Of course it's another port timing question

Octavio /
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I'm doing a port map for a stock Puch cylinder and I keep getting hung up on if this one detail is important or not. The rings start 4mm down from the top of the piston crown and since they are what make the seal it seemed logical to do all my timing calcs from their TCD position. All the port timing material I've read seems to tell me to just measure from the piston crown TDC position. So the question is: Does that 4mm make a difference? It seems like it should.

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