Bottom Bracket Size Guide: How to Measure for a Mid-Drive Kit (With Photos)

If you’re planning to install a mid-drive kit, the most important step is confirming your bottom bracket (BB) size. In most cases you only need two measurements: BB shell width + BB type/thread.

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1) What you need (tools)

Tape measure or ruler

Phone camera (for a clear BB photo)

Optional: caliper (more accurate)

Tip: If you’re not sure, just send a clear photo of the BB area and we’ll confirm it for you.

2) Step 1 — Find the BB shell

The BB shell is the frame area where the crankset passes through (center of the bike). If you can remove the crank, measurement will be easier — but it’s not always necessary.

3) Step 2 — Measure BB shell width (most common sizes)

Measure the frame width across the BB shell (not the crank).

Common widths you may see:

68mm / 73mm (very common on MTB/hybrid)

83mm (some eMTB / heavier frames)

100mm / 120mm (often on fat bikes)

✅ If you get 68–83mm, you’re in the most standard range for many mid-drive kits.

4) Step 3 — Identify BB type (threaded vs press-fit)

A) Threaded BB (BSA/English thread)

Usually has visible threads inside the frame

Often marked as BSA or “English thread”

Very common and easiest for mid-drive installs

B) Press-fit BB (BB86/BB92 etc.)

Smooth inner surface, no threads

Bearings are pressed into the frame

Still possible to convert, but may need adapters

Fast check: If you can see metal cups pressed into a smooth shell → likely press-fit. If you see threads in the frame → likely BSA/threaded.

5) The 3 photos we need to confirm your fit

If you want us to recommend the correct kit fast, send:

A clear photo of the BB area (both sides if possible)

A photo showing the BB width measurement

Your bike brand/model + wheel size

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6) Common mistakes (avoid these)

Measuring the crank arms instead of the frame shell

Measuring with the chainring still blocking the view

Not showing the ruler “start point” in photos

Assuming all bikes are BSA (many modern frames are press-fit)

7) Quick recommendation (most riders)

Commuting / delivery riders: mid-drive + higher capacity battery (48V/52V)

Hills / heavier loads: choose the setup with higher torque and the right battery capacity

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