If you are serious about stock racing then this is a must.
http://www.trinityrc.net/shop/index....product_id=695
This is one of the only ways to set up two motors identically. The markings on the can are great for a reference point but they can vary quite a bit from motor to motor. I can tell you first hand that none of the D4's I own pull the same amp draw at 50 degrees timing. This is due to differences in build tolerances. This is a great tool to have in your arsenal.
I believe there is a 20% off sale with code 112814 on the site right now if you spend over $100. Grab them while you can.