SM-BB70 Limit Screw Adjustments: How to Get Them Just Right

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The Shimano SM-BB70 [paid link] is a Hollowtech II bottom bracket, commonly paired with mid-range Shimano road cranksets like the FC-5700 [paid link] or FC-4600 [paid link]. As a bottom bracket, it doesn’t have “limit screws” in the traditional sense, which are found on front and rear derailleurs. However, the performance of this bottom bracket can directly affect drivetrain alignment, crankset behavior, and front derailleur limit screw adjustments. Improper installation or misalignment can create the illusion of poor limit screw tuning—especially when chain rub or shifting issues arise.

This article clarifies the role of SM-BB70 [paid link] in the broader context of drivetrain tuning and guides you through the correct setup and diagnostic process. While the bottom bracket itself has no physical limit screws, it plays a key role in ensuring other components operate within their limits correctly.

Understanding the Role of SM-BB70 in Drivetrain Limit Adjustments

The SM-BB70 [paid link] is responsible for supporting the crank spindle and maintaining precise lateral spacing of the crank arms. If this spacing is off due to incorrect installation—especially with inadequate or excessive preload, shell facing issues, or frame compatibility problems—it can affect the front derailleur’s ability to keep the chain on the correct chainring. This often leads to false assumptions about derailleur limit screws needing adjustment when the root issue lies with bottom bracket alignment.

Even minute misalignments at the bottom bracket level [paid link] can tilt the crankset slightly, altering the chainline and thus complicating the performance of limit screws on front derailleurs.

Installation Precision is Crucial for Limit Screw Performance

To get the most out of your front derailleur’s limit screws, ensure that the SM-BB70 bottom bracket is installed with precise alignment and torque settings. Here’s why:

  • Shell Facing: If the frame’s bottom bracket shell isn’t properly faced, the SM-BB70 cups will not align correctly. This creates side-loading on the crank spindle and introduces uneven crankset alignment.
  • Correct Torque: Under- or over-tightening the bottom bracket cups can introduce unwanted lateral force or movement over time. Shimano recommends 35-50 Nm for SM-BB70 cup installation.
  • Spacer Configuration: While SM-BB70 is typically used with road bikes that don’t require spacers between the BB shell and the cups, incorrect use of spacers (especially if borrowed from MTB setups) can misplace the chainline.

All these factors, if neglected, can force you to overcompensate with derailleur limit screw adjustments—potentially resulting in inaccurate gear shifts, dropped chains, or chain rub.

Symptoms of Limit Screw Misadjustment Caused by SM-BB70 Misalignment

When the bottom bracket is misaligned, it may mimic common limit screw issues:

  • Chain rub in extreme gears: This may be due to a skewed chainline caused by poor BB installation, not incorrect limit screw setting.
  • Front derailleur fails to shift cleanly to big ring: If the crankset isn’t centered due to BB misplacement, even a correctly adjusted high limit screw won’t guide the chain effectively.
  • Chain drop on small ring shift: This could be due to crankset lateral shift rather than a low limit screw issue.

These issues typically prompt riders or mechanics to repeatedly adjust derailleur limit screws without resolving the root problem: bottom bracket misalignment.

Corrective Actions to Ensure Accurate Limit Screw Tuning

To avoid chasing phantom issues, follow this process to ensure your SM-BB70 is not contributing to false limit screw problems:

Frame Preparation

  • Ensure the BB shell is faced and chamfered correctly. This is crucial for Hollowtech II bottom brackets and prevents cup misalignment.
  • Clean and inspect BB shell threads before installation.

Bottom Bracket Installation

  • Grease the BB threads or use anti-seize if required.
  • Use a torque wrench [paid link] to tighten each cup to 35-50 Nm. Do not over-tighten.
  • Confirm crankset spindle inserts smoothly without force or misalignment.

Chainline Confirmation

  • Measure the chainline after crank installation. For most Shimano road setups, it should be around 43.5mm.
  • Use a straightedge tool or laser alignment tool to check for crankset misalignment.

If chainline is off, re-inspect BB cup seating or crank preload settings.

Front Derailleur Re-Evaluation

Once the BB and crankset are perfectly aligned, revisit the front derailleur:

  • Adjust height and angle.
  • Reset high and low limit screws with the correct reference.
  • Test shifting through full range with cross-chaining to confirm no rubbing or hesitation.

The Impact of Crank Preload on Perceived Limit Screw Accuracy

Crank preload, applied via the Hollowtech II preload cap, is another source of false shifting issues. If too tight, it can compress the BB bearings and shift crank arms laterally. If too loose, it can introduce play, affecting front derailleur indexing.

To ensure correct preload:

  • Use Shimano’s preload cap tool.
  • Tighten cap until play is removed—not beyond that point.
  • Torque crank arm pinch bolts to 12-14 Nm only after preload is set.

Improper preload can lead to subtle crank drift under load, appearing as limit screw drift during high-torque pedaling.

When to Reassess Your Limit Screws After BB Work

After any SM-BB70 installation, adjustment, or service, always reassess your derailleur’s limit screws. Even slight realignments can alter chain trajectory. Perform a full drivetrain diagnostic:

  • Check gear shifts from inner to outer chainrings.
  • Confirm there’s no chain rub in cross-chain positions.
  • Ensure the derailleur cage doesn’t over- or under-travel.

A perfectly aligned bottom bracket creates a neutral foundation for accurate limit screw settings that won’t drift with torque or terrain variations.

Conclusion

Although the Shimano SM-BB70 bottom bracket doesn’t have limit screws, its proper installation and alignment are critical to how well your drivetrain’s actual limit screws perform. Many issues blamed on derailleur tuning are actually rooted in bottom bracket misalignment, crankset offset, or preload inconsistencies. Addressing these foundational components ensures your shifting is smooth, consistent, and fully within spec.

If you’re constantly readjusting your limit screws, take a closer look at your SM-BB70 installation. It may be the hidden source of your drivetrain frustrations.