What are the stock dimensions of the silicone tubing used on the K40? Mine is a little too short for where I need to place all my water cooling, and I’d like to add some inline flow meters, etc.
Not sure about a standard, but mine is 8mm in diameter. Why not just measure the one you have and match it?
@funinthefalls asked because my calipers are at work and I’m not back to work until Tuesday, it’s a hour drive. 
Would 8mm OD, 5mm ID seem about right?
That is what mine is. I standardized all my lasers to 8mm/5mm tubing.