I have attached a summary PDF file of the manual thumb addition I did for my Kubota B26 which has the BT820 backhoe. Kubota does not offer a manual thumb for this backhoe and I could not justify the expense of adding the backhoe third function hydraulics to my machine. I purchased the pins and thumb Weldment from Kubota (just in case I want to add the hydraulics in the future). I thought I would share what I did as I have seen posts on other forms about others wanting to do the same. I mimicked the Kubota 2650 manual thumb arm design. The dimensions should help you get close. If I was ambitious I would tweak the neck offset on the inner arm to tighten the retracted position of the thumb a little bit but it is not worth it in my situation. I took my sketches and wooden prototype to a machine shop to get the welding done and they were a little too professional. They custom machined the end bushings which was beyond what I had expected and made everything greaseable. The tractor was 3 hours away so the tack weld fit-up test was skipped.