ORIGINAL: DarZeelon
There was a model of the HB.61 engine that was used to power military target drones.
It had only a rear bearing installed; i.e. no front bearing was used in it and a bronze bush was there instead.
The only front ball bearing size listed for this engine, is a 608Z, single shielded, 8x22x7 mm.
Hi Dar. Some models of the drone HBs did have front bearings because I have two of them. I think this is an exception to normal procurement though, and my good fortune.