If one of the pistons needs replacing on your pump, you can check which thread you will need by pulling up the handle on the pump and unscrewing one of the metal pistons with your hands, turning clockwise to unscrew. (Sometimes these can be a bit tricky so some marigold gloves can help with grip!) You can then compare the thread on the end of the metal piston with the image below (fine thread).
Here is our video on how to assemble the parts of the pump: https://redpaddleco.com/en-gb/paddle-boarding-know/inflatable-paddle-board-maintenance-videos/how-to-maintain-your-pump/