Will a Mid-Drive Kit Fit a 68mm or 73mm Bottom Bracket? Measure Before Ordering

PRE-PURCHASE FIT MEASURING GUIDE + CHECKLIST UK + US

The Quick Answer

A 68 mm or 73 mm bottom bracket can take a mid-drive kit, but width alone does not prove it. Most aftermarket mid-drives, including KirbEbike’s Z16 1000W and TSDZ8 500W/750W, are built for threaded BSA/English shells. Check five more things: that the shell is threaded, that its bore is about 33.7 mm, that nothing blocks the motor, that the motor and chainring clear the frame, and that the chainline lets you use your gears.

Will a 68mm or 73mm Bottom Bracket Fit a Mid-Drive Kit?

Usually, if it is a threaded BSA/English shell with nothing in the motor’s way. The 68–73 mm figure is the shell’s width; a mid-drive also needs the right bore and thread, clear space around the shell, and a chainline your gears can live with.

68mm vs 73mm Bottom Bracket: What Actually Changes?

Shell width is the frame shell itself, measured from outer face to outer face. It is not the crank spindle length and not the distance across installed cups.

Thread and bore per Park Tool; widths per Sheldon Brown. The motor’s own instructions decide the spacer arrangement.

Why “68–73mm Compatible” Does Not Guarantee Fit

Because other shells share those widths. BB30 and PF30 press-fit shells are also made 68 or 73 mm wide, but they have smooth bores of about 42 mm and 46 mm instead of a threaded 33.7 mm one (Sheldon Brown, Park Tool). And even on the right shell, the motor can hit the chainstay or down tube, a cable guide can sit in its path, or the chainring can end up too far out to reach the largest rear cogs cleanly.

How to Measure Your Bottom Bracket Before Ordering

Step 1: Measure the Shell Width Face to Face

Remove the cranks and bottom bracket first, so you measure the frame shell rather than cups or spacers. Use a digital caliper across the two outer faces. About 68 mm or about 73 mm: continue. Any other width (83, 100 or 120 mm, for example): stop and check the exact motor version.

Step 2: Identify Threaded BSA vs Press-Fit

Look inside the empty shell. A BSA/English shell has internal threads; a press-fit shell has a smooth bore (Park Tool). Thread-together cups can make a press-fit frame look threaded from outside, so judge the frame shell, not the installed bearings. On an English shell the drive-side cup uses a left-hand thread, as Park Tool’s English tap and facing set shows (Park Tool). Italian shells use a different 36 mm × 24 TPI thread, a 35 mm bore and are usually 70 mm wide (Sheldon Brown).

Step 3: Measure the Internal Bore

A BSA/English shell has an inside diameter of about 33.7 mm (Park Tool). KirbEbike’s installation drawing for the Z16 shows the part that passes through the shell at Ø33.2 mm, so a standard BSA shell leaves only about 0.5 mm on the diameter. A bore of about 41 mm or more points to a press-fit or T47 shell; an Italian shell is about 35 mm.

Step 4: Inspect for Obstructions Around Shell

Check under and around the shell for cable guides, weld beads, bosses, internal sleeves, suspension pivots, kickstand plates and anything that is out-of-shape in the down tubes or chainstays. The motor housing should be able to lie flat against the frame.

Step 5: Check Motor Housing and Chainstay Clearance

The motor body should swing up into position without being forced against the frame. Then check the chainring against the chainstay on its own: the housing can clear while the ring still rubs. KirbEbike’s side-view drawings measure about 225 × 145 mm for the Z16 and 204 × 126 mm for the TSDZ8, which helps you judge the space under the down tube.

Step 6: Check Chainline Before Calling It Compatible

Chainline is the sideways distance from the frame’s centreline to the middle of the chainring. If the ring sits too far out, the chain runs at a steep angle to the largest rear cogs, which causes noise, poor shifting, dropped chains and faster wear. There is no single perfect number: measure it after fitting and test every gear.

68mm vs 73mm Mid-Drive Installation: Spacers and Hardware

What Spacers Actually Do

Spacers take up the difference between the shell width and the motor’s mounting length, or move the motor out to clear the chainstay. Luna Cycle, for example, notes that a BBSHD built for 100 mm fat-bike shells needs extra spacers on a 73 mm shell. Each motor has its own arrangement, so follow its instructions.

Why a 73mm Shell May Need a Different Spacer Stack

On KirbEbike’s Z16 drawing the chainline is 47 mm on a 68 mm shell and 49.5 mm on a 73 mm shell, because the wider shell pushes the drive side out; the TSDZ8 drawing gives 49.5 mm and 55 mm. The fixing plate and lockring position also change with width. Use the kit’s own parts; do not improvise with washers or clamps.

Direct Fit vs Spacer Fit

Threaded vs Press-Fit Bottom Brackets for Mid-Drive Conversions

BSA / English Threaded: The Straightforward Case

British and ISO threaded shells use 1.370 in or 1.375 in × 24 TPI threads in 68 mm and 73 mm widths (Sheldon Brown). This is the shell both KirbEbike mid-drives are built for. “Straightforward” still means checking the motor body, chainring, cables and battery space.

Press-Fit: When an Adapter May Work

BB86/BB92 (PF41), BB30 (PF42) and PF30 (PF46) are different standards, and width alone cannot tell them apart (Park Tool). An adapter is only valid when both the frame standard and the chosen motor support that adapter path. KirbEbike lists both of its mid-drives for 68/73 mm threaded shells and does not list a press-fit option, so a press-fit frame needs a specialist assessment before you order.

When a Workshop Assessment Is Sensible

Have a bike shop inspect the frame if the shell standard is not clear, the frame is carbon, an adapter would be required, the frame's threads are damaged or the shell's faces are not even, the internal cables pass over the motor path, or the suspension pivots are in the way of the shell. It is also a good idea if you don't have crank and bottom-bracket tools or if you can't measure chainline after installing.

Chainline vs Chainstay Clearance: The Fit Check Buyers Miss

Physical Fit vs Drivetrain Fit

Physical fit means the motor passes through the shell and tightens against the frame. Drivetrain fit means the finished bike shifts cleanly across the cassette. One KirbEbike Z16 owner fitted kits to two 12-speed trail bikes and could not use the lowest two or three gears: the motor fitted, the drivetrain did not fully.

Smaller vs More Offset Chainrings

A smaller chainring sits further from the chainstay, which can solve rubbing; a different ring offset moves the chain in or out. KirbEbike sells a 38T aluminium chainring for the TSDZ8 (£40; owners describe the stock ring as 44T), and one owner reports it gave a good chainline on a Trek mountain bike.

What to Record on a Test Installation

Frame make, model and year; shell width, type and bore; spacer stack; chainring size; chainring-to-chainstay gap; final chainline; the lowest rear gear that shifts cleanly; and any cable you had to reroute.

KirbEbike Mid-Drive Fit: TSDZ8 vs Z16

From KirbEbike’s mid-motor kit page (Z16 and TSDZ8 options), its installation drawings and the mid-motor collection, November 2026.

TSDZ8 500W/750W

The TSDZ8 option (500W or 750W) is drawn for 68 or 73 mm threaded shells, with a 49.5 / 55 mm chainline and 170 mm overall width. Owners report fitting it to bikes such as a Giant Talon mountain bike, and one notes that some frames need extra spacers for a perfect fit.

Z16 48V 1000W

The Z16 is listed for 68/73 mm threaded shells on steel or aluminium frames, but do not assume the same hardware or chainline as the TSDZ8: its drawing gives 47 / 49.5 mm. KirbEbike asks buyers to check frame clearance and battery space, and at 6 kg and about 225 × 145 mm in side view it is the larger motor of the two.

68–73mm Mid-Drive Alternatives Compared

Each maker’s or retailer’s published fit data, November 2026. Verify the exact version before ordering.

Mid-Drive vs Hub Motor When Your Bottom Bracket Is a Poor Fit

Mid-Drive vs Hub Motor: Fit Requirements

When a Hub Kit Is the Cleaner Conversion

On a press-fit, carbon or heavily obstructed frame, a hub kit avoids the bottom bracket entirely and needs no adapters or frame changes. See our rear hub vs mid-drive guide and conversion kits with battery.

Printable 68–73mm Bottom Bracket Fit Checklist

Print this page or save it, then tick each line with the cranks and bottom bracket removed.

One “No” means stop and verify. For battery space, use our battery frame-fit guide.

What to Send Support If You Are Still Unsure

• One full side photo of the bike
• Close photos of both sides and the underside of the bottom bracket
• A caliper photo showing the shell width
• A photo of the inside of the empty shell, if you can
• Frame make, model, year and size
• Your current crank and bottom bracket, if known
• The motor kit and chainring size you want
• A photo of the chainstay area and any cable guides or pivots near the shell

Send these to KirbEbike support. Do not file, grind, clamp or modify the frame to force a motor to fit; if fit is uncertain, verify before ordering or use a workshop.

Conclusion: Width Is the First Pass, Not the Answer

A 68 mm or 73 mm measurement tells you the motor might fit. A safe order needs the rest confirmed: a threaded BSA shell, a bore of about 33.7 mm, nothing in the motor’s path, clearance for the motor and chainring, a chainline your gears accept, and the right hardware for your width.

Measure, compare the figures with the exact motor’s fit data, and send photos if anything is uncertain.

Frequently Asked Questions

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