A C4 stock style shock(incl koni and bilstein) has the top fitting like this, the shaft thread is shorter and it is wider under the thread. The bushing is tight fit on all stock type shocks.

You could try to assemble it in different order. First put the koni bushing upside down and then my bushing in top of the koni and last the camberplate, washer and nut.
Edit.
Added better pick of the C4 standard shock shaft thread:

You could try to assemble it in different order. First put the koni bushing upside down and then my bushing in top of the koni and last the camberplate, washer and nut.
Edit.
Added better pick of the C4 standard shock shaft thread:


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