Ok so I can’t get to anywhere that supplies coach bolts any time soon so over to stw hive mind.

I have a project which involves bearing with internal diameter 15mm I want to fix them in place with as flat a head as I can, so coach bolts but I want them to move, side to side as little as possible. So I need a coachbolt where the side of the square is as close to 10.61mm as possible.

What diameter coachbolt am I looking for?

Ok I’ve answered my own question the square is as big as the diameter so 10mm

If you don’t need mega strength (and it sounds like you don’t) then another option is a counter sunk bolt. Once tightened the bearing will sit on the taper of the head and the you’ll get the head pretty flush. M10 will probably be about right, or maybe an M8

What diameter coachbolt am I looking for?

If you’re looking for fine tolerances you’re probably not going to get that for the square bit of a coach bolt – they’re designed to crush their way into wood rather than be a close fit to anything – they’ll vary from brand to brand and will often be a bit tapered too

How long does it need to be? Some crank bolts are m15 and have a pretty low profile head

Some shield anchors are an m12 thread inside a 15mm sleeve.

Think i’d so smaller and have some inserts 3D printed. Not exactly sure what it is you’re making though.

Ok I’ve answered my own question the square is as big as the diameter so 10mm

not on the diagonal it isnt…14.14mm

Why not go M12 and file a bit off each corner of the square to make it fit, shouldn’t need much, a mm or so? (Unless you’ve hundreds to do, of course)

Connect2 possible way to go.

Jam bo the circle is 15mm diameter so the diagonal of the square to fit in it is 15mm. a squared= b squared + c squared. a squared = 225 so both b^2 and c^2 = 112.5 √112.5 = 10.6

So lengths of each side of a square that fit in a 15mm circle is 10.61mm.

the square bit isn’t going to support much of he width of the internal race on the bearing. Coach bolts are also not threaded all the way to the head so what’s going to be engaging with the other side of the bearing?

15mm bolts are a bit of a rarity but you can get 15mm OD bushings that would have an ID thats a bit more of a common size bolt size. A flanged-button bolt would give you a similarly low profile head to a coach bolt.

I like nickjb’s suggestion. An M8 countersunk bolt head is 16mm, so just enough to engage your 15mm bore on the taper, and probably less than 1mm protruding above the bearing.