How do you interpret that caliper to read 3.94 mm?
What piece of kit produced the bearings?
It does take a bit of getting used to, that’s for certain. Here goes: first off there are two scales on the beam, both start with “0”. The lower scale is inches so that’s straightforward, the upper scale is metric and you can see three hash marks but not a fourth. Now look at the dial face, and for metric we’re looking at red numbers and the red pointer (black is inches) . There are two zeros on the red scale though — what gives? Okay, when the calipers are zeroed out and fully closed the pointer is at the top zero where you would expect it, right. As we open the caliper the red pointer will rotate clockwise and at the point where it reaches the bottom red zero that’s 1mm. Coincidentally, the first hash mark division will also show on the upper beam scale. Open the caliper further so the red pointer now reaches the upper zero again (we’ve made full revolution of the red pointer now) and the beam scale shows two hash marks so 2mm. See how this works? For every 1/2 rotation of the red pointer you’ve opened the jaws 1mm. So for our three 3.94 mm dimension we’ve gone one full revolution for 2mm plus a half revolution for an additional 1mm but we’ve stopped just shy of the second complete revolution and the red number scale reads “ .9” and two small hash marks. If we look at the dial face again it shows that each scale division on the dial is equal to 0.02mm, so two of those means we’re at .9 + .04 = .94 plus the 3 whole mm equals 3.94mm. Easy eh? Want to double check my math? Convert 3.94 mm to inch and you get 0.1551... inches which is where we’re at at the black Imperial scale as well.
As for the source of the bearings, I think I bought them at Oak Bay Cycle but can’t remember what for. I can remember going there and the guy taking some out of one of those little plastic parts drawers to sell, but why I needed them I can’t recall. None of my bike stuff is a match, so I may have been fooling around with something else and just bought the balls there. Would have been 40 years ago by now I’m sure. It’s weird that I had the six though as I’m sure I would have asked for an even number to begin with, which makes me think I never even used any.
-frank
