The pistons for GM 60 degree V6 motors appear to have
89mm bore and 23mm wrist pins. I haven't found any
further dimensions in my googling so I haven't found if
the wrist pin -> piston crown dimension is similar to
the B20 piston. There seem to be lots of different
versions of this motor that have different displacements
but with identical bores -- this would indicate to me
that at least one version of this motor has an 80mm
stroke and would have to have a wrist pin -> crown
dimension identical to the B20's pistons. I'd also assume
you can make up some of that by shifting the center
of the wrist pin hole in the rod down a bit while enlarging
it that extra mm.

Similarly there's a ford piston for the 170cc I6 from
1961-1972 that's pretty much identical to the B20
piston except for a 23mm wrist pin; I found a place that
has those for sale for $16.25 each.
(google ford piston 1119)

Has anyone ever used BMW pistons in a volvo? It looks
as if at least some of the BMW motors have 3.5 bore by
3.15 stroke with a 22mm wrist pin which looks like a
pretty good match to me...

chris