I recently acquired a Delton log car at a train show.  I want to replace the plastic wheels with Bachmann metal wheels.  Do I need the 24.5mm or 31mm size?  Alsio how is the diameter measured-from the tip of the flange or from the rail contact surface?

the 31mm wheels are standard.  The diameter is normally over the tread.  gj

Thanks a million for the info.  I just ordered a set of 31's from Bachmann.
Cheers,
Dick Watkins