Hello Peter: I don't think you need to lock the crank! Pull your plugs and for a 117.968 eng you place crank at 0 deg tdc (cyls1&6),90 deg (5&3),180deg (4&7),270 deg(8&2). should find these marks on balancer. Just remove arms with cam lobe pointing up and Dont Change There Positions! Load the cyl with compressed air/ leakdown tester and remove valve spring dont push valve into piston!
NOTE:there were two styles of valve guides depending on year one has a flat retainer at seal 5.2 mm groove and later version a round wire 3.3mm groove at guide.