I am at work and do not have my bike here to measure this item. Does anyone know the length of standard chainring bolts and how long they need to be to run a single chainring with a chainring guard on both sides of the chain ring?
OP, if you are using a guide in place of a chainring you can use your same bolts. If you are just running one ring then you need shorter bolts.
You are right Cynikal. A single ring guard with some type of downtube mounted chain retention device would take the normal bolts that a double ring setup does. No guards would take shorter bolts.
Double guards require 15mm bolts, with 3.5mm spacers (as was stated above) so the inner guard allows appropriate clearance for the chain. Also, at least on my setup, the inner guard is dangerously close to rubbing the chainstay. You may need to play with bottom bracket, if you can.