Diesel pump - plunger depth Q

[whereisgibson]

I have the top of the pump off at the mo, but noticing that there's only about 5mm on the plunger from left alone to fully depressed - is this right?

Also, how do you know you're taken it out too far? Will it obviously not go back in? I'm wondering if my plunger is sitting on top of the pin and flexing it... or something. (I've not seen inside these things.... yet). Twiddled the boot around 180 degrees and not sure if I can now get the top back on...

Help!

 

[Cptsideways]

There are some pics on one of my threads explaining whats it there & how to get it back in