Izikrufu zokutywina, ezikwaziwa ngokuba zizikhonkwane zokuzitywina okanye iziqhoboshi zokutywina, zizinto ezikhethekileyo eziyilelwe ukubonelela ngesitywina esikhuselekileyo kunye nobungqina bokuvuza kwiinkqubo ezahlukeneyo zemizi-mveliso kunye nezoomatshini. Ezi zikrufu zibonakalisa uyilo olulodwa olubandakanya into yokutywina, ngokuqhelekileyo i-O-ring eyomelelayo okanye iwasha, edityaniswe kwisakhiwo sesikrufu. Xa isikrufu sokutywina sibotshelelwa endaweni, into yokutywina yenza itywina eliqinileyo phakathi kwesikrufu kunye nomphezulu wokukhwelana, ukuthintela ukudlula kwamanzi, iigesi, okanye ungcoliseko.