Will a 2813 fit into a 2836 case with 2836 dials and can a 2813 crown + stem be used with an eta 2836? Many thanks
The 2813 would likely fit into the 2836 case but the stem height is different. When you try to put the stem in there will be a vertical angle created such that the crown will have an angular component of force pushing it against its threads unevenly top to bottom. This will make turning the crown extremely difficult and risk damaging its threads and/or the tube threads.
You can use a 2813's crown on a 2836 movement, but you will have to swap out the stem. They are different in shape and not interchangeable.
Also, the datewheel is in a different position between these two movements so if a cyclops is involved, a crystal swap or cyclops reposition will be needed. The dial feet are not in the same position either so they would have to be clipped and replaced or dial dots used. Use the proper movement in the case it was designed for.
Posted response primarily for the benefit of others since the question is 7 months old.