Key Way design - "When installing a shock absorber, strut or cartridge it is often difficult to prevent the piston rod from rotating when tightening the top mounting nut. However, KYB piston rods have been designed to allow for a Spanner (Figure A), Hex Key (Figure B) or Key Way
(Figure C) to hold the piston rod in position during fitment. For example the Key Way design locks the piston rod in the top hat mounting assembly, preventing the piston rod from rotating."
CORRECT INSTALLATION OF SHOCK ABSORBER, STRUT OR CARTRIDGE
It drove me bananas until I worked it out and still catches me out. The only fail safe way for me is to do it with the strut out of the car and to ensure the spring is compressed enough that no effort is required to engage the key way onto the piston rod flats. Otherwise you're fighting the spring while trying to locate the key way onto the rod while also trying to tighten the nut. Once the key way is engaged you only need to hand tighten the nut with the final torque once mounted back on the car. (Before putting back in the car check the piston rod isn't spinning while holding the top hat steady as this is a sign the key way is not engaged).
As the photos provided by JohnGD show there is very little little metal available to engage the key way. What's also interesting is that despite my having KYB struts and tops hats the key way design is the same style as the Monroe in the photos (ie only a few millimetres of metal available and nothing like what's shown on the KYB website).
Looking at the design I would have thought that the key way would self-engage as the nut is tightened but it hasn't been the case for me. I'm surprised I haven't stripped the metal away on more than one occasion.
Let me know if you need more photos. I have top hats and struts in my living room from last week's strut change over.