Start with a conical blank for your boring tool rather than a round or square one.

Only the tip needs to get into the small end so the size of the shank further down is defined by what will fit in the tapered bore. Starting by step drilling would make a bit more room too.

I know nothing about them but there are “D-bit” style reamers. Relatively easy to make and possibly up to converting a step drilled hole into a taper.

Clive

I suspect lack of clearance is pushing the cutting edge off the surface rather than the tool deflecting under load

One way to get better access is to cut the female taper with the big end facing out and with the topslide still set at the same angle run the lathe in reverse and cut on the back side to do the male taper, if you need to use tailstock support on the crankshaft then just mount it with the small end of the taper facing the headstock.

A milling cutter also makes a good boring bar when you get down to these sizes, hold it in the toolpost with one flute on ctr height. Not quite parallel to the angle of the topslide so the side does not rub Not a flywheel but a similar size taper being cut for a venturi. 5mm dia cutter will work for your 6mm min hole. At 5mm it will be stiffer than most small boring bars

This one was cut with the tool cutting on the far side, Crankshaft taper being used to test fit the distance it goes in upto the red line.

Would it be possible to cut two matching male tapers?

Then one can be sacrificed to lap the female one to a better fit after boring as best you can by other means.

That avoids the need for heat treating a D-bit.

The tapered valves and the D-bit used to make the tapered, blind and flat bottomed holes are show along the bottom of this picture:

The valves and D-bit were machined at one setting on the lathe, although to get the best surface finish they were then ground on the cylindrical grinder. The tapered seat was stepped drilled before using the D-bit to form the taper. Internals shown here:

Andrew