What is BCD (Bolt Circle Diameter)?

BCD is the diameter of the circle that passes through the center of each chainring bolt hole. It determines which chainrings will fit your crankset. Common BCD measurements include 110mm (Shimano compact), 130mm (standard road), 135mm (Campagnolo older), and 64mm (track single rings).

How do I measure my chainring BCD?

For 4-bolt chainrings, measure from the center of one bolt to the center of the opposite bolt. For 5-bolt chainrings, measure from the center of one bolt to the center of the second bolt (not opposite) and use a BCD chart to calculate the actual measurement.

Which brands make small chainrings for junior compliance?

Brands that make individual chainrings in unusual sizes include Miche, TA (Specialities), Stronglight, and Taillens. These specialist European suppliers often require international shipping and may have import duties.

By Cycling Team UK · Updated: 2026-04-16

A Guide to BCD and Chainring Compatibility

You've found the perfect chainring size to get your child's rollout under the limit. You order it, it arrives, and it doesn't fit. The bolt holes are in the wrong place. Welcome to the world of BCD.

What Is BCD?

BCD stands for Bolt Circle Diameter. It's the diameter of the imaginary circle that passes through the centre of each chainring bolt. It's measured in millimetres and determines which chainrings fit which cranks.

Think of it like a light bulb fitting — a bayonet bulb won't fit an Edison screw socket, even if they're the same wattage. A 110mm BCD chainring won't fit a 130mm BCD chainset, even if the tooth count is right.

Common BCD Standards

BCD sizes you'll encounter

110mm (compact) — the most common modern road standard. Used by Shimano (4-bolt and 5-bolt), SRAM, and many aftermarket brands. Supports chainrings from about 33T upwards.
130mm (standard) — older road standard, still found on some bikes. Shimano, Campagnolo, and SRAM all used this. Minimum chainring size is typically 38T-39T.
104mm — mountain bike standard. If your child rides an MTB crank, this is likely the BCD. Good availability of smaller chainrings.
144mm — track standard. Used on most track chainsets. Chainrings available in a wide range of sizes from track specialists.
Direct mount — some modern chainsets (SRAM DUB, Shimano Hollowtech) use proprietary direct-mount interfaces instead of bolts. Chainring choice is limited to that manufacturer's offerings.

How to Measure Your BCD

If you're not sure what BCD your child's chainset uses, you can measure it:

4-bolt chainsets

Measure the distance between the centres of two adjacent bolts (not diagonal). Multiply by 1.414 (the square root of 2). That gives you the BCD.

5-bolt chainsets

Measure the distance between the centres of two adjacent bolts. Multiply by 1.701. That's your BCD.

The easy way

Look up your chainset model online. The manufacturer's specs will list the BCD. Search for "[chainset model] BCD" and you'll usually find the answer immediately.

The Minimum Chainring Size Problem

Each BCD has a minimum chainring size — go smaller and the teeth would overlap the bolt holes. This is often the real constraint for youth gearing:

130mm BCD: minimum ~38T — too big for many youth categories
110mm BCD: minimum ~33T — works for most youth categories
104mm BCD: minimum ~30T — works for all youth categories

If your child's bike has a 130mm BCD chainset and you need a 34T chainring, you're out of luck — you'll need a new chainset with a smaller BCD. This is one of the most frustrating (and expensive) discoveries parents make.

Bolt Count Matters Too

Even within the same BCD, chainsets can have 4 or 5 bolts. A 5-bolt 110mm chainring won't fit a 4-bolt 110mm chainset. Always check both the BCD and the number of bolts before ordering.

Modern Shimano road chainsets (105, Ultegra, Dura-Ace from 2015 onwards) use a 4-bolt 110mm BCD. Older models and some other brands use 5-bolt. SRAM road chainsets have moved to direct mount on newer models.

Where to Find Compliant Chainrings

Once you know your BCD and bolt count, you need a chainring in the right tooth count for compliance. The main suppliers for unusual sizes:

Miche — Italian, good range of 110mm and 130mm chainrings in odd sizes. Available from specialist UK cycle shops and European suppliers.
TA (Specialities) — French, excellent range including very small sizes. Often the go-to for youth racing. Can be ordered from French bike shops.
Stronglight — French, similar range to TA. Good availability in Europe.
Rotor — Spanish, direct-mount and bolt-on options. Higher-end but good quality.

Tips for Getting It Right

Identify your BCD and bolt count before shopping for chainrings
Check the minimum chainring size for your BCD — make sure the tooth count you need is physically possible
If your BCD won't accommodate the chainring size you need, budget for a new chainset rather than trying to force a mismatch
When buying from European suppliers, double-check the BCD is listed in the product description — don't assume from the photo
Keep the old chainring bolts as spares — they're easy to lose and annoying to replace