Data Sheet
The data sheet for the Diode can be found here online. The assembly instructions can be found here.The part number is D1230N18T. Note the assembly instructions says the plastic housing must touch the heatsink.
Important Notes
Max Temp: 180C Max Current Average ON @100C: 1230A Threshold Voltage: 0.81V Max repetitive reverse voltage: 1800V Max reverse current: 50mA Forward Voltage @ 800A: 1.063V
This is a plot of the current given a voltage. I followed this page to understand some of the symbols here.
Data
Here is the diode response as a function of applied voltage. I used a delta elektronika power supply and a Keysight volt meter, since the voltmeter is battery powered to prevent ground loops. I measured the voltage drop across the diode by placing the probes on the connectors of the heatsink. I used low gauge wire so I could go up to the max current of the power supply 51amps. The diode was placed in the mount such that the plastic housing touched the heat sink as required by the assembly instructions. Here is the plot of the voltage versus current
The diode threshold rating was 0.81V but as we see significant current is conducted before ever reaching that voltage, likely due to manufacturing differences. The voltage also dropped as a function of time when taking these measurements when on for a prolonged period of time, so the voltage after a couple seconds was recorded for each current. After leaving the current on at 51amps for roughly 5 minutes the hottest temperature measured was 55 degrees C, though it looked like it was still rising slowly.