Large Airstone specific to D1 spas - aftermarket **different diameter** 3/8" NPT thread, 68mm long, 19mm diameter Cannot use supplied extractor split pipe for removal/insertion - a length of 1/2" pressure pipe with a split cut down the end will slip over this airstone to allow unscrewing from the base of the filter housing. Alternatively a very short length of 3/4" CVT tube can be glued over 1/2" pressure pipe and provide a softer option for gripping the airstone.