It is necessary to cut off the head of the native key, but not at the root, but on distance of about 5-7 mm. To do this, mark the place of the cut with a marker. Holding the key with pliers, saw off the head with a grinder. If there is a vice, use them, because this method is extremely dangerous.
Next, you will need a spanner key, in which hole the cut-off head is inserted as closely as possible. I took it at 8, I don’t need it that much, especially the second half of it will remain completely working.
Insert one into the other.
Fixing with arc welding.
Hammered the dross with a hammer after welding.Grind to shine with a grinding wheel on the grinder.
Our handy key is ready.
Testing: tightening the cartridge very well, it's just a song!
It's a shame that I suffered a long time with an ordinary key. I recommend you do the same. As a result of a longer lever, you can easily create a good tightening force, without unnecessary strain on yourself.