What's the purpose of the hole in the rear spindle bearing (5210) on a 10EE?

What is the purpose of this hole in the 5210 K used as a rear spindle bearing on the 10EE?



Notice that the oil reservoir for the rear spindle bearing is not underneath the 5210, it is adjacent to it.

Another difference between the 5210 K and a normal 5210 bearing is that the semicircular cutout in the side of the outer race is not present on the 5210 K. Instead, the semicircular cutout on the inner race is larger. I believe the semicircular cutouts are used during bearing assembly. i'm speculating here, but could that mean that Monarch did not want the bearing to be installed so that the semicircular cutout was at the bottom, thus retaining more oil in the bearing if the reservoir went low?

5210 K bearing, no semicircular cutout in outer race:


Normal 5210 bearing with semicircular cutout in outer race:

The semicircular cutouts on the 5210 allow more balls to be put in then otherwise. New Departure calls it full complement, as in more than normal complement. MRC/Fafnir/SKF call it a filling notch.

The K best I can tell is described in the MRC catalog as standard width. There were/are double row bearings made in a wider width to be drop in replacement for single bearing pairs.

The hole in the outer race I vaguely remember reading in a bearing catalogue as an oil hole, not required to be used, but allows oil directly to the inside of the bearing. In our headstock there is no need as the oil bath comes in from the side.

Keep in mind the New Departure 5210 everyone else calls a 3210.

Looking back in a 1943 New Departure book, the 5xxx bearings they made back then, they say they were made with internal preload at the factory via sizing the balls to the bearing. The modern 3210 is not made like that, they rattle like anything.

My rear bearing in my 10EE did not have that hole in the bottom.

Odd, On mine, I distinctly remember that the oil hole in the bearing spanned over the oil resevoir for the 3rd oil sightglass on the headstock. When I replaced the bearing, it did not have the hole, and I found no use to fill that resevoir any longer as it served no purpose. . I figured that they (Monarch) figured that on later models, there was enough splash from the main ccompartment and the special resevoir was redundant.