Hey Hogweed -
Seems like I used my 12 point 12mm on mine but it was a while ago. I don't have any special gizmo in my toolbox so it WAS something standard and I'm pretty sure that's what I used. I remember using the impact on them w/o issue. I have a fuzzy memory of buying a new socket for that just to be sure my old one wasn't worn out. Either way, it worked.

Be sure to mark the flywheel's relative location to the crankshaft (assuming it'll go back on that 616). Some are reputed to be match-balanced and might not be marked. Mine went on my 617 so it was sort of a moot point. BTW - I just bolted mine on the 5 cylinder and everything was balanced just fine but I guess others have had different results.

Dan