I researched the D4 OMI manuals in our ACMOC library and found the seat tank holds 30 U.S. gallons. The OMIs for the models with the side tank did not list the fuel capacity. I was able to find the capacity of the side tank in the ACMOC library D4 sales brochure, 25 U.S. gallons.
Craig
What was the advantage of a side tank.My dad had a D2 with direct start and a seat tank.A neighbor had a D2 with a side tank and a starting engine.Another neighbor had a D4 with a side tank and direct start.Prefered driving my dads D2 over the other two because of the seat tank.By the way, these were all used for farming orchard trees in the 60s.