This is an adjustable pressure regulator typically used to limit the pressure of fuel supplied to Dickinson diesel stoves and heaters in one of three situations:
A — In a gravity feed installation, where the fuel tank is especially large, or high above the stove's oil metering valve (carburetor) creating pressure in the fuel inflow line of greater than 3.0 psi - which has the potential to overflow the fuel metering valve.
Install this regulator in the fuel line to regulate the fuel flow pressure to the valve, keeping it below 3 psi.
You will certainly need this regulator if your gravity feed tank is over 30 gallons, or more than 8 feet above the fuel meter.
B — In an existing installation where the fuel tank is below the stove, and you have to change the location of the tank or stove increasing the distance between them to greater than 4 - 6 vertical feet, or 15 horizontal feet.
In this situation, you will need to retrofit your existing FRD-2 (DIK 20-000) pump with a stronger spring - effectively changing it into a higher pressure FRD-4 pump - so it can pull the fuel the greater distance to the stove. However, once the fuel reaches the height of the pump, the pressure will have to be regulated back to 3 psi so as not to overflow the fuel metering valve. That's where you use this regulator. (The stronger spring you will need to buy along with this regulator is #DIK FRD-SPRING).
Effectively, you will be creating a DIK 20-002 assembly, see below.
C — You are installing a Dickinson stove or heater with a fuel tank below the the stove, and realize you purchased an FRD-2 pump (#DIK 20-000) suitable for installations where the fuel tank is within 4 vertical feet of the stove (or 15 ft horizontal), but you find out the distance is actually greater than that - so you need a DIK 20-002 pump/regulator assembly.
Depending on where you live, it is sometimes cheaper to buy the regulator and stronger spring (as in "B", above) and retrofit the pump you have, rather than to pay to return the 20-000 low pressure pump and purchase the 20-002 high pressure one. You'll have to do the math for your location. Don't forget to consider shipping and Customs fees...and the hassle of doing the mechanical work.