O rings on spindle hubs

[The Tuul Crib 7,340]

 I am reworking my 48 inch deck and noticed on the PDF that on the top of the hub was an O ring. Was kind of curious if this O-ring is really necessary. I'm assuming since it's there should put another one on it .  As we know these always can be found about anywhere. 

[gwest_ca-(File Mod) 11,761]

Not for 48" deck but likely for the same reason.

Garry

[pfrederi 19,016]

WH# 971015 is a standard O ring Size Dash # 015