How is axle to crown length measured on a single-crown fork? From the top of the crown (where the steertube starts, where the lower bearing race is) to center of the axle? I'm going to have to temporarily ride with spacers BELOW my headtube for awhile, until I get a proper fork, and I need to know how to get the height as close as possible.

Measured from the crown race seat (the face on the crown that the installed crown race bottoms out on) to the axle centerline.

What you are proposing to do is sketchy and not adviseable.

Yea, I started lookin at the bearing race and its interface with the fork... Very sketch. Ehh, maybe it'll be close enough for a few rides before the fork gets swapped.

^^ The ONLY thing (IMO) that comes close to being an acceptable spacer for the crown is this:

https://www.universalcycles.com/shopp...&category=1696

But it's only going to buy you 5mm (?) and needs to be compatible with your lower headset bearing.

+1

The fork that came factory on the bike measures 518 A-C. The only fork I have available for the next few weeks measures 498.

If I crank down the pre-load to effectively eliminate sag, and accounting for proper sag from the factory fork, is it a safe set up? I just need something to ride for 2-3 weeks until my proper fork comes in.

20mm difference is only gonna change the head angle around a degree or so... I personally wouldn't worry about the change too much. Ride the bike easy for a couple miles to get used to the difference. Stiffening preload won't really be unsafe, just uncomfortable. I would set it up with proper sag for the fork.

What kind of bike?

It's an '03 Jamis Komodo. I'm more worried about structural integrity than ride quality, naturally. I don't mind having the ride be a little crappy for a few rides, it's a pretty crappy fork anyways. I just don't want to have the headtube shear off, then have to start my replacement frame search all over again.

No worries, you won't harm the frame by running a stiff fork.

Stiff, yes, short....?

A shorter fork is fine. The suggestion you had at first is just plain dumb. Don't do it.