Once again there is still a lot of room for improvement in my port grinding but it is my first attempt.
Now to my way of thinking there is no point in opening up the exhaust port if you don't open up the exhaust outlet as well. Our new exhaust port has an area that is the equivalent of a circle of about 40mm diameter.
The standard exhaust outlet on the RD400 is only 38mm diameter. So if we don't open it up the 38mm size would be a restriction. So we mounted the barrel on the bed of a mill and used a fly cutter to open up the exhaust outlet to 41mm.