To add....
On a ship, let's say there's a gasket that needs to be replaced. The stokers head up to the stoker workshop on 3 deck, grab a strip of gasket material, bring a bit of that along with a hammer and a knife down to the space.
They isolate the valve, drain the line into the bilge, unbolt it using the 4-8 old bolts that are on the valve, they split the valve, scrape off the old gasket, place the piece of new gasket material over the valve body, use the hammer to trace out the edges with light taps (which also cuts the edges of the new gasket to match) then take away the extra material, maybe trim one spot with their knife, then re-assemble the valve using the old bolts again.
When they drop one of the bolts and nuts into the bilge, they assemble it anyhow with the 7 remaining bolts, head up to the stoker workshop again, grab a random/close size bolt and nut from the 'spare' drawer, and go install it. "Close enough" for most of the valves on the ship. They de-isolate, test for leakage, call it good enough, and carry on with their day.
Or.
They order a new valve because the old one is leaking by slightly (meaning the valve seat/stem gasket inside is worn). They isolate the valve, drain the line into the bilge again, undo the cover of the valve with the handle on it (leaving the valve body in-line with the piping) and strip the new valve stem/handle from the new part they received, and put that on the old body that's still in-line. The new valve stem/handle has a new gasket that they transfer over, and they dump the old part in the box with the good bottom half that they never used, and send that back to return stores in about 3 years and 2 crews later when they find it under the bench in the stoker workshop.
Or something like that.