I looked at this site to find out how to tighten up the central 10 x 12mm bolt ( ref 90105-VA3-J01)at the bottom end of the crankshaft and under the cutting blade. With the blade removed I had to replace the central bolt and found when trying to tighten it, that the whole assembly revolved. No good suggestion on the site was found to stop this rotation. It then occurred to me that at the other end of the crankshaft is the hand starter assembly. Remove this assembly from the top of the engine cover and there is a large nut on the top end of the crankshaft. Using socket spanners of the correct size for the different bolt and nut sizes, you can tighten up the central bolt at the clutch end.
My owner manual for my Honda HRB 475 gives the torque setting as 5 to 6 kg.m
The wood block reference, is stop the cutting blade rotating when tightening the two blade bolts. These bolts are shorter than the central bolt.
This response is long after the original question but it may help fresh enquiries as mine was.