1. G1/4 is the actual hole on a waterblock item.
2. X/Y ID refers to the Inside Diameter for the water to run through.
3. X/Y OD refers to the Outside Diameter of the tubing
I have myself recently gotten into the watercooling. Overall, if you shop around, most everything is G 1/4 threaded. So you shouldn't need to worry about that. All you need to be concerned is if all your fittings are the same. Eg, tubing and fittings.
The MCP 655 is not interchangeable in terms of fitting. It is a hard 1/2" ID. So it requires 1/2" ID tubing. Though with a lot of effort such as dipping the tubing end in boiling water, some people have managed to fit a 3/8" ID tubing onto the MCP655. There is a mid step tubing available at size 7/16" ID.
If you are going to use compression fittings instead of normal fittings, then the tubing OD matters. It has to match the compression OD fitting size. Else you just wasted a fair chunk of money as watercooling equips are non-refundable/tradable back to the seller like ncix.com.
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