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This is unfixable in my opinion (unless you weld it). It'll just break in a different spot a year later. I've broken my swivel tilt control mechanism twice on my chair and my recommendation is to buy a new unit.
This is the replacement that I'm currently using (it's beefy and badass). You'll want to take measurements of your bolt hole spacing on your chair. Using those measurements, find a mechanism that fits correctly with your chair.
If and when you eventually receive the package, you'll want to take some big pliers and grip down on the gas cylinder that's connected to the mechanism and twist it off to break the friction fit.
Let me know if you have any questions.
Good point, thanks.
Essentially I have a very old chair and I need to bolt a steel plate to the bottom of it, and onto that steel plate I will then bolt a standard chair base (like this one https://www.amazon.com/dp/B072YFNQSR/_encoding=UTF8?coliid=I2KQW74R1BRPV2&colid=3VXFQEB3HVION ).
The chair itself is fairly heavy, combined weight of the chair and the occupant will probably be around 375lbs