You need to be carful that you get a Block Heater that has the same threads as your Engine.
Some People bought Block Heaters that Had an OD that was also too small and the Adapter Sleeves were not available.

When I was under the Car I looked and could not find a Block Heater.
Years later I found the Block Heater Cord tucked up under the front Frame.
I plugged it in and it worked.

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84 300D, 82 Volvo 244Gl Diesel