BCD what is it?
The holes on the spider arms used for attaching a chainring can have various dimensions, concerning the bolt circle diameter, commonly abbreviated as BCD. This measurement is sometimes referred to as pitch-circle diameter (PCD). Cranks designed to mount one or two chainrings will almost always use a single bolt circle diameter. Cranks designed to mount three chainrings will almost always use two different bolt circle diameters; the larger one to mount the two outer rings and the smaller one to mount the inner ring. Most modern three-chainring cranks use 104mm bolt circle diameter. In a chainring with 4 bolts the BCD is the distance between two oposite bolts. Check up on the information below which BCD you have in your cranckstet.

Mountain three-chainrings
104 BCD (Shimanao, Race Face, Truvativ and other)

Mountain two-chainrings
88 BCD (Shimano XTR 985 model)

Mountain two-chainrings
120 BCD (Sram XX, X0, X9, X7 w/o removable spider)

Mountain two and single chainrings
94 BCD (Sram new version X7, X9, X01, XX1 with removable spider)

Spiderless (Direct Mount)
It is a new Sram, Race Face and Specialized standard, which occurs in cranksets where you can remove a spider. All these standards have different mount shapes and are not compatible between each other.

All SRAM X7 (new version), X9, XO, XO1, XX1 we offer solution with direct mount chainring.