PSA: When removing clutch cover...
Remove the 02 sensor cable from the cover before you pull it off. It's a little bit harder to take the cable out while the cover is in place, but if you leave the cable in place you WILL snag the cable as you pull the cover causing the left side to release first and sorta hinging on the right side of the cover (where the O2 stopped you from moving). Same thing in reverse for assembly, cover first, then O2 cable.
Ok great...Why does it matter if you pull/assemble the cover straight off/on? I suppose you could damage the gasket or alignment pins by pivoting funny, but more importantly there is an oil seal that mates with the crankshaft that you want to mate cleanly. One of the two times I pulled my clutch cover I sliced the oil seal that mates over the crankshaft. Could have been tonight, could have been 150 miles ago. Who knows?
$3 for a new seal is a pretty easy pill to swallow for my negligence, but it sucks to have the bike out of commission while I wait.
I'm sure this is just common sense to all you guys and gals that have been wrenching on bikes for decades but the Z125 is an affordable and easy to work on motorcycle so I'm sure I'm not entirely alone in cutting my teeth on basic engine work with this bike. If I save even one person from slicing an oil seal open it was worth my time to draw attention to my idiocy.
(As an aside I'm fairly certain the only reason the crankshaft oil passage opening was sharp enough to cut the seal was because I didn't have a deep 19mm socket to remove/assemble the primary gear retaining nut and my 1/2" drive was riding against the opening. If you're doing clutch work soon, ensure you have a deep 19mm before starting. As Greg E mentions in his video, you certainly CAN get by with a std 19, but it's not the right tool for the job.)
Dropped a gear... ...still here