Anyone who has tried to disassemble an OE front strut has cursed the fact that the shaft retracts itself back into the strut body as you compress the spring. The net result is that it is hard to get all spring tension off the top hat, and thus very hard to get the strut and spring back together again.
The reason for this is that OE struts have a rebound spring internally that prevents the strut from fully extending. Aftermarket struts, as a rule don't have this feature so are easier to assemble. The attached image is from a 3-series strut, but the design appears to be typical. In the center of the picture is the rod guide and seal with the cylinder cut off. The cylinder appears in the right side of the picture. It is much smaller than the outer body of the strut because the strut is a twin-tube design.
