If you separate the upper & lower of a TRS and do nothing else to it can you just put it back together or is the some sort of preload you need to set?

Set the upper half on the lower half and insert a .025 feeler guage between. Shims are behind bearing cup sticking out of upper housing.Subtract to close up gap/add to increase gap.

if no shims were added or taken out went separating upper & lower can it just be assembled as is?

I'd check it to make sure it is shimmed correctly. Most of the ones I've played with were not.

Thanks, appreciate the help!

Most likely nothing has changed, but as Biggus says more likely it was wrong to begin with. This is not only important for preload on the vertical shaft bearings, but if there is any clearance it will change the running location of the pinion gear.

My reality check is to assemble the upper & lower, leaving the nuts loose. Measure the rotating torque on the prop shaft, then tighten the upper/lower and note the increase. It is not much, specially with used bearings but look for a minimum of 3 inch lbs.

Note - this is the change resulting from tightening the lower, not the overall preload.