Pony motor water pump shim question

[chas036]

I have a 720 diesel with a pony motor and I am replacing the pony motor water pump with a rebuilt pump. When I removed the old pump, there was one paper shim behind it. So when I installed the new pump (without the cover so I could see the impeller), I put one shim and tightened it down. I tried to move the impeller with with hand, but I could not get any movement , no backlash what so ever. My question is, what is the purpose of the shims, and should I continue to add shims until I get a slight backlash when I try to move the impleller with my hand?

 

[Partspring]

Add shims until you get a little backlash the replacement pump is not the same length from the mounting flange to the bevel gear.