I've got a well-made old cordless drill, it works well but has too much runout (a long straight bit wobbles side to side too much). What part(s) would likely need to be replaced to fix the bad bearing that causes runout? Motor? Transmission? Chuck? Or is it unfixable?
I need a drill with a 43 mm collar, hard to find, my old one has that size collar. Drill is a DeWalt DC987, if that matters.
Depends on why it has runout.
Often debris jammed (or internal damage) inside the chuck. That can be as "simple" (often not so simple) as disassembling, cleaning out the debris, (and/or filing/grinding away the offending defect,) then reassembling; depending on what's going on.
That is: "not bearing runout" which tends to be pretty hard to spot unless it's extreme. It's also your most likely to be "reasonable" repair effort.
Otherwise, you have only a couple of actual bearings. It's hardly ever cost-effective to replace the bearings rather than the whole tool, in part because they can be difficult to find, or the part of the case they mount to may be damaged, rather than the bearing itself, or the worn bearing may have caused damage to the motor by letting parts slop around and hit/scrape each other.
