Philippe,
You could - I believe - use adaptivity to control this. Create the washer in
the context of the Assembly, and instead of giving the washer a blind
thickness, use the To option and select the face of the opposing part. Then
use a mate constraint with an offset between the two components to drive the
thickness of the Washer.
Hope that helps,
--
Phil Kenewell
----------------------
Gage Designer
North American Lighting, Inc.