pat

Grady Clay · 02-19-2006, 06:10 PM

Pat,

Why did you replaced the bearings in the first place; a failure, an excursion or proper maintenance?

Steve has identified one of the possible causes.

Have you checked all the other hardware; trailing arm to torsion bar tube, parking brake backing plate to trailing arm, trailing arm to spring plate and spring plate to chassis?

Even things like loose CV joints can sometimes give these apparent symptoms.

When you removed the axle, did you leave the stub axle undisturbed or did you undo the big axle castle nut? If you undid the nut, was it too easy?

Even it you undid the nut; you can torque it back to spec. The bearing will appear “loose” if the nut isn’t tight to spec on the stub axle.

When you reassembled everything 1K ago, what spec did you torque the axle nut to? Was the torque wrench calibrated? Did you have to do anything to position the castle nut so you could install the cotter pin? Was there any sign of deformation of the part of the parking brake backing plate that retains the bearing?

As you dig deeper into this, loosen the four bolts that hold the parking brake backing plate to the trailing arm and see if the bearing is loose in the trailing arm. You will need to re-tighten them to get the hub out.

If you have to expend a bearing for peace of mind, it’s worth the effort.

If you find the bearing isn’t a tight press-fit in the trailing arm, there is a Locktite product that may help. Of course this depends on your application. If this is a severe problem, the solution is to replace the trailing arm.

Careful measuring of the bore in the trailing arm will tell.

It is not unusual to have to use a press to remove a bearing. Many times you can tap it out with the P-tool. You should be able to install a new bearing easily with differential heating/cooling – heat the trailing arm with a torch and cool the bearing in your freezer or with dry ice.

Hopefully it is something simple.

Best,
Grady

patkeefe · 02-19-2006, 07:14 PM

I was quick to agree with Pat on his assessment of bad bearing/improper installation. In deference to Grady's wealth of knowledge, I am reconsidering my remarks. Grady does indeed point to the multitude of symptoms and fixes. However, if Pat has only the stub axle in the bearing, and wiggles it around and gets a clicking sound, there is likely a problem with the bearing.

I had mine replaced by a machine shop last year as a PM item. They pressed one in crooked, and it shot the bearing in no time. I then had to build a removal tool to get the bearing back out, and buy another bearing (now, I'm up to three bearings in two weeks). Also, I had to clean out the shoulder area where the bearing seats in the inner trailing arm side of the hole, as some of the aluminum was smooshed over this shoulder. Luckily, I was able to get mine back in with the differential heating/cooling method Grady described.

So when the $15 per hour kid in the machine shop realizes (if he does indeed realize) that he just scrapped out a $900 trailing arm and a $200 bearing, do you think he is going to admit to it?

I believe that is a sealed double row ball bearing (if I remember correctly). Any misalignment of the bearing will kill the ball carrier.

The bearings are a light press fit in the trailing arm. If a machine shop has the bearing just drop out, it should raise a red flag. Likewise, if it can be pushed in by hand, or is not going in just right, that also should raise a red flag. My point here is that anyone who is a machinist will spot a potential problem in about one minute, and notify the client before dropping a high dollar bearing into the trailing arm and sending it out the door.

Of course you could have installation error, which I can see as being only insufficicent or excessive torque of the big castellated stub axle nut.

It is also possible that he stub axle itself is too small an OD, and that is what is causing the clicking sound, by being loose on the inner diameter of the bearing. Recall, Pat stated the bearing functionally make no noises and rolls smoothly. So this could simply be an axial play issue and the bearing may be ok after all.

Pat

Pat S · 02-20-2006, 03:42 AM

Thanks for all the comments guys.

Grady,
Storz is the shop who removed the old bearings and pressed the new ones in. So I think that part is okay, they would have definatly said something if they noticed something wrong.

Quote:

Why did you replaced the bearings in the first place; a failure, an excursion or proper maintenance?

I had the whole suspension off and was replacing everything... shocks, bushings, bearings, etc... My guess was that the old ones had been in there 30 years so I might as well replace them while I had the arms out. Plus I did have some bearing noise that I couldn't locate at the time... It turned out to be a streched intermeadiate plate and worn bearing on the pinion shaft in the transmission that has since been rebuilt

When I pulled the hubs for that time they were very tight. and the race for the hub stuck to the hub while the race in the arm stuck solidly in the arm.

Quote:

Have you checked all the other hardware; trailing arm to torsion bar tube, parking brake backing plate to trailing arm, trailing arm to spring plate and spring plate to chassis?

I didn't check the e-brake plate, but I did remove the castle nut and pull the axle, and I checked the spring plate. With just the e-brake parts, and the hub on the arm I was able to still get the same amount of play.

Quote:

When you removed the axle, did you leave the stub axle undisturbed or did you undo the big axle castle nut? If you undid the nut, was it too easy?

Even it you undid the nut; you can torque it back to spec. The bearing will appear “loose” if the nut isn’t tight to spec on the stub axle.

When you reassembled everything 1K ago, what spec did you torque the axle nut to? Was the torque wrench calibrated? Did you have to do anything to position the castle nut so you could install the cotter pin? Was there any sign of deformation of the part of the parking brake backing plate that retains the bearing?

I think this is going to be the problem. The castle nut was tight but it didn't seem to take as much pressure to loosen as I recall putting on it to tighten the last time. I was able to get it off with just an 18" breaker bar. No where near what I expected. I didn't use a torque wrench (didn't have one that went to that spec) so I used a 3 foot breaker bar with me standing on a scale method to tighten to what I though was the right spec. But on one of them (I can't remember which side) I did have to adjust the nut to get the cotter pin holes to line up and I remember backing off the nut to make that happen rather than overtightening it.

Since that nut was looser than I expected I reassembled everything and retorqued it to 340ft lbs. (Again an "estimate" but I do have a wrench that goes to 300ft lbs now and I used it first just to be sure I was at least there. I also retorqued the other side and it needed just a couple degrees of a turn to be right). Anyway there was no change in the play, so I suspect at this point that nut was undertorqued and that it did damage the bearings.

I plan to pull the old bearing out and replace it again. When I check the bore in the arm is there a spec to check it against or am I just making sure it is round and that the bearing fits tight? Same thing for the hub?

Thanks,
Pat

captainrb · 02-20-2006, 07:26 AM

It seems that with that much play you could 'see' the move ment. I have replaced my '73 trailing arms for aluminum one from an '81 SC. Thers is a small diference between '73 and '74 and later modle. But don't beleive they will interchange,the differece is to great. At first off I thought you had improperly torqued the axile nut.

Driver8 · 02-20-2006, 09:39 AM

The torque spec for the castellated nut is 217 -231 ft-lb. If you over torqued the nut that may have caused damage (stretching) to the threads of the axle. The 340 ft-lb torque spec is for the self locking nut which does not use the cotter pin.

Pat S · 02-20-2006, 10:10 AM

Thanks,
I don't know how I missed that. My guess is that it would also put more stress on the bearings hence the problem I'm having.

It's good to know there was a problem and now I know how to avoid it when I install the new ones. (even if I did cause it)

Pat