This may be true on the RIVEBE but I don't see the relevance in the context of the M6F specific post. I noted three but there are actually six issues / potential issues with an M6F and any width
UT60 in P/P
1. The context of the poster's remark was visual ..... "not wanting to cover up the pretty M6F MoBo".... the only relevant dimension "visually" is thickness.... you may be able to fit the thicker rad in there but you won't be able to "see" anything behind it. I'm measuring about 29 mm from the bottom of the XT45 to the Heat Sink ... with the 27mm fan, it "misses" blocking the view of the Top of the MoBo by 2mm. Add the 15mm and the top 13mm of the MoBo is blocked.
2. Forgot to mention this one, but the M6F is equipped with a diagnostic digital LCD that displays the error or Q-Codes. The bottom of this display is 1.5" (38mm) below my XT45..... add the pull fan (27mm) and the extra thickness of the UT60 and you are at 42mm. So access to a key feature is taken from you with a UT60 .... yes it fits, both 140mm and 120mm will fit, but again "visually, you will not be able to "see" the diagnostic panel. Start button gets difficult perhaps but shud be easy enough to press if ya gotta.
3. The plug for the EPS cable is as inaccessible with the 120mm width rads as it is with the 140mm. Look at the pic above which shows the cable clearing the rad but once ya put the fans on, how would you access the plug at the end of it. The extra 10mm you get from using a smaller rad is not enough for you to slip a hand behind it and remove / replace the EPS cable plug.
4. If when ya built it ya decided ya didn't need wireless ..... or if after ya built it, ya found out that having both wireless and wired "active" at the same time doesn't work and ya want to take it out, you can''t get ya hand in there to install / uninstall the unit.
5. It's obvious that the RAM fits but if ya wanted to re-seat, by using the levers, it's tough access when in the up position..... of course you can press in place and whereas I always prefer to do both tro minimize warping, the N6F backplate should help here.
6. I think it will fit once done but installing / uninstalling ya fittings / tubing for the MoBo VRM block becomes difficult with the thicker rad