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  1. Z

    Pentair Pool Heater ASTM Thread vs Buttress Thread

    I just learned something today after struggling and searching for a few days. I thought this info may help someone. I replaced my Pentair Mastertemp 250HD with a slightly used ($1500) Pentair Mastertemp 400HD model 461021 ASTM, an exact fit so it would be an easy install. However. I could not...