On the M20 engine as used in the E30 325, the pressure on the timing chain tensioner is the result of a spring that pushes against the water pump.  To prevent the spring from flying off into space, it is wound around a steel pin that is held in position.

At the water-pump side of the pin, there is a detent (three-sided metal wall) that boxes in the blunt end of the pin.

At the tensioner side of the pin, there is a hole into which the sharp end of the pin fits.

The tension on the spring is significant. To get the pin into position, I had to push very hard, with much of my upper-body strength.

Let’s think about this pin.  It’s made of steel.  It’s about 2 inches long.  It has a sharp end.  It’s kept in position by a spring with A LOT of energy. If this sounds like a dangerous combination to you, than we’re in agreement.  I made a point of wearing safety glasses while working on this task.  Even so, I was very wary.

To get the pin into position can be very tricky when you’re installing the water pump, if the timing belt is already in position, exerting counter-pressure.  We ended up loosening both bolts that keep the tensioner in position.  That enabled us to get the spring and pin into position, and we could then pry the tensioner back into position well enough to get both bolts back into place.