Theory American Bottom bracket

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Your Theory bikes American Bottom bracket to mid Bearing kit is made for any frame that has an American bottom bracket and installing cranks with a 19mm, 22mm or 24mm spindle. This kit pops in any American Bottom bracket shell and allows you to run a 19mm, 22mm, or 24mm sealed bearings. Most American BB frames have unsealed bearings and 1 piece cranks. So for anyone who is upgrading from the 1 to 3 piece cranks, this is the perfect starting point. Just select bearings that match your spindle size and you'll be all set. Pick up a set today to complete your crank system and get rolling. 

Upgrade an older American bottom bracket BMX frame to a modern 3 piece crank setup with the Theory American Bottom Bracket.

Designed for BMX frames with the larger American BB shell, this conversion kit uses press in American cups to hold modern sealed Mid style bearings. It's available for 19mm and 22mm BMX crank spindles, making it an excellent solution for riders upgrading from an older one piece crank or loose ball bottom bracket to a modern 3 piece BMX crankset.

The complete Theory American Bottom Bracket includes precision sealed bearings, American BB cups, an internal spindle spacer, dust/cone spacers, and adjustment spacers to help properly position your cranks and dial in chain alignment.

This setup is especially useful for old school BMX restorations, entry level BMX upgrades, Big BMX builds, and frames that still use an American bottom bracket shell but need compatibility with modern BMX crank systems.

Instead of replacing a perfectly good frame simply because it has an older American BB shell, convert it.

Old school American BB shell. Modern sealed bearings. Modern 3 piece BMX cranks.

That's exactly what the Theory American Bottom Bracket is designed to accomplish.

Product Use

The Theory American Bottom Bracket is used to convert a BMX frame with an American bottom bracket shell to run modern sealed bearings and compatible 3 piece BMX cranks.

It's ideal for:

Converting from one piece cranks to 3 piece cranks

Replacing loose ball American bottom bracket bearings

Old school BMX restorations

Entry level BMX upgrades

Big BMX upgrades

Wheelie bike builds

Custom BMX builds

Street BMX bikes

Park BMX bikes

Cruiser BMX builds

Installing 19mm 3 piece BMX cranks

Installing 22mm 3 piece BMX cranks

Modernizing an older American BB frame

Why BMX Riders Use the Theory American Bottom Bracket

Convert to 3 Piece BMX Cranks

One of the biggest reasons riders use this kit is to upgrade an American BB frame from an older one piece crank system to a modern 3 piece BMX crankset.

That gives riders considerably more crank choices without requiring a new frame.

Sealed Bearings

Traditional entry level American bottom brackets commonly use loose ball bearing systems.

The Theory setup uses precision sealed bearings, providing a cleaner modern bearing system that's easier to maintain.

19mm and 22mm Options

BMX cranks don't all use the same spindle diameter.

Theory offers versions for 19mm and 22mm BMX crank spindles, allowing riders to select the bearing size that matches their crankset.

Keeps American BB Frames Relevant

Older frames and some BMX bikes still use the large American bottom bracket shell.

This conversion lets riders keep those frames while upgrading the drivetrain around a modern 3 piece crank.

Great for BMX Restorations and Custom Builds

The Theory American Bottom Bracket is particularly useful when rebuilding an older BMX while still wanting the performance and convenience of modern sealed bearings.

Theory word:
American Bottom Bracket full kits in 19mm or 22mm also sold as American cups only

5 Problems You Can Run Into With Other Bottom Brackets

Loose Ball Bearings Requiring More Adjustment

Traditional loose ball American bottom brackets can require additional adjustment and maintenance as components wear or loosen.

Incorrect Spindle Size

Installing a 19mm bottom bracket on a 22mm spindle or vice versa will not work. Bottom bracket bearing size must match the crank spindle diameter.

Poor Fitting Conversion Cups

American to Mid conversions depend on properly fitting cups to securely locate the bearings within the larger American BB shell.

Incorrect Crank Spacing

Poor spacer selection can affect crank positioning and chain alignment.

Buying the Wrong Bottom Bracket Standard

American, Mid, Spanish, and Euro are different BMX bottom bracket standards. A bottom bracket designed for the wrong frame shell will not solve your compatibility problem.

5 Reasons Riders Choose the Theory American Bottom Bracket

Converts American BB Frames to Modern 3 Piece Cranks

Keep your American bottom bracket frame while upgrading the crank and bearing system.

Precision Sealed Bearings

The sealed bearing design provides a modern alternative to traditional loose ball American bottom brackets.

Available in 19mm and 22mm

Choose the version that matches your BMX crank spindle instead of being limited to one size.

Complete Conversion Kit

The complete kit provides the cups, bearings, center spacer, cone/dust spacers, and adjustment spacers required for the conversion.

Great Upgrade for Older BMX Bikes

It's a straightforward way to modernize the drivetrain of an older BMX, Big BMX, cruiser, or custom frame equipped with an American BB shell.

Specs:

Outer size: American

Bearing outer size: Mid

bearing inner diameter: 19mm or 22mm

Material: Aluminum

Complete Kit Includes

2 precision sealed bearings

2 press in American bottom bracket cups

Internal spindle spacer

Drive and non drive side dust/cone spacers

Adjustment spacers/washers

Important: Select the bottom bracket bearing size that matches your crank spindle: 19mm or 22mm.

Still Running an American Bottom Bracket? You Don't Need to Replace Your Frame.

Your BMX may have an older American bottom bracket shell.

That doesn't mean you're stuck with an old loose ball bearing system or one piece crank forever.

The Theory American Bottom Bracket lets you convert a compatible American BB frame to modern sealed bearings and 19mm or 22mm 3-piece BMX cranks.

Keep the frame.

Upgrade the bearings.

Upgrade the cranks.

Build the drivetrain you actually want.

It's an especially useful upgrade for older BMX bikes, Big BMX bikes, cruisers, restoration projects, and entry level frames that came equipped with one piece cranks.

Choose 19mm or 22mm based on your crank spindle, install the appropriate conversion kit, and bring that American BB frame into a modern BMX drivetrain.

Old frame. Modern cranks. One simple conversion.

Why Buy From Stacked BMX Shop?

Stacked BMX Shop specializes in the BMX parts riders need to repair, upgrade, and modify their bikes, including the small compatibility parts that can make the difference between a crankset fitting and sitting unused in your garage.

Not sure whether your bike has an American, Mid, Spanish, or Euro bottom bracket?

Not sure whether your crank spindle is 19mm or 22mm?

Get the fitment figured out before ordering.

Convert your American BB and get your 3 piece BMX cranks rolling.

Shipping: Shipping is based off of your orders weight, dimensions & your location. Ships next business day, excludes weekends & holidays.  More shipping info here.

We accept all major credit cards.

Frequently Asked Questions

Q: What is a Theory American Bottom Bracket?

A: The Theory American Bottom Bracket is a conversion style BMX bottom bracket that uses press in cups inside an American BB shell to accommodate modern sealed bearings for compatible 19mm or 22mm 3 piece BMX cranks.

Q: Can I convert my BMX from a one piece crank to a 3 piece crank?

A: Yes, if your frame uses a compatible American bottom bracket shell. The Theory American Bottom Bracket conversion kit allows many American BB frames to use modern 3 piece BMX cranks when the correct spindle size and compatible components are selected.

Q: Does the Theory American Bottom Bracket come in 19mm and 22mm?

A: Yes. There are 19mm and 22mm versions. Select the size that matches the diameter of your 3 piece crank spindle.

Q: What's the difference between an American and Mid BMX bottom bracket?

A: An American BB frame has a much larger bottom bracket shell and traditionally uses cups pressed into the frame. A Mid BB frame allows Mid bearings to press directly into the frame. The Theory system uses American cups to adapt sealed Mid style bearings to the larger American shell.

Q: Does the Theory American Bottom Bracket use sealed bearings?

A: Yes. The complete kit includes two precision sealed bearings, providing a modern sealed bearing system for your BMX crankset.

Q: What comes with the Theory American Bottom Bracket kit?

A: The complete kit includes two press in American cups, two sealed bearings, an internal spindle spacer, drive and non drive side cone/dust spacers, and adjustment spacers.

Q: How do I know whether I need a 19mm or 22mm bottom bracket?

A: Measure or verify your 3 piece crank spindle diameter. A 19mm spindle requires the 19mm bottom bracket version, while a 22mm spindle requires the 22mm version. Don't choose the size based solely on the bike or frame.

Q: Will the Theory American Bottom Bracket fit every BMX bike?

A: No. Your frame needs a compatible American bottom bracket shell. Frames using Mid, Spanish, or Euro bottom bracket standards require the appropriate bottom bracket for those shell types. You can also remove the bearings form the cups and use just the bearings if you have a mid bottom bracket.

Q: Is the Theory American Bottom Bracket good for old school BMX bikes?

A: It can be an excellent option for compatible old school BMX frames when the goal is to retain the American BB frame while converting to a modern sealed bearing 3 piece crank setup. For historically accurate restorations, however, riders may prefer the original crank and bearing style.

Q: Why convert from a one piece crank to a 3 piece BMX crank?

A: Riders commonly make the conversion to gain access to modern BMX cranksets, sealed bearings, different crank lengths and gearing configurations, and a more contemporary drivetrain setup while continuing to use an American BB frame.