Yes, it’s normal. Yes, it can be adjusted and should be checked periodically. The green bible does cover it. This is a good starter video to get the concept down. On a rover you can use the hand crank to rotate the pistons/valves to the right spot to check the gap.
If your truck is running smooth/quiet. Let it be. If you’re getting misfire, backfire or other abnormalities, checking the valve gaps is one of several regular tuneup check points.
1968 Series IIa
1997 Defender SW (Original Owner - Sold)