Recently I had to replace my top PTC, PC-4M5, which was not grabbing the PTFE tube anymore, after 1 year of printing. Unfortunately I could not find the exact model that Flux uses but found in ebay similar M5 and modified them to ease filament insertion. Attached pictures below show the mods, which include enlarging the hole and chamfering it with a countersink bit.
Also, as you see the replacement, on the left has 10mm nut but it fits the opening of the top plate, Flux original has a 8mm nut.
1- Original Flux PTC connector is top right, has a 2.5mm hole;
2- Replacement PTC unmodified, bottom right, has a 2mm hole, filament insertion is very difficult because of the blind entry.
3- Replacement chamfered with a countersink bit, bottom left, still 2mm hole. Easier to insert filament but maybe a bit narrow, although printed well and I heard no clicking due to friction.
4- Replacement PTC, chamfered with a countersink bit + enlarged hole to 2.5mm, top left. Easiest to insert filament, even easier than the original PTC (one could also chamfer more if necessary).
Hope this may help, if you are planning to change the PTC.