360 units with sayco 3 stem shower in all units. approx. 1 in 5 diverter removals end with a stem that will not remove. The packing unscrews perfects but the stem itself, though movable, will not dislodge leaving our only option of tearing into tile. Why is the stem stuck so bad that a puller, hammer, vise grips, etc. will not remove. Please help because this could save us thousands if someone knows any tricks! P.S. If you don't have an answer don't waste your time or anyone else's time by responding with a stupid answer that will not solve this issue. Thanks

Is this the diverter stem that is in those units?

Yes! The threads release easy! The lower portion of the stem will wiggle every way but will not budge in or out. No visible corrosion just will not pull! Any ideas hkdtroud?

Which nut are you loosening A or B?

B. I have replaced going on 200 of these! Its 1 out of 5 that get stuck

First, why are you having to replace so many of these?

Second, have you ever taken note of the position of the stem before starting to remove the diverter? Is the stem in the retracted position (as shown in the photo) which is presumably the tub position? Or is it in the extended (shower position)?

We are replacing all of them as part of a renovation. They have been in the wall since the 80's. Also, the most effective method of removal I have found is with stem fully open/retracted. The faucet wrenches seem to cause this issue more frequently so a 21mm deep socket while reattaching the faucet handle to stem and turning simultaneously to the left. If the (B) portion screws off without full removal of stem. We are screwed lol

If the (B) portion screws off without full removal of stem. We are screwed.

most effective method of removal I have found is with stem fully open/retracted

Exactly what I was thinking. The stem must be retracted and must turn with the body of the diverter as the body is unscrewed. If the stem does not turn with the body as it is unscrewed, you are extending the stem as you unscrew the body. Since the threads between the stem and the body are courser than the thread between the body of the diverter and the valve body, the stem would travel father inward (extending) than the body would travel outward. That would therefore push the stem up against the valve seat with great force.
I suspect that excessive force is driving the head of washer screw into the interior opening of the valve seat. Assuming that the head of the screw is slightly larger the opening of the seat and given that the screw is brass and softer than a steal seat, once forced in, given the shape of the screw head, it would be very difficult to remove.

My suggestion would be to retract the stem as far as possible, tighten the bonnet nut (A) as much as possible, that would help insure that the stem turns with the body of the diverter as you unscrew the body.
After retracting the stem, I would use the socket or tub wrench to break loose the diverter body. Then use the diverter handle to unscrew the assembly.
If unscrewing the assembly with the diverter handle is too difficult you could use a pair of vice grips or even a small pipe wrench on the stem shank. Grip is about 1" from the diverter body. Granted you will scar up the stem shank but that should be covered by the chrome trim.

Thank you for all of your great information. I will be cutting a valve body open today to confirm your suspicions of the brass screw jam. I will keep you posted as to my findings!

HK, what would happen if he used nut B to break it loose a quarter turn or so, and then just put the handle back on the stem to finish unscrewing it by turning the stem? That would ensure that he was not allowing the stem to screw back in.

That was what I was suggesting, John

Now that I read it again, that is indeed what you suggested. Oops.

Well, it was a long and convoluted post John. Not very well composed, just what came out the end of my fingers. I was glad you posted to state the suggestion explicitly. I often wonder if what I'm saying is what I think I'm saying.