Cut the old belt, then wrap it round the new path, overlapping the ends. Draw a line across both belts, then open it out and measure the distance between the two marks. That is the length of the belt you require.if anyone has a shortcut to which belt to order to no longer drive the pump, just the alternator I would be most grateful. The instructions for the pump say you can just leave the mechanical impeller in place. To start with I may do this but want to long term remove it. The housing is alloy isn’t it? I’ve only welded steel before alas.