I ordered the MK3 kit first, later I ordered the MK3 to MK3S upgrade kit. But it all started out as an MK3.
MK3 kit assembly.
The MK3 kit is much better then the MK2S kit I assembled in June 2017. I own a BQ Hephestos 2 as well, I disassembled it for pieces. But it has a similar more sturdy frame, the XY plane is screwed onto the XZ vertical plane. Both sides of the XY plane have a fixed and correct Y axis length. That alone was a major source of frustration to get the MK2S assembly perpendicular.
Top view of the MK3 frame.
I was excited for the extruder of the MK3 because it is using bondtech gears. Both sides of the filament have a gear and the extruder motor moves both with the same speed but opposite direction. Both gears have a special filament guide as well.
Gears for 3D printers.
Below is a more detailed image.
Two issues I encountered. Not with the kit but the gears I got from here, there and everywhere. The gears I have are the ones for the motor (they have a small screw). The screws that tighten the gear to the axis cannot stick out or they bump into the extruder housing. Again, not for this kit, but in general. I have a few dozen extruder bodies printed and sometimes I need to replace an extruder and then those things happen.
Second thing is the other side with a gear doesn't have that screw because it is placed on a small rod and a plastic thingie underneath. It has a name but I don't remember it. Whenever I build a spare bondtech extruder I cut a nail to fit the width I need for the second gear. And I use some tube to make sure the void between nail (rod) and the gear is filled.
Bondtech gear in red circle.
I think BondTech gears are a great improvement over the MK2S extruder. But the extruder kept ticking especially after a print that took a few hours and then jeping the extruder heated and starting another print. I looked it up on various fora but couldn't find a solution that worked. It turns out that the reason is the MMU specific change they made to the heatbreak of the MK3 extruder.
When I bought the MK3S upgrade for my MK3, the PTFE tube that fits in the heatbreak is not a straight cut but conic to make sure no blob forms in the heatbreak.
That was for me the issue that made the extruder tick after a long print. It was hot enough to melt the filament at the top of the heatbreak and it formed a blob that prevented the extruder to push filament through. Usually when I let the extruder cool down, the next print doesn't suffer from the ticking problem (ticking is because the motor skips the filament and ahs too much resistance to feed it through the hot-end).
Here is the MK3 in action:
Original Prusa i3 MK3 printer in action.