I'm trying to work out the mechanical design of the kingpins. I'm a total newbie to vehicles and Cyclekarts but quite handy in the workshop.
This what I've come up with so far _

kingpin 15mm dia. x 72.05mm, bored 10mm
pivot bolt 10mm dia. x 100mm
housing 30mm dia. x 60mm long, drilled 16mm, recessed each end to take bearings.
bearings 15mm x 24mm x 5mm type 61802
bottom thrust bearing 15mm x 28mm x 9mm type 51102
top bronze washer 30mm x 3mm x 15.1mm hole
grease nipple in housing

'C' shaped axle mounts from 31.75mm x 3mm (1.25" x 0.125") Should this be thicker??
Stub axle M15 socket head bolt

The kingpin is nipped by the long bolt between the arms of the 'C' and rotates in the main bearings in the housing. The housing is welded to the bent main axle (from Gemini Karts).

Now - have I got this right???

Please, please correct me where wrong and advise accordingly. Thanks.

Nylon bushes are fine but mine wore over time and I've now replaced them with bronze. Yet to see how well they wear.

Just a couple of amendments to what Stefan has said.
Pit bike wheels come standard with 15 mm spindles. They are a bit on the light side, 17 mm works just fine (Refer to Chris Brown bearing chart)
Honda wheels as standard are 12 or 15 mm. The same applies. Again, check that OD of the bearings, I have had some with 32mm OD bearings, Learn from my costly lack of knowledge!
If using Gemini stub axles, both  varieties are 17 mm (Honda or Pitbike), the difference is in the length, at least the ones they sold me on each occasion that was the difference.
I have used plain steel on the 10 mm kingpin, also used bronze, and also kart spindles with bearings and 8 mm king ins. All have worked OK for me. These cyclekarts are not a fit and forget, just make it so you can change things readily along the way.
David F-R, your method sounds like an engineering masterclass, I suspect that very few would feel the need to go as exotic as that.

Pit bike wheel takes 17mm and I think Honda is normally 20mm

….When fitted to a Cyclekart

I know it's now a sticky on the Tech Forum, but it's easy enough to post ny tabulation again, so here goes.

Chris

David i'm not sure if your 3mm c shape axle support will be thick enough without some side webs. Do you have a pic?

Quite right Jim. I've upped it to 5mm. No pic as yet - it's all hanging by a loose thread in my brain.
It will have a web back to the steering arm and I'm considering a front web on the yoke providing that it doesn't foul the main axle - I shall know as I build.
Some metal and bearings ordered today - so much for waiting 'til Autumn. I have lots of off cuts, short ends and other steel from my loco building (still very active) so plenty to try with.
Chris, I've sorted the bearings, thanks, and ordered some.

The ones im doing i think are similar. I used 1/4 inch and have put a build up of weld up the back.

Those look a lot more elegant than mine will be. Nice to see them, thanks.