Starting in the last of the K2, the Rocker Shafts were locked down with the little 5mm bolts, with 2 being added to each shaft. The reason: Honda was attempting to quiet the engines as so many were being used with fairings, and the rattling noise from the rockers rotating in those towers was their concern. Prior to that, the shafts were allowed to spin free (and lasted MUCH, MUCH longer...).
The big bolt that holds the middle of the rocker tower(s) down has only 7 threads on the pre-1973 heads. This caused many to become stripped when people did not 'read the instructions' in the Shop Manual, which clearly stated to torque these particular 6mm bolts to only 70 in-lbs, lest they strip. In the later heads, Honda added 16 or more threads to stop this chronic issue.
If the big bolt is the one you're saying is now a 1/4-20, that's the right size hole to rethread with new 6x1mm Helicoils, which I do in almost every pre-1973 engine I rebuild. I install 2 of the short (1/4" long) Helicoils against one another, and then the bolt can be (accidentally) torqued to 115 in-lbs without damage.
If just the big bolt is used, the max torque it should receive is that which is just shy of pinching the rocker shaft from rotating. I usually find this to be around 60 in-lbs or so. These bolts were intended to hold the rocker towers in place while the top end was assembled, after which they serve no purpose as the cam bearing nuts and bolts (
hold the towers in place for runtime. On some engines, the bottom of these bolt holes got drilled thru (like some of the bearing studs) and can become a source of oil leak, which IMO probably causes the overtightening impulse on occasion?
If the rocker shafts have the little 5mm holes drilled and tapped, then they must also have the oil grooves underneath the rocker bearings, or the shafts will wear very quickly. On the late K2 thru K3 engines, these grooves were often misaligned, which caused poor oil film under the rockers. That's why I show in my book how to augment either the shaft grooves or the rocker oil holes, so the groove is open to receive the splash oil from above.
