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The rear derailleur tension adjustment screw on my 1997 or 98 Veloce 9speed bike is broken. The correct replacement part seems to be the Rd-At115 screw.

However, there seem to be at least two more such screws with part numbers RD-DA004 and Rd-Sr014.

Here's my question: What is the technical difference between these three screws (that seem to do the same thing)? Diameter (outer? inner?), steepness of the tread? Something else I've overlooked?

Also, if they are indeed different, what would be the reason to change the specification of this rather trivial part?

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RD-AT115 has a screw thread pitch of 0.8 mm, the standard size for a 5mm metric thread. The later RD-DA004 and related rear adjusters, including RD-SR014 all have a thread pitch of 0.5 mm, standard size for "M5 fine".

The older, typically 8 and 9s RDs did not need such a fine adjustment to cable tension, simply because the precision required of the system as a whole was not as high and the rear derailleur return spring has, over the years, been gradually reduced in strength - in fact there are two slightly different strengths in use according to the derailleur model and year, and have been since 2002.

This has meant that the pitch of the screw and so the amount of variation in tension per full turn that the screw induces, has needed to be changed.

There are also materials differences that affect the overall price of the derailleur but not the function - so some adjusters are made of a simple Magnesium Zinc Aluminium alloy, others are made in polished stainless steel.

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