i have a double roller timing chain set for my 350 chevy (PO put it on so i have no directions and don't remeber how it was when i took it off... i know shame on me) the crankshaft gear has 3 key ways in it each with the own mark O, A, R, and if you count 4 teeth counterclockwise from each key way theres the matching mark (ex. count 4 teeth counterclockwise from R key way theres the R mark) and the gear also has 2 DEG stamped in it i can draw picture if needed but don't have the time at the moment
Nate
