A replacement chainring can appear correct on the workbench and still be completely wrong for the crank. The bolt count may match, yet the holes will not align; or it may fit physically but compromise front shifting or chainline. This chainring bolt circle diameter guide explains how to identify BCD accurately before ordering a road, gravel, MTB or track chainring.
Bolt circle diameter, usually abbreviated to BCD, is the diameter of the imaginary circle running through the centres of a chainring’s mounting bolts. It is expressed in millimetres: 110 BCD, 130 BCD and 104 BCD are familiar examples.
BCD is a primary fitment dimension for conventional bolt-on chainrings. If the BCD and bolt layout do not match the crank spider, the chainring will not mount. It also influences the smallest chainring that can be produced, because the bolt circle occupies physical space near the centre of the ring.
It is not, however, a complete compatibility description. A 110 BCD chainring is not automatically interchangeable with every other 110 BCD chainring. Bolt count, bolt spacing, chainline, tooth profile, shift design and the crank manufacturer’s mounting standard still matter.
Do not confuse BCD with chainring tooth count. A 50-tooth and 34-tooth road pair can both use 110 BCD, while a 52/39 combination on a traditional road crank commonly uses 130 BCD. Tooth count determines gearing; BCD determines the mounting geometry.
The cleanest method is to remove the chainring, place it flat and use vernier callipers to measure from the centre of one bolt hole to the centre of another. Measuring from the edge of a hole introduces enough error to make nearby standards difficult to distinguish.
With an even number of evenly spaced bolts, use two opposite holes. The centre-to-centre measurement is the BCD itself. A four-bolt ring measured directly across from one mounting hole to the other at 110 mm is therefore 110 BCD.
Five-bolt chainrings have no directly opposing holes. Measure the centre-to-centre distance between two adjacent holes, then multiply it by 1.701. For example, an adjacent-hole measurement of roughly 64.7 mm indicates 110 BCD. An adjacent measurement of about 76.4 mm indicates 130 BCD.
For reference, the calculation uses the number of equally spaced bolts:
| Bolt pattern | Measure | Multiply by |
|---|---|---|
| 3-bolt, evenly spaced | Adjacent hole centres | 1.155 |
| 4-bolt, evenly spaced | Adjacent hole centres | 1.414 |
| 5-bolt, evenly spaced | Adjacent hole centres | 1.701 |
These calculations only work where the bolt holes are evenly spaced around a true circle. Many current crank systems use asymmetric four-arm layouts, hidden fixing bolts or proprietary interfaces. In those cases, measuring one adjacent pair and applying a standard formula can produce a misleading result.
An old chainring is useful evidence, but it is not definitive. It may be an incorrect part fitted by a previous owner, or it may have been modified for a different drivetrain. Check the crank spider, count the fixing points and inspect whether all holes are equally spaced.
If a chainring is still fitted, remove the chain and rotate the crank to access each fixing point. A clear photo of the crank’s rear face, its model marking and the mounting pattern is often as useful as a measurement when confirming an order through a workshop or specialist supplier.
The following dimensions are common reference points, rather than a complete list of every historic or proprietary design.
| BCD and layout | Typical application | Fitment considerations |
|---|---|---|
| 130 mm, 5-bolt | Traditional road double cranks | Usually suited to standard road chainring sizes; a 34T compact inner ring will not fit. |
| 110 mm, 5-bolt | Compact road and older gravel cranks | Widely used for 50/34, 52/36 and similar combinations. |
| 110 mm, 4-arm | Modern road cranksets | Often asymmetric or manufacturer-specific despite the nominal 110 mm figure. |
| 104 mm, 4-bolt | Legacy MTB and some trekking cranks | Common on older multi-ring MTB systems; assess chainline and ring position. |
| 96 mm, 4-arm | Shimano MTB cranksets | Typically asymmetric, so dedicated chainrings are required. |
| 144 mm, 5-bolt | Track cranks | Usually paired with 1/8-inch or track-specific chainring systems. |
Older MTB cranks may also use 94 mm, 64 mm or mixed outer-and-inner BCD arrangements. A triple crankset can have a larger outer bolt circle and a smaller inner circle for the granny ring. Treat each mounting position separately rather than assuming one measurement applies to all rings.
The most common compatibility mistake is treating a nominal BCD as the entire specification. It is only one dimension in a wider system.
First, confirm the bolt count and layout. A five-bolt 110 BCD ring cannot fit a four-arm 110 BCD crank. More subtly, two four-bolt cranks can share a nominal diameter but use different arm shapes, timing positions or hidden-bolt arrangements.
Next, identify whether the ring is designed as an outer, inner or single-ring application. On a double crankset, the outer ring normally carries shift ramps, pins and locating features engineered to work with a specific tooth difference and front derailleur. Reversing ring positions or fitting a generic single-speed ring may sacrifice shift quality, even when every bolt aligns.
Drivetrain speed and chain profile also require attention. Modern 11-, 12- and 13-speed chains have different outer widths and tooth engagement requirements. Narrow-wide chainrings for 1x use have alternating tooth profiles intended to retain the chain, while 2x rings are designed to release and receive the chain during shifts. They serve different jobs.
Chainline is equally relevant, particularly for gravel, MTB and power-meter installations. A chainring can mount to the spider but sit too far inboard or outboard for the cassette and frame. This increases cross-chaining, noise and wear, and can create clearance issues with a chainstay. Offset is especially critical on direct-mount systems.
Many current MTB, gravel and road cranksets use direct-mount chainrings. Instead of bolting a ring to a spider on a defined bolt circle, the ring attaches directly to the crank with a splined, threaded or proprietary interface.
A three-bolt interface should not be described as a three-bolt BCD. The bolts retain the ring, but the interface geometry is determined by the crank manufacturer. SRAM-style three-bolt, Shimano direct mount, Race Face CINCH and other systems need a chainring made specifically for that interface, plus the correct offset.
Before selecting a direct-mount ring, identify the crank model, interface generation, required chainline and intended chain standard. The number of bolts alone is not a purchasing specification.
A smaller BCD generally permits a smaller chainring because there is more room between the bolt circle and the teeth. This is why compact 110 BCD road cranks can run small inner rings suited to climbing, while 130 BCD road cranks are associated with larger standard combinations.
There are practical limits, not universal rules. A 110 BCD five-bolt crank is commonly used with 34T inner rings, while 130 BCD generally starts around 38T or 39T. Tooth shape, bolt recesses, chainring thickness and the manufacturer’s design all affect the minimum viable size. Confirm the stated size range for the specific crank and chainring rather than relying solely on a BCD chart.
For workshops and serious riders, the efficient process is to record the crank model, bolt layout, BCD, ring position, drivetrain speed and desired tooth count before searching a catalogue. That specification prevents most returns and keeps the discussion focused on the right options, whether selecting a Praxis replacement chainring or another premium drivetrain solution available through DSG PRO Imports.
A careful measurement takes minutes. Replacing an incompatible ring after it has marked a crank, delayed a build or left a customer without their bike takes considerably longer.