A lot of the ones I have seen have the back tube extend up and the pivot mounted on a piece of tubing that slides up and down on the back tube so you can adjust height without taking things apart. The spiffer ones use a crank and a track so you can adjust on the fly.
Also having the force to return the hammer up supplied at the very end of the lever arm will take a lot more spring than if you can attach it somewhere in the middle.
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Thomas
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