If I could do it over, I’d pop the y-rails off to make it easier to get epoxy in there.
Also I just went to the hardware store. As long as you can thread the countersunk head in enough to gently squish the o-ring, and is not too long that the screw bottoms out beforehand, you should be fine.
The silicone is a bandaid for now because there’s a smidge of concrete exposed under one of my y-rails, but it does keep chips out which is nice. Probably isn’t leak proof and traps a little bit of coolant so I’ll eventually get around to popping the rail off, sanding around the bald spot, and pour a bit of epoxy in there. Then just leave it open or put some mesh or something to keep chips out.
But I’m reluctant to do that because I spent *HOURS* making sure there was no twist or bow in my rails using a Starrett #199 Master Precision Level.
For those interested: If you pickup a used one, it’s super easy to calibrate it with a surface plate. HAAS makes a great video on levelling machines (to eliminate bow and twist). But it won’t help you with checking your twist across both y-rails because we have a gantry mill. So to do that, you can use a cheap 4ft level, lay it across both y-rails at the front, of the machine, and place the Starrett level in the middle, and somehow keep it fixed in place. Then mark the 4ft level with a sharpie so when you move it to the rear, you can keep it in the same spot left and right. It doesn’t need to be level, the (Starrett) bubble location just needs to match the front.
Doing that, my tram is dead nuts doesn’t move at all no matter where I jog my spindle to it stays dead on. Which is contrary to what ALOT of people seem to experience during fly cutting.