I've got my outer tie rods off right now doing repairs, but am just about ready to put a new set on. The short outer tie rod came with the zerk, castle nut and cotter pin, the long side did not, but had the zerk already installed. Can i re-use the old castle nut (* new cotter pin of course).?

 

As long as its not all bashed up I don't see why not. This isn't a highly loaded component, but as you say, a now cotter pin is a must.

 

From an engineer's perspective:

The default answer is "replace all fasteners when parts are replaced".

There are a couple reasons for that:
- Some fasteners are torqued slightly beyond their yield limit during proper installation (like the head bolts). This weakens them slightly, so multiple re-uses would become a problem.
- There are a LOT of clueless gorillas out there grossly over and under torquing fasteners and ruining them. But even though we are far from able to control them, we engineers get blamed when they screw things up.
- Some nuts are deformed slightly to be self locking. Each time they are removed or installed, they lose some of the deformation and don't lock as well. These MUST be replaced to provide proper locking function.

But in this particular case:
- A tie rod castle nut should never have been torqued beyond yield point, and they are not self locking.

So if you are confident NOBODY had previously under or overtorqued them, you should be quite fine to re-use them.

 

Since these are not torqued down as much as other bolts and nuts, You can re-use them no problem as long as there is no sign of damage (ie: threads messed up, nut dinged up or rounded off, or castle part of the nut bent over).

It is a must to put in a new cotter pin though.

 

you'll be fine to reuse that fastener..

so long as its not ready for teh scrap heap..

 

Since castle nuts are generally installed on 0-lash parts, they'll last forever as long as you don't chew them up using the wrong wrench or hammering the cotter pin down...

It's too bad Ford doesn't build engines with safety wire anymore...