>I agree with Richard.....If the piano has not been restrung in the last fifty years, the best solution is to have  it restrung to >modern 440 pitch.  I am curious what the difference in in string tension is  between a viennese piano and a modern piano is. 

I suppose the definition of a Viennese piano is one with a Viennese action which is usually a Walter type with brass Kapsels. Certainly Boesendorfer were still building them in 1910 and it seems Schimmell still sold pianos with Viennese action as late as 1960s. so a "Viennese'" piano can have the same tension as a modern piano. In the beginning you have Stein with the top strings at say .28mm up to a very late instrument  say 1mm for the top strings possibly 4x the tension.

Richard 

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