I read once that the finest line in model railroading is the one that separates running clearance from slop. I made four driver bearings on the weekend, starting with a 1/8″ drilled hole in the end of some brass hex. Only after I’d completed all four did I examine them critically on an axle.
They were decidedly on the wrong side of that line: the holes were easily three or four thousandths of an inch oversize (actually the axle diameter is less than the nominal, so it’s not all the holes’ fault). Does this matter? To tell the truth, I don’t know. However, it seems to me that if the connecting rod can shift the wheel rather than rotating it, then the wheel will hesitate before going around.
I never perfected the creation of boring tools before, and I was tempted to hope for the best. But then Future-Me reached back through time from the test track where he was trying to get 622 to run smoothly, and slapped me. Hard. “You could have at least tried!” he cried.
That smarted. So, I got out a length of 1/8″ drill rod and fashioned a tool that ought to do the trick. Then I made a holder to sit in my tool post. And I turned up a new bearing.
And you know what? It was better.
I can do better, so I should do better. With the Railway Modellers Meet looming, I don’t really want to spend time redoing parts, but that is what must be done.
I really should reach into the past and slap Past-Me for going ahead and finishing four bearings without checking the first one for fit.
Pembroke:87 
I never rely on a drill’s stated size to determine the final hole diameter, expecting things to be slightly oversize. I also make sure I drill out in progressively larger steps. When working on wheel centres requiring a force fit onto a ⅛” axle, I typically use a centre drill to start with, then 1.5mm, 2mm, 2.5mm, 3mm and 3.1mm before finishing off with a 0.124” drill bit bought specially for the purpose and not used for anything else. I used to go 2mm, 3mm, 0.124”, but had some issues with the drill wandering slightly and cutting a bigger hole, only by a thou or so, but enough to produce a fit which wasn’t tight enough nor at the same time loose enough to be a running fit.
If you need a precise hole, you need to get a very small boring tool, purchase a reamer or made a D shaped bit, which may be what you did.
I should have known better!
Yes, I fashioned a little boring tool for the second go.
For a running fit I suggest you drill 1/64 under-sized and then use a 1/8″ reamer which is a tool for just this application. If after repeated reamings you need a looser fit, you might try a bit of finger pressure on one side of the reamer while the chuck spins then, if a still looser fit is needed, try putting a thin strip of paper in one flute of the reamer.
md
I went looking for a reamer before and came up empty, but now I find this one: https://www.mscdirect.com/product/details/02310084
Maybe for the next engine I should get this.
Of course, the K&S tube I am using is actually .123”.
more than you may ever want to know about why drill bits always drill over-sized / why their resulting holes are not round.
https://www.mmsonline.com/articles/the-dynamics-of-better-drilling
md
Fascinating