If your 501E has no rods and springs, I tend to think it's already set up for 20oz bottles. The rods and springs when set in the correct holes are for 12oz cans, Red Bull, 16oz Energy such as Monster etc... However, depending on the product, the rod/spring need to be installed in the appropriate hole. (1 of 3 available hole options)

A can shim (1 on the right side of the column) is required if going that route obviously.

I don't have any experience vending 16.9oz bottles out of these machines, only 20oz. What color cam motors are on your machine? If it's dropping 2 at a time and then none, the cam does not sound to me like it's set for the right product. Bottle neck guides are not required on this model. Try dsvendinginc.com for parts for this. They'll have any shim, rod/spring etc...needed for this machine. Part numbers are on the set up decal located inside the machine. Hope some of this info helps you out. I'll answer any other question(s) you may have. Good luck...

I have installed these kits in several machines of this model and have never experienced a problem. Any vending issues to me sound like the kit may not have been installed correctly. I believe I have additional extra kits on a shelf in the warehouse should you be interested in one or more.
Chris