The book says to replace when metal "innards" start to show. That would be fine probably for corn or beans (heavier crops) but I would suggest changing them when the "nubs" are wore off in small grains. We changed ours from the top rather than from the shoe. Worked ok for us - sure helps to have someone to turn the separator (by hand, of course!) for you. The bolts that hold the sections in are threaded right into the shaft. If you replace them from up above, you can remove that baffle along the left end of rolls to make access easier to those sections. I cannot remember the exact cost of all new sections for our R52 (replaced them last summer),but $860 seems to pop into mind. Another tip - while you're dinking around in there, check the tightness of the bolts that hold those distribution auger paddles to the shaft. We seemed to have a heckuva time keeping those things in the machine on our R50 and the 52 as well. We have replaced them all with the new style so hopefully we won't need to worry about them anymore. Had one set come loose on the 50 while doing cob-mix high moisture with no chaffer or sieve in - ended up in the clean grain auger tube where it exits the bottom of the shoe. Ouch! As long as you have the chaffer in like most sane people, they will mosey right on out the back. Good luck- Tom langan