You were right Livens, the plunger and spring were indeed for the locking pin. It is there for disassembly purposes only to give a way out for the locking pin to release the center hub from the gear. So It indeed can be filled with flowable silicone without issue. and if you don't ever plan on disassembling the gear completely it can even be welded shut.

as for the plunger o-ring, I just went to napa and popped on whatever fit.

 

Thanks hhn2002, really cool of you to post these pictures. This needs to be a sticky on this forum, to let anyone with a VVT gear know they should probably just plug theirs up before it starts leaking. In fact I will be doing this to my practically brand new VVT gear just in case!

I still think its a silly design. Couldn't you just pull the cap off of the gear and then pull out the locking pin? Why did there have to be a big hole in it with a oring that served no purpose other than to close the hole back up?

 

The locking pin was situated so that the only way to gain access to it is radially like it is now. In the standard vvt gear, and I believe at least one other volvo vvt gear the locking pin moves along the axis of rotation but I believe because of space constraints and for the sake of a thinner profile they designed for it to move radially. They needed a way it plug up the hole and I guess the first thing that came to mind was a plunger/oring combination. Honestly though it was not designed with longevity in mind. They should have used a screw type oil cap, just like the large one used on the front end of the gear. But it is what it is and now we know.