Well, on the basis of what you guys say here, plus some other positive comments, I've just ordered a pair too. Very nice people to deal with, was my initial thought - great sense of humour!
So, yup, a bit of top-end wear to deal with hereabouts, after 50,000+ miles in 13 years in my hands and I don't know how many before.
I was expecting gross bore wear as well as worn-out pistons. Was even worrying, as you do, whether when I got there the bores would come clean at +60 or have to be sleeved . . . I knew from when I had the engine open a few years back the one and only time (crank endfloat job) that the time was coming for some top-end investment. But, wanting to get every last mile out of the 67-1421 BSA +1mm parts, I just put some rings in, new shells and D/side main down below just because it was apart, and some ex valves & guides . . . and kept on going. Which she did, with hardly a hiccough. (The t/side bush had 1.5 thou clearance at that point, which also pleased me mightily.)
But to my delight several years on, bore wear max is 4 thou tops, and in most places is between 2.5 and 3 thou. Very hard to believe frankly, or maybe I'm just too pessimistic with these things! But - the pistons are up to 8 thou undersize most places below the rings. Not scored, not damaged, just tired, with signs of blow-by on the side faces of both pistons, and that golden-brown sticky colour on the top ends of the con-rods where oily stuff has happened. So the pistons took the hit, and the bores have remained clean. Nice. Wasn't smoking at all, but was using a drop, and had become noisy.
A trial fit of a 71mm brand new STD B31 piston (why the heck have I got one of them sitting around???), just to see if I could believe my measurements, gave me skirt clearances of 4 - 4.5 thou, and the 71mm rings supplied with that piston gapped at 11 thou. Not definitive of course, but a useful indication. So I have forced myself to believe the evidence of my eyes and measuring sticks.
And am more impressed than I have ever been with the hardiness of these motors. With any luck these IMD bits will see me through the next 50K or to the grave, whichever comes first!