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My 750F2 project.

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McCabe-Thiele (Ted):
I'm certainly no machinist - it did make me scratch my head as to  how you end up with a barreled shaped bore when using a boring bar of the right size range?

Bryanj will know.

K2-K6:
With no idea what their machinery is like, it's speculative to assess from me.

Usually associated with feed rate in general machining, the tool has to be buried into the material it's cutting, with a wedge shape of the base material being sheared out of its structure as the tool is forced through it in removal.
If the entrance feed was cautious to preserve face of bore, speeded up through mid part, then slowed for exit, this could produce a barrel effect. Basically with higher feed it has more aggressive cut width on each revolution, with the tool being "pulled" further into the wall as it travels with that difference in feed. Its a bit like stripping wallpaper, when you get too much angle on the blade and it's goes "oink~twang" taking a neat little triangle of your plaster with it  :)

You'll get it on a lathe too, with a longer tool extension being a little flexible if reaching a long way in. On that you try to offset that little wedge of removed material by grinding a back rake on top of tool face to minimise that wedge in size, the angle of attack effectively.

Drill bits in lathe headstock do it too, you have to go at it really slowly to stop the drill nose "walking" around in a circle if biting too hard, especially as it's likely there's imbalance across the drill cut blades from one side to another.

It shouldn't happen obviously from a good machine shop, often you'll see experienced machinists going about something in a certain way that may look odd, but experience in that process gives them ways to mitigate shortcomings in process.

Bryanj:
If those are sizes in thou over the size of the piston its been bored too big anyway

Cb750r:
.0025" is what JE calls for for these pistons.

From what I understand this is likely to bad honing setup/ technique. Either way these sleeves are toast as 65mm is max bore on stock sleeves.
 
I may try these on a cheap set of 65mm pistons in these if I build up all my leftover engine parts into a second engine some day. But these cylinders sure aren’t going together with all the expensive bits I’ve assembled for this build.

royhall:
0.0025" is way too much for a Honda. At that it would burn oil for fun. Maximum on those is 0.001" and sometimes less than that. Plus they may not all be the same. Pistons should be allocated to a bore then measured for each bore. Fairly likely that the pistons are not all the same size. If you should get a slightly undersize piston on a 0.0025" bore you will hear the slap. As for the barrel effect, from my time as a machinist that is usually too high a feed rate. Rushing.

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