yes marks can be off. i recently built a 360. used a Mancini roller set and the marks were 4 teeth off. that put my valves 68 degrees adv. till it broke a valve face off and exploded my motor. each tooth on the crank gear is 17 degrees. i am not saying anything bad about Mancini because everybody gets a bad part sometimes. it's weather the make it right or not that counts. at this point they are working with me. so compare yours to other brands to make sure yours is marked right. i will enclose a pic (if i can downsize it enough) to show correctly marked and not.