When the oil pump cover is fixed with normal nuts, a small leakage occurs through the threads, so originally special nuts were used for this purpose, which incorporate a seal.
This was not used from the beginning, for the old Volkswagens where the oil pump was fixed with M6 threaded studs, standard M6 nuts with loose washers were used. For sealing, you had to use a liquid seal, no guarantee for a perfect seal. Volkswagen therefore changed this arrangement in the 1967 revision, so that the nuts were equipped with a sealing washer at the factory.
With the nuts shown here, we go one step further, the nuts have been fitted with a collar (flange nuts) that better holds the seal in place. Also, the nuts were often mounted with the seal up (the idea of a locknut), which negated their function as a seal. Now that the nut has been given a base, it is clear to everyone that this is the side that meets the oil pump cover. These nuts are available in M6 and M8, so even older engines can benefit from a perfect seal.
These nuts are not supplied with engine gasket kits, oil pump covers or oil pumps, so you will always need to order them extra.
Note: These nuts are for single use only. If they are reused, the nut seal will no longer do its job and leakage will occur.
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