I am attempting to pull the compressor clutch in order to check the air gap. How in heck do I hold it from turning so that I can loosen the 12mm nut?

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Steve
'87 300TD - 132K - Soon 4-Sale
'84 300D Turbo - 122K - Driving
'77 VW Type II - 77K - Restored
'08 250EX Ninja
English Bulldog (Brier) - My best friend. Passed away 12/02/04 while in my arms.