Output shaft play on 18 speed

You should have no play at all. Check and make sure it's not the yoke that's not worn on the shaft. If its the actual shaft you have a issue with the bearings

 

make sure the yokes nut is tight. I've seen brand new trucks with questionable play and the dealers response was "is the seal leaking" .....well no....."then don't worry about it"

 

Pretty sure there is a bulletin on this for the 18s. Get a new nut and torque to 500ft/lb. Thats eatons solution if there aren't any other issues such as leaks.

I think the bulletin covered trannys with a nut with gray locking compound built between certain dates but the same repair would be my advice either way.

And yes, any play is too much play.

 

Eaton has had problems with the locknut holding the yoke to the output shaft and it loosening. Eaton has actually come up with a different part number locknut to cure this problem. It is something I chased for a long time on my own truck. If the nut has been lose for any amount of time than the yoke will have wear on the splines. You need to replace the yoke and locknut to keep this from happening quickly again. Most people install a new locknut and considered it fixed, but it will loosen again if you can slide the yoke ON/OFF the output shaft of the transmission by hand. The yoke is actually a press fit on the splines of the shaft. The press fit comes when you use/tighten the locknut to press the yoke into place. If the yoke will move on the shaft without the locknut in place, the yoke is worn and a new locknut will not hold for more than 6 months from my experiences in the past

Unfortunately I don't have the Eaton part number for the revised locknut, but any good parts place should be able to get one for you

 

The nuts come loose if the yoke is worn on the splines of the output shaft. If you have new yoke, new nut , torqued to spec and you still have up and down movement, you have issues with the two bearings that support the shaft itself.

You will never keep a nut tight on a worn yoke.