They should both be 14mm, is it possible the wrench might be getting hung up on something else, preventing it from going on? If all else fails, you can always get a cheap 14mm from a parts store or something and file it out a little larger

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint