Pertaining to the Princess 1493, there are two types of machines. While one will play only large center hole 45 rpm records, the other will play a combination of 45 and compact 33 rpm records. That said, depending on which machine you have, there are two idler wheels to choose from; one for each type machine. First, let's determine what you have. The image below shows a Princess with a Mech-O-Matic Intermix platter. It is indicated with three RED fingers. When a record with a small center hole is placed on the platter, it pushes down the fingers, causing a mechanical speed change. A platter with no fingers will only play the large hole 45 rpm records.
Which idler wheel goes with which machine? If you have the platter with fingers, the idler part number is 35831. This wheel has a 4.7mm bushing (.185 Dia.) If you have the platter with no fingers, the part number is 30789. This wheel has a 5.4mm bushing (.212 Dia.) Obviously, one will be too small and not fit on the shaft and the other will be sloppy. This will not work.
The rubber tire on an idler wheel will harden over time and will need to be replaced. While these idlers are in production, you may choose to have yours rebuilt. Both are an options that work.
Be absolutely certain that this rubber tire never has an oily surface. Use a clean cloth and alcohol to clean the rubber tire and motor shaft. A drop of oil on the bearing is necessary.
WHAT'S UNDER THE PLATTER?
As mentioned above, there are two types of platters. This one includes the mechanism that will play 45 and 33 rpm. Looking closely, you will see two steps on the motor capstan. The wider step is for 45 and the narrower step is for 33. The speed change mechanism will raise and lower the idler wheel for the appropriate speed. On the right side of the picture you will see what makes it happen. Single speed machines do not have these extra parts.