It's 4mm extra stoke in the crank.
5mm longer rods so you have to use the 795 series pistons for the extra rod length. The hole on the piston is moved 5mm to compensate.
You either need a spacer plate to compesate for the extra stroke or some head work done.
You will want porting done to compensate for the extra stroke too or you won't be happy with it. Then you'll want carbs, intakes, filters, .. it can get very expensive once you get the bug.