The 1990-1996 Nissan 300zx OEM Turbo to Downpipe Nut screws into the turbo exhaust housing studs and holds the downpipe in place. The Z32 turbos have two of these nuts on each turbo and these nuts are made from hardened steel to hold up to the extreme heat.
When upgrading to aftermarket turbos they often do not include these nuts and we recommend getting them when you purchase the new turbos. For 4 Bolt turbos like the Sport 500, Sport 600, and Gt525 you will need 2 nuts per turbo. For the 5 Bolt turbos like the Sport 530bb, Sport 550bb, Sport 700bb, GT600r, EXP90, etc you will need 3 nuts per turbo.