Water pump on s4 GT and GTS models
Water pump mounting holes layout.
The pump takes two sizes of bolts: 9x L(ong) and 5x S(hort). (ps: the above picture shows 9x Long so the picture is wrong... PET says 8 for both pump types)
** I must correct this next time I renew the pump.
The Long is M6x25, 8.8 grade Zinc plated or Cadmium if possible (10.9 or 12.9 is harder, has more strength in the length direction, but wont like ant force sideways) . The 8.8 takes a Torque of 9.5Nm (where the 12.9 grade takes 16.4)
The Short is M6x20 (see comment above)
All bolts ate fitted with a washer !!
Use Optimoly TA on the bolts, and clean the holes with a proper M6 tap (thread chasing) carefully. Make sure you -follow- the thread and not make a new one :)
Some say: use stainless steel bolts, but this is not a splendid idea. The bolts have less strength compared to carbon steel. Corrosion on the threads is the biggest problem and antiseize is a lot better than nothing, but if the bolts do get stuck you really need a strong bolt to work with :)
Lecture on bolts:
"Strength" of a material can have many meanings, like the proverbial story of
Eskimos having 43 words for snow.
One meaning is stiffness, the "spring rate" of a material, also known as modulus; there are even several variations of that property, Young's modulus, bulk modulus, etc. The interesting thing about this property is that all carbon steels have the same stiffness, for all practical purposes. While acting only as a very stiff spring in the form of a bolt or other component, they all have the same level of this sort of "strength". Aluminum has one-third this sort of "strength". Stainless steel is a bit weaker than carbon steel. Carbon fiber composites are much stiffer. Many things turn out to be stiffness-limited, so this property is often more important than the sorts of strength that govern outright failures (such as ultimate tensile strength). Not bolts, usually, but stiffness is a factor - torquing a bolt is the same as applying a spring preload.