They may be, but depending on what age/series RCBS dies you have the thread size on the decapping stem varies. You'll just have to take your dies and see which ones fit.
Most of my Lyman dies have a 5/8 thread (I'm guessing, no thread gauge in my pocket), but, one has a fixed decap stem (non-adjustable) whereas others have an adjustable and rebuild-able stem/expander/pin.