S-Cam question

Correcting my previous post with the numbers. Multiple Meritor kit 8042 bushings brand new out of box were measured at 1.522" & two brand new meritor cams were measured at 1.492" (only 2 thou off their 'throw away' spec).
The equivalent Sirco Bushing measures at 1.512"
The Meritor cam has much larger grease passages, Meritor on left, Sirco on right

 

The tubes well enlarge or go out of round over years of use and more frequently on axles with spring brakes. The downward and side force pressure the chambers, slacks, and shoe force against the drums put a lot of stress on the tubes. Lack of grease and wear on bushings is not the only reason they get loose.
To understand the force on tubes I am talking about lay under the truck and watch the s cam tube and bracket move as someone apples the brakes for you. Them tubes move a little more than just sit still while the shaft rotates when activated.