Strictly in the "FWIW" dept, at about a thousand hours I pulled the starter off of our 23 hp diesel (Universal M25XP) and took it in for a preventative maint. rebuild.
BTW, the inner of the two bolts required a swivel socket and copious swear words.
It had been working OK, starting the engine on either battery.
A mechanic friend advised me to go ahead and do this, as a p.m. idea.
About $135 later it was back, rebuilt, with a new solenoid (they are evidently not a rebuildable item).
The shop is a highly-regarded local marine starter/alternator shop and the proprietor is always willing to explain stuff. He liked the idea of "fixing" this before it might let me down in the midst of summer vacation cruising.
The punch line: it cranked the engine a LOT faster when reinstalled
So, what slight little lesson there is to be learnt here is that sometimes these parts wear very slowly and performance degrades... slowly...
It's kinda like pulling the injectors and testing the spray pattern every so many hours, it pays to have an engine maint. sched.
As Forest said, "That's all I know about That."