Demystifying Subaru EJ20 cam positions and unloading
One of the steps when replacing cam seals on a Subaru EJ20 engine is to remove the cam shaft sprockets.
Before you remove the cam shaft sprockets you need to remove the timing belt, and the first step when removing the belt is to put the engine into the "service position" which results in all 4 pistons being the same distance from the crankshaft. This means it is impossible for the valves to come into contact with the pistons which could cause damage.
However, in the service position one set of exhaust valves and one set of inlet valves on the left hand side of the engine are engaged - this means the cam lobe is pressing the valve in. This also means that when the belt is removed its possible that the camshafts can spin around due to the tension of the valve springs.
If you are just swapping the belt only, there is a special tool to lock the 2 left hand cam gears in place, however, when replacing cam seals you will need to take the cam gears off and therefore unload the cams.
The Subaru factory service manual has an explicit warning about only rotating the cam gears in one direction as the inlet and exhaust valves can actually hit and damage each other if they are turned to a particular position.
This advice was confusing to me as logically you could still hit the valves together. There is much debate on different forums about this, so below is my addition to explain things.
The factory manual guidance assumes the cams have already unloaded. The image on the right is from the manual, and it is only in reference to aligning the cams up before putting a new belt on,
This can be confirmed by looking at the markings on the diagram - the top intake cam has unloaded anti clockwise, and the bottom exhaust cam has unloaded clockwise. Hence the task to realign them is in the opposite direction.
To help clarify this for myself, I made the 2 diagrams below in inkscape - these came in handy to keep nearby when doing the cam change.
The images below how the cams are positioned in service mode before the belt is removed, and the arrow highlights which way to unload the cams so they are no longer under tension. The cam lobes are marked "radiator" and "firewall" so there is no ambiguity as to which they refer to.