PCD (Pitch Circle Diameter) Defined

PCD, or Pitch Circle Diameter, is the diameter of an imaginary circle drawn through the fixing holes of a chainring. Since it's not possible to measure this directly, you can instead measure the distance between the centres of consecutive fixing holes and convert to PCD using this table:
Distance between fixing holesPCD
34.1mm58mm
43.6mm74mm
50.6mm86mm
55.3mm94mm
64.7mm110mm
71.7mm122mm
76.5mm130mm
79.4mm135mm
84.7mm144mm
Chris Juden of the CTC offers this additional information in the June-July edition of Cycle:
Chainrings are sold by number of bolts and PCD or BCD, which is the same thing and stands for Pitch Circle Diameter or Bolt Circle Diameter. Easier than diameter to measure is the centre-to-centre distance between two adjacent bolts. That can then be multiplied by the appropriate geometrical factor to get the PCD: in the case of 4 bolts x1.414 or 5 bolts x1.701.


4 Comments

by Ken Hawkins on 07 September 2019
The word Pitch in engineering is used to describe the distance a screw thread advances in 1 revolution. The term BCD, when used by manufacturers such as Campagnolo is the diameter of a circle which dissects the centres of the bolts of the crank arm. The distance between adjacent bolts has no relevance, and the word pitch has no relevance
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by likeybikey on 15 August 2020
@Ken Hawkins

Maybe Campy calls it BCD, but Shimano calls it PCD. The only reason I found this page is because I wanted to make sure I understood what Shimano was talking about. https://bike.shimano.com/en-EU/product/component/slx-m7100/FC-MT610.html

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by santosh kumar singh on 15 May 2008
I think the definition could be more generalised.
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by bob on 11 January 2008
this is really hard :@
Reply
by Deepak on 05 June 2007
The PCD - distance co-relation is for 5 equispaced circles.
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