How do I take the slack out of a door closer chain?

Question

There is a cylinder with a spring inside. Presumably when the spring is compressed it will pull the external chain attached to it to be taught, I.e. not slack.

But I can't keep the spring compressed to then insert it back into the door cavity and faceplate on top.

Here's a sketch of what I mean by the spring inside the cylinder.

if the spring is as illustrated in your diagram, then the top picture should show a tensioned chain .... you are missing something ... the rectangular cutout in the door edge indicates that you are missing at least one piece — jsotola, Jan 03 '22 at 23:32
Thank you @jsotola with regard to the missing part. I will consider carefully disassembling same mechanism on another working door to see what that missing part could be. I had disassembled the original door to be able to manoeuvre furniture and may well be able to identify that part in the same box where I stored the cylinder, after comparing with the working door. — therobyouknow, Jan 03 '22 at 23:55
To answer your question is the spring compressed when not connected to anything. Answer is no. But when I push a screwdriver into the end of the cylinder I can feel the tension of the spring compressed. — therobyouknow, Jan 03 '22 at 23:57
then you probably drew the diagram incorrectly ... the spring probably compresses when you pull the chain — jsotola, Jan 04 '22 at 00:04
+1 jsotola thank you, now that's a thought! Maybe I connected it up wrong or there's a missing part as you say. — therobyouknow, Jan 04 '22 at 00:09

score 2 · Accepted Answer · edited Jan 18 '22 at 13:49

Looks like I need to twist the chain to increase the tension, according to this Q&A: How to stop self closing doors from slamming

Thanks to jsotola for their input.

There is another faceplate, which I had while working on this, but not shown in photograph. But, as jsotola suggests, I should check for other missing parts.

Key things: