OldIHC

I guess this is the correct forum for this question.
Has anyone had good result from using a tumbler to clean rusted bolts, washers etc?

Seams to me you would wreck the threads.

If your tumbler is water-tight, add some EVAPORUST. I use this product and I can recommend it. I like the ultrasonic cleaner action with Evaporust. Check it out on You Tube.
And another thingee. Corrosion may leave some pitting in your fasteners. Each pit is a stress riser and will weaken the fastener significantly. I doubt it would cost $100 at Fastenal to replace all of the bolts and nuts on an OLD IHC.
If you think about tumbling fasteners dry, you will be relying on contact to knock the dirt and corrosion out of the threads. CB suggested thread damage. I agree.
I used to ash the fasteners in a solvent to cut the grease then chase the threads with taps and dies. This is time consuming you will still finish with a compromised fastener.

I like ultrasonic and I love Evaporust.