How to measure case back allen screw?

[firebynight]

I'm trying to open a case back secured with allen screws, but I've tried multiple sets (none of them are actual watchmaker specific) and no size fits exactly. I'm afraid to strip the screw with an ill-fitting allen key. How do I measure the screw and make sure I have the correct size? Is there something with special sizing of allen keys used on watches/case back screws?

Thanks! 

[Tudor]

If you use your calipers you can measure across the flats of the socket. That’s the size. Convert the direct reading to the other (inch to mm or vice versa) and get the closest size under. 
 

I would be very surprised if they had unique size hex heads made.  

Edited September 18, 2020 by Tudor

[rodabod]

I’d expect metric, but don’t rule out imperial. 

[firebynight]

Sorry for being so obtuse, @Tudor @rodabod -- 

 What are the flats that I should measure? 

1. from rounded edge of the head
2. from angle to angle of hexagon
3. from edge to edge of hexagon

The main problem with this is I'm having difficulty even getting into that space with my calipers to measure any of these spots.

I ordered a hex set from Esslinger. And none of those fit.

I even resorted to going to the hardware store to try and see if they had any allen keys that would fit. Nada. Closest I found was a torx marked TT8, but then even among other TT8, they seemed to differ and not fit.  

[Tudor]

Number 3. Flat to flat.

I see it is not very deep a socket either, so once you find the right size, you'll want to dress the end of the tool to keep the corners sharp to avoid rounding out the hex.

Maybe heat the screw with a soldering iron first too, in case there is any threadlocker in there.

[rodabod]

If you can’t find one that fits exactly then buy an oversized one and stone the edges down on a diamond lap until it fits. 

[Tudor]

That’s a great idea. I’ve done that with a bench grinder and larger cap screws but no reason it wouldn’t work on a small one too. 

[firebynight]

 

On 10/6/2020 at 5:58 PM, rodabod said:

If you can’t find one that fits exactly then buy an oversized one and stone the edges down on a diamond lap until it fits. 

This is a great idea, thank you @rodabodwill keep that in mind if all else fails!