I have a hard time believing this was your fault....I have done many valve adjustments and have never needed the third tool.....I am really believing this issue was present before you did the adjustment....granted it may not have been completely broke but I imagine it was on its way or the valve was bent....you putting torque in it....to loosen the nut, was all it need to break at the fracture....

I was really thinking your issue was valve guide or seal related....I suppose I was thinking correctly....

Now let's look at the bright side, with a used head and hopefully you'll get it rebuilt.....you will have two very nice w123s on your hands...

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