A dipstick assembly includes a lock system. A cap has a blade, cable or extension for measuring fluid levels. The locking system has the ability to limit the assembly of a locking dipstick to one radial position. The cap has an engagement end. A cup has a receiving end. The engagement end of the cap is removably sealably disposable in the receiving end of the cup. The lock system may include a pair of slots on the cap and a pair of lugs on the cup, and a tab on the cap and a shoulder/recess on the cup that cooperate to limit the system to one way assembly. The tab and recess also provide a visible lock indicator to confirm that the slots and lugs, which are not visible, are fully engaged and locked.