Maybe you are over thinking this. Just make the bearings a good fit on the shaft leave a bit of play in the bearing seats. Install the crankshaft and just nip up the bearings with a good running fit.

Or, yes a cheap adjustable reamer would do the job if so desired. The first third of the blades are tapered so you could adjust it to fit through both bearings and ease them out just that bit. But, it is easy to make these little engines a bit close in the clearances and they end up running a bit tight, so the previous posters advice might be better way to go. As Mr Harley said to Mr Davidson: A little extra clearance never got in the way.

Boring is possible if you have a tiny boring tool but for 7mm that's not so easy. How would one "nip up" the bearings? They are made from a solid extrusion and IIRC there is no split in them or provision for nipping up.

I suggest that you just drill the bearings. Get a couple of good drills say 6.9 and 6.8mm. Carefully stone a tiny rounding on the outside corners of the cutting edge to make a "Dagenham reamer" – this reduces the tendency to cut over size. Try on a bit of scrap brass first, with luck you'll find that the 6.9mm one will give you the size you want. Slow feed.

Or you could make yourself a D bit by turning turning down say 8mm silver steel to the size of your shaft.

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