Ive have been looking at watercooling components and it seems that some have 1/2" or 3/8" threads while others have G1/4" threads. Are "G" thread sizes directly comparable to the normal " sizes? If so, does that mean that a converter has to be used to narrow or widen to a different diameter? Thanks.
"G" size threads are the part you attach the fittings to. The 1/2, 3/8, etc is the size of the tube. G1/4 is standard for threads. So if the thing you are connecting is G1/4, you attach a G1/4 fitting to that of the size you want, say 1/2 or whatever. The first number is the inner diameter, the second number is the outer diameter which is needed if you use compression fittings. This means you can use whatever size fittings\tube you want since they all screw into the same thread size (assuming your device is standard).
G1/4 Threads are the part that connects to the actual block itself. 1/2" and 3/8" are the part that connect to the tubing and the tubing size. I recommend G1/4 with 1/2" if you don't have any kinks. If you do, then switch to 3/8". Basically your water cooling fitting converts it.
G1/4" threading has a diameter of 1/4", so any tubing size above that doesn't lend itself to more flow. So as far as what size tubing to get, it doesnt matter as long as its above 1/4". Its actually strangely regional, North America tends to go for larger tubes like 1/2" while Europe typically kicks around with 3/8" or 1/4".