What breaks first inside a 4L60E?

What fails, in order: heat first — once the fluid breaks down the clutches follow quickly. Then the 3–4 clutch pack, and the sun (reaction) shell, which takes out 2nd, 4th and reverse when it lets go. Heavy vehicles, towing, sticky tires and poor tuning all speed that up.

Signs a used one is already on the way out: burnt-smelling fluid, slipping shifts, or missing gears.

The upgrades that matter (Speedway):

• 3–4 clutch pack: stronger frictions and steels — one of the most common failure points.
• Hardened input shaft: keeps it from twisting or breaking with hard launches or sticky tires.
• Sun shell: a beefed-up shell.
• Valve body / shift kit: firmer shifts cut clutch slip and heat.
• Torque converter: stall matched to the cam and powerband; for high torque, one with an anti-ballooning plate.
• A large external cooler: heat is what kills these.
• Tuning: shift timing and line pressure calibrated for the build.

More: how much power a 4L60E can handle.