Before opening a gear meter please note that the cover needs to be remove carefully as it is possible to break the gear axels if the cover is removed incorrectly.  A broken axel can not be replaced and a new meter will be required.

When replacing gears in a gear meter note the position of the magnets circled in the picture.  They should be positioned so you can see them when placed in the meter. See picture.  If one or both of the gears are replaced upside down the meter will not measure correctly.  Also note that the gears when placed correctly should both rotate around each other freely.  When not lined up correctly the gears will lock up part way through a rotation and this will cause a meter failure and possible breakage.