I'm assuming they are. I just got a bag full of water bottle bosses/ inserts to add a third bottle mount to my frame, and want to add mid mound fork inserts to hold front panniers.
I figured it couldn't hurt to ask before brazing them in.

Thanks,
Ron

I always thought they were 6mm, but I did a survey of production bikes and they seem to mostly be 5mm, like water bottle mounts. I think 5mm will work fine.

Both rack and bottle fork mounts are M5 typically. I do like M6 for the crown though. More meat and better fits for fender clips. As more forks are thicker walled (can you say disks?) I get less concerned about poking lot's of stress risers in my blades Andy

I wish we had this discussion before I bought the Paragon 6mm bosses

Proper lowrider bosses will be much stronger than bottle bosses.

https://www.cycle-frames.com/bicycle...IDER-BOSS.html

I have some of those low rider bosses and decided not to use them. That's a really big hole to put in a fork, and it's unnecessary. I got these https://www.paragonmachineworks.com/...fork-boss.html I wasn't particularly happy about the size of the hole I had to drill. The next fork I build with low rider bosses is getting 5mm.

Thousands of Long haul truckers are circling the globe using a 5mm boss without issue, so I'm pretty sure that's a suitable approach. OTOH, the attachment at the dropout is pretty important. I beefed up the eyelet on my dropout, it looked like a failure waiting to happen.

+1 this. Those look like the low-rider mounts we used at Trek. They go completely through the blade, so they're anchored in two places. Unthreaded, with a bolt to pass through the mount secured with a nylock nut on the other side.

Thanks all for your feedback. I can see the benefits of from both sides of the 5mm vs. 6mm spectrum. I wrote a big long letter post discussing the assets of both, and then conveniently / well/ mistakenly deleted it... and am too tired to reiterate, however, thanks.

I did get a surly big trucker honkin' something or other front rack today. Mocked it up only using the lower eyelets and one pipe clamp on an upper. Even mounted the Jandd expedition mongo packs on it. They are huge. I would have NEVER considered mounting them there, yet, it was amazing how stable the bike was with the oversized bags hanging off the front (even missing one of the upper attachments). Also- the surly rack kit only came with 5mm hardware. I almost wonder if you'd have to ream out the mounting slots to accept 6mm.

This week, I should get a new fork in the mail from fleabay to put the braze on's on.... I will mock up the bike, strip it down, braze it up, paint it up, then ride off into the sunset! (ps- these pictures are aweful- and I used the rarely used "flash" on the point and shoot, but was happy to see the reflectors on the packs still work!

Thanks again.
Ron

My miyata 1000 had m6 through-fork bosses on it, they were threaded. I remember having to drill out my lowrider rack to fit the bigger bolt.

I was looking through my braze on collection for something else and realize I have 10 of the bosses that go through the fork. I might use them on the next fork.