Iran recently put one of its three Kilo class submarines back into service after a period of refurbishment by an Iranian shipyard. Russia insists that refurbishment and upgrades of its Kilo class subs be done in Russian shipyards. But the Iranians believed the Russians were charging too much and feared the Russians would not return the sub. So the Iranians did it themselves, even though the Russians would not provide the technical data normally needed for such an upgrade. According to the Iranian press release, Iranian technicians fabricated replacement parts for internal (pumps, compressors, engines, and the like) and external (sound absorbent tiles, control surfaces) components and installed them. While some of the technology needed here (pneumatic, engines, electronics) is pretty common stuff, other items are not. So it's unknown how adequate the Iranian replacement parts were. Iran often exaggerates its accomplishments in the area of military technology.
The Kilos weigh 2,300 tons (surface displacement), have six torpedo tubes, and a crew of 57. They are quiet and can travel about 700 kilometers under water at a quiet speed of about five kilometers an hour. Kilos carry 18 torpedoes or SSN-27 anti-ship missiles (with a range of 300 kilometers and launched underwater from the torpedo tubes). The combination of quietness and cruise missiles makes Kilo very dangerous to surface ships. The new Iranian components have probably made these boats louder and easier to find. U.S. and allied anti-submarine forces will be keen to play with the refurbished Kilo when it ventures out into international waters (which it may avoid doing to prevent anyone from "hearing" how this Kilo has changed).