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.

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