One can look at the pictures of the block and know that they are not 3/8" NPT or BSP. If they were, it would be possible to use a 5/8" ID barb on them. They are not, however.
How big is the temperature difference? I am not comfortable enough with the block design to throw a number out. I AM, however, quite surprised at the vehemence of the watercooling community to "free" performance. It has been shown several times by Bill (the original 462 data, the innovatek blocks, and again with the -U vs. UH swiftech blocks) that substantial performance can be gained by changing fittings with an eye to waterblock design parameters. I also wrote an article on changing fittings and researvoir placement to make better use of existing pumps. FREE performance! And the community just argues with the Darcy equation rather than assimilating it into their own design plans. Splendid.
Glad to see you guys have managed to keep the thread civil. I don't think moderators are really even needed around here
