Way back I converted an Gasser Compression tester setup to diesel by switching the Gauge.
It worked OK but the setup used Tire Valve Stem Schrader Valves as the valve in it. I found that the heat of compression developed by a Diesel can burn up the seal in the Schrader Valve.
I suspect that the Harbor Freight Tools also use the Schrader Valves.

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84 300D, 82 Volvo 244Gl Diesel