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That is for after the internal hex fitting has been destroyed. Once you give up on saving the retainer, there are lots of options.
I rather use it before the hex fitting is destroyed, it's just another way to fix the retainer and loosen it from the body.
Why would you wait until you have destroyed the hex fitting when you have this tool?
From the ebay offering: " This tool will save the first stage body when the hex socket in the DIN retainer is stripped:"
Obviously it would work before then, but it looks like it might be a bit rough on the retainer. Hot and cold water and looooong soaks don't do any damage.
Something too few people seem to try is impact tools. Almost all the fiddling with heat and whatnot is no replacement for an impact wrench, as any mechanic will tell you.
Once I listened to a mechanic who was laughing at how much work I was putting into taking apart seawater welded pieces by hand, I have not looked back. Impact tools and easy-outs for the odd port plug and DIN retainer that stripped out because someone used a metric/SAE in a SAE/Metric hole.
Obviously this is trickier for assembly, but then again it was probably air-powered impact tools that assembled the parts in the first place