SUBARU DIFFERENTIAL PRE-LOAD NUT SEIZURE.
Problem:
Our client ran into trouble when assembling the pre-load nut into the case. The belief was that the technician had cross-threaded it.
A replacement nut was ex-Japan.
Cause:
The technician had not cross threaded it but it would appear that the assembly had been attempted without any lubricant on the threads. Because the case and the nut are both manufactured from aluminium, the assembly ran into the “similar metals” anomaly where two similar metals can be incompatible when sliding together. Aluminium with aluminium is a very bad design combination where sliding can occur; such as with this thread. A snowball effect starts from a small seizure and continues picking up material, from one component to the other, until seizure locks the two materials together.
Cure:
Always use an assembly lubricant on threads; particularly when the two components are made from aluminium.
The repair was made using heat to encourage thin oil into the seized thread. The pre-load “T” bar design tool was then used with heat and force to unwind the nut. The thread on the nut took most of the seizure and with a lathe we were able to re-machine 80% of the thread. The case thread was less damaged and a thread file tidied that up.
Given that the condition of the two threads was more likely to seizure now, we recommended that assemble proceed with a suitable anti-seize lubricant on both threads.