how do i know when it needs changed it has two groves in the bottom but not sure how to tell if to deep thanks for your help i have been geting a lot lately thanks to all of u

According to manual " The groves in the base plate ( spectacle plate) resulting from basic operation are irrelevant as long as the hard facing is in good working condition.

cutting ring and plate can't be expected to wear equally and will need to be replaced accordingly.

That said Ive been told "really needs to be messed-up" well I just replaced both cutting ring and plate and mine as I felt what I saw qualified as pretty messed up.   also note that 8mm gap where cutting ring slides into rock-valve is max. more cutting ring is worn-out..  Dont forget to replace spring with cutting ring..

here is a pic of my plate

and pic 2 should i replace it

Hell I see the problem, the rest of the pump is Missing!!!!!!!   

Now that I,ve opened by mouth, Ill open my eyes. KIDNEY PLATE-ok back to the book.

  Replace plate only if scoring deeper than 2.5 mm. are found in the areas the kindey seal contacts..scoring outside that area doesnt matter..p. 36 service manual.

I think you're ok if you clean up the plate with a grinder and a sanding disc and install a new kidney seal and pressure spring, I have seen worse; as long as you dont have problems pumping,

You shoul change this mirror plate the grooves seem deep enough and the output is not round anymore, it will worn out your insert inside the transition piece, if you ground down the material to make it flat again it will have to be machine to keep the surface even, that will cause an extra space in between the rock's cutting ring and the spectacle plate and then the pressure spring won't be able to do its job

Just my 2 cents