There is a fellow in Minneapolis who has made a cottage industry of shortening bicycle cranks. He's reasonably priced and does a very nice job. Not every crank can be shortened and there is a limit to how LITTLE he can shorten a crank because he has to leave some metal above the old hole. I doubt he can shorten a 170 mm to 156 mm. If you're interested I can find his information but it might take me awhile.
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My greatest fear is all of my kids standing around my coffin and talking about "how sensible" dad was.