Posted 25 March 2016 - 05:48 PM
SR3s were built like tanks, unless mistreated you had to run seriously huge amounts of film through them before wear became an issue with most parts of the camera. We're talking TV series shooting for hours a day over months, repeated for years. Not something an owner/ operator or Indy filmmaker generally has to worry about.
In my experience looking after a rental house inventory I never had to replace an SR3 gate or the pressure plates in a mag. SR2 gates sometimes, usually just the side rails though. But SR3 gates just didn't wear, they had sprung sapphire guides and a very hard chromed surface. If you cleaned one with something metal maybe you'd scratch it..
I also wouldn't worry too much about movement parts if the camera is steady and quiet, it will take thousands and thousands of feet before wear becomes an issue, unless the movement gets contaminated. Maybe a spare claw if you're concerned about parts availability down the road, though I suspect qualified technicians who can diagnose wear and properly replace movement parts will run out sooner than the parts themselves.
Definitely the mag clutch can wear, as Kenny mentions, and the mag drive gear can get damaged teeth (get the flexible semi-opaque ones not the hard plastic ones). Usually it was little things like the mag footage counter cover panel getting cracked, or the rubber inserts on the mag doors tearing. Mag door latches. Running light bulbs and housings. Memory batteries. Pico fuses. Eyepiece optics might get scratched, viewfinder Pechan prisms sometimes yellowed, RS sockets got burnt when someone plugged their incorrectly wired power cable in. Generally it was damage rather than wear, which can be anything really. Bits that stick out can get hit. The mirror sometimes got scratched when someone stacked rear filters on a lens.
I would actually recommend saving your money buying parts that may never need replacing, and spending it instead on regular maintenance with a tech who really knows SR3s. Once every few years, or if the camera gets contaminated in a dusty or corrosive environment. Wear becomes a problem when a camera doesn't get regular maintenance (just like a motor car really).