There are 2 types of reamers. One is SAE ane the other Metric. Which to use depends on the prop shaft.
http://www3.towerhobbies.com/cgi-bin...&I=LXG941&P=ML is for Metric and
http://www3.towerhobbies.com/cgi-bin...?&I=LXG940&P=M is for SAE. You need to determine which is needed for YOUR engine. There are other manufacturers of reamers too of course.