my vote is *very* bad alignment or improper installation of new bearing.

 

Turns out I went to my mechanic and it was the Axle nut, it was loose. This isn't the first time it has happened either. He said the 350's have a terrible design because the cotter pin is at the end of the axle, while the nut is way back. The cotter pin does not press up against the bolt to prevent it from moving but rather just prevents the bolt from falling off the axle.

 

Your mechanic must have a poor understanding of how the cotter pin works on the 350Z. If you look at the axle flange end, the axle nut cannot move if it's been properly torqued and pinned. The cotter pin is essential to locating the nut and ensuring it will not turn once it's properly installed. It has nothing to do with "pressing up" against the bolt.

Glad you found your problem, but if this isn't the first time, that axle nut is not installed properly.

 

Wheel bearings are easy to install .... On our cars

 

Apparently, properly installing them are a problem if AtomicX's pro mechanic has been installing the cotter pin ABOVE, rather than through the castled axle nut. There's not a well designed part or component on our cars that we can't screw up if we don't understand how it works.

 

Which brings up another question for the OP- were you still using the OEM axle nuts when the wheel bearings were worked on? If so, and if your pro wrench was installing the cotter pins in the slotted hole AFTER he installed the castled nuts, then THAT could be the cause of the wheel bearing failures.

The purpose of the cotter pin is simply to locate the nut, not provide any type of clamping force to it.

 

Maybe I'm confused by what you're trying to say, but looking in the FSM, it says to remove cotter pin, then remove axle nut. As for installation instructions, it says the reverse of disassembly...which would be axle nut and then cotter pin.

 

The cotter pin goes through the castle nut and a slot in the threaded axle stub. You cannot remove the nut until you pull the cotter pin. However, if you simply put the castle nut back on and then insert the cotter pin into the slot above it, two things will happen: 1) the pin will simply rattle around because it's not properly located inside the castle nut, and 2) the castle nut will eventually work its way loose and cause other damage.

 

That's why I love threads like this. OP comes in askng for advice, and after a few days, bails on the discussion. He's probably still going to the same shop that doesn't understand how a castle nut and cotter pin works.