Adjust rear derailleur (limits + indexing)

Skill level 2 of 4 · Tune-upAdjustModerate25 minRear derailleur (Shimano MTB-style)

Moves the chain across the cassette and keeps it tight. The cable pulls the parallelogram inward to bigger cogs; a spring pulls it back out. Two limit screws (H and L) stop it from throwing the chain off either end. The B-tension screw sets the gap between the guide pulley and the cogs. The cage length sets how much chain slack it can take up (total capacity). For example, a Shimano Altus RD-M310 handles up to a 34T cog and 43T capacity.

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Tools

Ball-end hex key set (1.5–10 mm): Park Tool HXS-1.2 Professional L-Shaped Hex Set ($$)
JIS #2 screwdriver: Vessel Megadora JIS screwdriver #2 ($)
1/4" click torque wrench (about 2–17 Nm) + bits: Pro Bike Tool 1/4 inch drive click torque wrench set (2.2–16.9 Nm) ($$)
Repair stand: Park Tool PCS-9.3 Home Mechanic Repair Stand ($$$$)

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Steps

Put the bike in the stand. Shift to the smallest cog and the chainring.
Loosen the cable anchor bolt so the derailleur sits on its H limit.
H limit: look from behind. Turn the H screw until the guide pulley sits directly under the smallest cog. Clockwise moves it inward.
Re-attach the cable: barrel adjusters in, then 1 turn out; pull snug and tighten the anchor bolt.
L limit: push the derailleur body inward by hand while pedaling to the biggest cog. Turn the L screw until the pulley lines up with the biggest cog and cannot go past it toward the spokes.
Indexing: shift one click from smallest to second cog. If it is slow or will not go, turn the barrel adjuster counter-clockwise (adds tension) a quarter turn at a time.
If it overshoots or makes noise, turn the barrel adjuster clockwise. Work through every gear up and down.
B-tension: in the biggest cog, set the screw so the guide pulley sits about 5–6 mm below the cog teeth.
Test ride. Small barrel tweaks on the shifter are normal for the first week as the cable settles.

Torque

Fastener	Torque
Cable anchor bolt	5–7 Nm
Derailleur mounting bolt	8–10 Nm

Diagram

Side view, chain side toward youRear derailleur

Rear derailleur: adjustment points

Signs it needs doing

Chain hesitates or clicks between gears (cable tension)
Chain falls off the smallest cog (H screw too loose)
Chain goes into the spokes (L screw too loose: dangerous)
Will not reach the biggest cog (L too tight or cable too slack)
Bad shifting in every gear after a fall (bent hanger)

Problems this fixes

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