The ER-16 and ER-32 collet chucks use an M12×1.75 bolt to snug their MT3 tapers in the Mini-Lathe spindle. As nearly as I could figure, I needed a 190 mm bolt to get enough thread engagement, but the nearest available sizes were either too short or too long.
Fortunately, making round things is what a lathe is all about:
The aluminum bellyband adds 30 mm to the length and aligns the bolt sections, with the threaded section from a long 5/16-18 bolt inside holding the metric bolt together:
Although I got it right on the first try (!), the bellyband lets me fine-tune the length as needed.
The original dimension doodle and some in-flight updates:
The fancy brass / bronze washer comes from a battered rod with mushroomed ends. A pair of V-blocks let me cut a chunk off one end with negligible drama:
It’s clamped firmly to the right block and a few licks with a file knocked off enough of the mushroom on the left end to put it flat(-ish) into the V; the near side of the right block is barely raised off the surface.
Face off the mushroom to get a flat spot for a center drill:
Some peaceful turning & boring produces a pretty washer:
The bore needed a bit of relief to seat the bolt head squarely on the outer surface:
And then It Just Fit™:
Loctite on the inner bolt threads should keep everything together.
The Smell of Molten Projects in the Morning 
In general, do you leave the collect chuck in place on your lathe, the three-jaw chuck, or what ever you last used? Just wondering about workflow.
The collets are so specialized they come out only when absolutely needed for a particular widget and, in point of fact, they’ve been needed only once since I got ’em, whereupon I discovered the drawbar bolt really wasn’t quite long enough for safe turning.
There’s also a slightly oversized four-jaw chuck in its box, awaiting corrective backplate surgery. I didn’t really use the hulking four-jaw chuck on the old South Bend very much at all, which is why it’s still on the to-do list.