Just take the pump apart and clean all the parts.
Look for any damage or wear.
Most important is the inside of the main housing, the part where the gears run in.
I know it is a 617 pump, but the concept is the same.
The inside walls should not be scratched to badly.
If they look useable, just smooth them out with some fine emery cloth.
Smooth out all surfaces on the gears.
Install the gears in the housing and measure the clearance.
On the 617 it should be 0.05 to 0.06 mm top and side.
I assume the 603 is similar.
Next you have to surface the lid.
It may look somewhat like this.
Move the lid over some emery cloth on an even surface.
You may want to start with a coarser grid and then move to a finer one.
Almost done (coarse grid in use).
Done (after fine grid used).
And a final check.
A note on assembly.
I always turn the driven gear around, so that the 'unused' surface of the driven gear runs against the drive gear.
The verdict on this may be split, but i had never problems with this practice.
The housing and lid have no dowel pins, so you have to align them manually.
To do this, assemble the pump and tighten the screws lightly.
Try to turn the pump shaft.
Now use a small hammer and hit the side of the lid lightly.
Turn the shaft again.
Repeat until you find the position where the shaft/gears turns most easy.
Just for the record.
This 3 gears are out of a 616 (left), 617NA (middle), 617Turbo (right).
Just the height is different, the diameters are all the same.
