I believe it'sw called a quick tach . It runs off the impulses from the injector lines . Don't have the site handy . but I'll look it up . John
Try www.tinytach.com
I am also interested in adding a tach to my BX23 but was wondering if anybody had used a tach other than the QuickTach - I was thinking of using a VDO boat tach
(waterproof) and was wondering if maybe this could be adapted to whatever sender is used on say a B series Kubota engine?