I have had 5 STS engines and if I remember correctly the rear bearing is a strange size. Here is the only place I know of in the US that still carries STS engine and parts.
http://nitrocountryrc.com/-c-45_46.html
I have one STS D21B with well over 5 gallons on it that I just put new bearings and a rod in it still going strong. What STS engine do you have? I have run 10 minuet qualifiers on it and never pitted. If you have problems on getting the rear bearing PM me. I live in Macon.