The heater comes with new studs in case the old ones are getting bad. You can get vise grips or pliers to change out the old ones.
Most of the time theses coils are replaced after quite a few years of use and have a buildup on the coil and the studs get corroded . In my case my original coil was only 3 years old and was leaking into the boiler, I didn't replace the studs ,which just unscrew from the boiler. You may need to heat up the area with a torch if they are badly corroded
The tankless coil will provide 3.25 - 4.00 gallons per minute depending on which boiler the coil is installed in.
14 Gals per min, never ran out of hot water. Not using a mixing valve and was still putting out hot without spurts of hot to cold. The coil itself may only hold like 1/2 gal or a gal(not sure) but with a high of 140 and low 120 boiler temp, never lost hot.