The needle is held in the piston by a wire spring clip that engages the grooves cut in the top portion of the needle. There are 4 sets of grooves, that determine needle positions 1 through 4.  The theory is,  gripping and twisting the needle 1/4 turn spreads open the wire clip, and allows the needle to move in or out to the next position.  In practice, it can be tricky. It’s hard to get a good grip on that little slippery needle, and you can’t tell what you are doing.  I think I’ve used rubber gloves to get a better hold of the needle. But, that’s how it works. I included a little chart I found somewhere that it quite useful. It is the resulting EXPOSED needle length in each of the four positions. Nice to be able to verify, yes, I’m in the correct position. 

@4949 Thanks, David. I have a pretty good understanding of how it's in there, but I can feel no resistance as I spin the needle/piston and cannot get the needle to release to next position. Thanks also for the measurements--they are from tip of needle to . . . ?

Reply to self. Bought a new piston which included needle which I can't change/check positions. Unlike the old one, this one has a slotted screw visible from top. See pic. Do I need to loosen this screw in order to remove/reposition needle?

Yes, you remove that screw, and the needle will fall out when you turn it upside down.  That particular needle uses the tiny c-clip to position it.  LOL, beware to do the work in an area just incase the clip pops off.  St.

@14724 Steve, Well, duh! I would have eventually resorted to removing the screw but THANK YOU for the info. Dr. Google was no help this time either!!

@14724 Steve, Well, duh! I would have eventually resorted to removing the screw but THANK YOU for the info. Dr. Google was no help this time either!!

LOL, no problem, I am not always as smart as Google.  I like this needle system better than the click and twist one. St.