SM-BB93 Limit Screw Adjustments: How to Get Them Just Right

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The Shimano SM-BB93 [paid link] is a threaded Hollowtech II bottom bracket designed for road bikes using a BSA shell. Although it does not feature traditional limit screws like derailleurs, many riders and mechanics still refer to precise bearing preload and crank fitment adjustments as “limit screw tuning” in the context of bottom bracket installations. Achieving correct adjustment is vital for minimizing bearing drag, eliminating lateral play, and prolonging component lifespan. This guide explains how to properly set up the SM-BB93 [paid link] bottom bracket with optimal bearing preload and crank arm positioning.

Understanding the Role of “Limit Screws” in the SM-BB93 Context

In bottom bracket terminology, “limit screw adjustments” may refer to the tension cap (also called preload cap) and the torque applied to crank arm bolts. These adjustments directly affect:

  • Bearing preload: Improper preload can accelerate wear or cause drag.
  • Axial play: Excessive play results in movement between crank arms and the spindle.
  • Spindle alignment: Critical for efficient power transfer and noise reduction.

Though not true limit screws, these adjustment points function as performance limiters if not set properly.

Pre-Installation Considerations for the SM-BB93

Before attempting any adjustment, ensure the following:

  • The bottom bracket shell is clean and properly faced.
  • Threaded cups are torqued to manufacturer specification (35–50 Nm).
  • Grease or anti-seize is applied appropriately to the shell and threads.
  • You are using compatible Hollowtech II cranks and following Shimano’s specifications.

Without correct preparation, even perfect adjustments can yield poor performance or long-term damage.

Tools Required

Correctly Adjusting the Crankset on the SM-BB93

1. Install Drive-Side Crank Arm

Begin by sliding the drive-side crank arm (attached to the spindle) through the installed SM-BB93 [paid link] cups. Ensure it moves smoothly without resistance. Resistance may indicate misalignment or contamination inside the shell or bearings.

2. Preload Cap Adjustment

Thread the non-drive crank arm onto the spindle and install the plastic preload cap using the Hollowtech II tool. This step is equivalent to adjusting a limit screw on other components, as it controls axial bearing pressure.

  • Tighten the preload cap until it is snug but not overly tight.
  • The goal is to remove lateral play without compressing the bearings excessively.
  • Over-tightening this cap is a common mistake and can cause bearing drag or premature wear.

The ideal torque is finger-tight—typically around 0.7–1.5 Nm.

3. Torque the Pinch Bolts Evenly

Once preload is set, tighten the crank arm pinch bolts using a torque wrench [paid link]. Shimano recommends 12–14 Nm for each bolt. Always tighten in alternating increments (left-right-left-right) to maintain even pressure across the spindle interface.

  • Uneven torque may cause misalignment.
  • Be sure to use a calibrated torque wrench [paid link] to avoid overtightening or stripping the threads.

4. Verify Axial Movement and Bearing Load

Check for side-to-side play by pushing and pulling the crank arms. If there is movement, re-tighten the preload cap slightly and repeat torque application. Spin the crankset; it should rotate freely and smoothly, with no gritty or resistant feel.

Common Errors in SM-BB93 “Limit Screw” Adjustment

Even experienced mechanics can misadjust the preload system. Watch out for these frequent mistakes:

  • Over-tightening preload cap: This is the most common issue. It increases friction and shortens bearing life.
  • Neglecting torque sequence: Failing to alternate torque on the crank bolts leads to uneven preload.
  • Skipping visual inspection: Dirt, thread damage, or debris inside the shell can prevent full engagement and proper adjustment.
  • Misidentifying play vs. crank arm slop: Sometimes play is due to worn crank splines, not adjustment errors. Always inspect the crank interface.

How to Know You Got It Right

After proper adjustment:

  • The crankset spins smoothly with minimal resistance.
  • There is no lateral play or knocking noise under pedal load.
  • There is no clicking or creaking from the bottom bracket area.
  • The crank arms stay aligned and secure over time.

These signs confirm that the “limit screw” equivalents—preload cap and torque settings—are optimized.

When to Readjust the SM-BB93

Though relatively low-maintenance, the SM-BB93 can shift slightly over time. Readjustments may be necessary if:

  • You experience creaking or ticking noises.
  • There is noticeable side-to-side movement in the crank arms.
  • The crankset doesn’t spin freely after cleaning or storage.
  • You install a new crank arm or change pedal configurations.

Periodic inspection and adjustment prevent long-term wear and preserve ride quality.

Conclusion

Correct “limit screw” adjustments on the Shimano SM-BB93 bottom bracket are about fine-tuning crank preload and torque—crucial elements that affect drivetrain efficiency and component longevity. While it lacks literal limit screws, treating the preload cap and crank arm bolts with the same attention ensures a secure and high-performing bottom bracket setup. With the right tools and technique, even home mechanics can achieve professional-level [paid link] results.