Metal expands when hot. A nozzle torqued cold loosens at 220 °C and leaks molten plastic up into the heater block — the blob of death, and the single most expensive beginner mistake on this list.
Phase 0 — Mechanicalabout 10 min
How to do it
Heat the nozzle to 240 °C.
Hold the heater block steady with a wrench so torque does not go through the heatbreak.
Snug the nozzle with a socket. Firm, not forced.
BURN RISK: the block is at 240 °C. Use tools, never fingers.
Re-do this every single time you change a nozzle.
How to read the result
No plastic weeping from the nozzle-to-block joint after a print.
Does your printer have this?
Look at the very tip of the hot end. You HAVE this if the brass tip is clearly a separate part screwed into the metal block above it — you can see a joint, and usually a hex flat on the nozzle for a spanner. You DO NOT have it if the tip and the heater are one sealed unit you swap whole, which is how most modern quick-change hotends work. If your nozzle changes are "unclip the old one, clip the new one in" with no tools and no heating, there is no joint to tighten and this step does not apply.