Would love to help but may need a bit more information. Buggies tend to come with different sized wheel bolts depending on when the buggies were made
or whether they are more light-use or heavy-use oriented. For example, very early Peter Lynn buggies have 15mm (rear) bolts, later ones went to 17mm,
and finally ended up having 20mm bolts. To complicate matters further, some buggies may come with 20mm bearings but have sleeves that go over the
axle bolts to stage them down to 17mm or 15mm. Front wheels are a whole other story. Some of the beefiest buggies I believe come with 20mm front
axle bolts, but I believe most come with 17mm, 15mm, and yes, 12mm.
On my PL BigFoot+ buggy, for example, my wheels are all the same and all take the same 20mm bearings. On the rears the bolts are also 20mm so there
are no sleeves. On the front, I can't remember just what it goes down to, but there are sleeves that slide into my bearings to accept (I think) a
15mm front axle bolt.
I see that you have "2 Flexibugs", a "PL Comp XR", a "PL Big Foot", and "land boards", but sadly that's not enough information for me to go on to
recommend specific bearings without possibly sending you down the same frustrating road you are currently on all by yourself.
Somebody else please jump in here with more experience. I feel as if I mansplained the problem without offering any real solution.
Born-Kites:
RaceStar+ (3.0m, 5.0m, 7.0m, 9.0m)
NasaStar-5 (2.5m, 4.0m)
NasaStar-4 (2.5m)
NasaStar-3 (3.2m)
Ozone kites:
Access (6.0m)
Flysurfer Kites:
Peak-5 (2.5m)
Buggy:
Peter Lynn BigFoot+ nose & tail; midsection VTT rail & seat kit; home-brewed AQR
NAPKA Member US2815
SWATK Member UT0003