With a "Sandseal" pulley, the retaining ring (or seal) does not last forever, so you always need an extra one (it is not a normal size found everywhere).
For the "Bolt-in" pulleys, the aluminum housing, including the retaining ring and spacer rings, are also available separately, so there is no need to buy a whole new set of pulleys for this. In addition, the fitted retaining ring is the same as that of the "Sandseal" pulley and can therefore also be replaced separately.
Useful information from Paruzzi magazine and from our YouTube channel :
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