Rivets

The rivet heading machine turns 1 Rivet Rod into 2 bundles of Rivets in 3 seconds at 0.3 N.m of drive, twice as many bundles out of the same iron as a cut nail machine, each half the metal of a nail bundle. Rivets are the only fastener a sealed pressure vessel accepts - a boiler shell above all takes rivets and refuses nails and strips outright.