Loose Rear Axle Nut = Unidentified Rear Knocking Sound

[portlandpiddler]

Wanted to share this in case someone has a similar problem -

 

91 Loyale, Manual, 4X4, 160k plus miles

 

After driving along a curvy river road, made a sharp left hand turn and heard a kind of knocking sound coming from the rear wheels that sounded sort of like a small rock was lodged in the tire, or like a rock was caught in the inside of the wheel, or some such nonsense, but that wasn't the case. It went away after a bit, but when I got into town and made some more 90 degree turns, it came back; was pronounced when in 1st through 3rd gears and seemed to go away in 4th and 5th.

 

Ended up driving a total of 300 miles and when I got to my destination and slowed down it was very clearly the rear left wheel - clunkaclunkaclunka with every revolution. Borrowed a torque wrench to tighten things up, and as I was pulling the cotter pin out on the axle nut, noticed that the axle nut was wiggling - less than finger tight. Ah-ha! The wheel basically was wobbling on the axle, if slightly.

 

Wrenching it to 145 ft. lbs seems to have solved the problem. Checked the bearings and splines and everything still seems solid.

 

 

Question is, for any experts out there, how the heck can a nut that's torqued down tight and has a cotter pin run through it come that loose? The threads and all the other metal looked in perfect condition.

 

Edit: I should add that this is a relevant discussion - just want to link the "knocking" issue to the castle nut discussion:

http://www.ultimatesubaru.org/forum/showthread.php?t=34364

[DaveT]

 

Question is, for any experts out there, how the heck can a nut that's torqued down tight and has a cotter pin run through it come that loose?

 

I'll guess that the shaft hadn't pulled all the way in, then it moved with use / temperature cycles. I found a front CV shaft on a recently acquired car that way - didn't look wrong from the outside, but then I noticed a gap between the inner seal and the CVJ.

[85Sub4WD]

I think that wheel bearing wear has something to do with it too.

[bgd73]

The bearings shrink over time in a way where they still look good and spin freely etc. . I just went through that very same thing you did about a month ago....Be ready to change them soon. New lube sped up the process of really doing bearings in!

[RONAN]

I`ve had the same thing happen to me with 2 subarus in the past. Always on the rear. What I found is that with wear, the cotter will get enough play in it to allow the nut to move, causing this. I`m talking about less than a 1/32" movement will create the pronounced clunking noise. Since my problems, I make it a practice of installing a new cotter that fits tight & folds around the nut whenever I need to do any brakework or maintenance.