Why does this matter? Tight valves burn. Loose valves clatter and rob power. The manual provides the "Special Tool" (MD998772) required to hold the rocker shaft, but a clever mechanic can do it with a set of feeler gauges and a 10mm wrench. If you hear a "ticking" that gets louder when cold, you are already overdue.
Good for routine maintenance, but dangerous for complex repairs. 4n15 engine service manual new