All valve rockers are tight against the face of a rocker shaft support stanchion, and these stanchions are not far apart. So there's not going to be much upward shaft bending flex at an unloaded rocker arm's centerline due to the force of some nearby valves being held open while the closed one is being checked. But it's more easily measured than calculated.

Spring force will be about 220 lbs (100kg) with D, K, or Isky 61 cam at full lift, Isky dual springs. Rocker push upward on the rocker shaft with 1.5 rocker ratio will be 2.5 times valve spring force.

The Camshaft however spans a long way between its middle and end bearings and will flex some, however open-valve downward force on any lobe at zero rpm will be only 1.5 times valve spring force. Intake valves are operated from near the middle of the camshaft span between bearings, and will be more affected during clearance checking than will exhaust valves, whose cam lobes are close to a shaft bearing.

Stiffness of the camshaft against bending flex varies as the 4th power of its basic diameter; so the thicker the better in this regard. Shaft bend could be measured during a rebuild when a dial gage could be set under it.

In use, all manner of irregular camshaft flexing will be happening, changing pattern and amplitude with engine revs.

How precise need intake valves be set? Exhaust valves?

Charles Greenlaw, Sacramento