You didn't say what machine other than 7045 fg, I have the IH mill and my stock spindle bearings went dry and destructed. I found that it took 1/2 hr to pull spindle and get it apart, washed and inspected. My bearings could have been reused but only for a short time as they were definately damaged. I went to the local McGuire bearing house and ordered up the less expensive of the "Angular contact" bearings > 7206 bgc3 & 7207 bgc3 for my spindle. I AM NOT SAYING THESE WILL FIT YOUR MACHINE ! At the same time I purchased some quality spindle grease too, this grease is less likely to melt off and sink to bottom of quil but eventually it will, I believe any grease will, so it would be recomended to drop spindle from time to time and grease it even if you only needle inject the top bearing. I also cut some leather from an old boot tongue and made a barrier that goes under between bottom bearing and on top of the threaded shield which can be seen under quil, this quite effectively prevents the grease from wicking out of bottom of quil and being lost. As for pre-load on these new bearings, info is basic and not specific to the spindle setup. I started with zero and tightened nut to first lock position, but this proved too loose, tested by running spindle and warming it then stopping spindle and grab tool and give it a shake ! if it wiggles then it is too loose ! I ended up taking two more positions on lock ring before the spindle would feel firm after warmup ! I now have been running it for a while and it remains quiet but i also give it a warm up time before going to full rpm from cold ! Starting at 1200 rpm and let it idle for a time. You will not necessarily benifit from going AC bearings, You will benifit from new bearings and keeping them greased by checking them from time to time. As easy as it is pulling quil i'd advise it, you don't really have to take spindle apart to grease it, just get access to top bearing and inject. bottom can be done by spinning off the threaded shield and injecting, but top will destruct first so get it greased. When i washed mine down for bearing change i used a clean pan and checked the fine in pan when i was done. I found lots of very fine cast iron left behind from the machining, that made it's way through factory assembly. This circulating with the grease caused enough heat to vaporize what little grease there was till it was dry then bearings howld and screamed like crazy. It was not mistakeable ! If your spindle is telling you it is in pain then fix it rite away !