The gear that is attached to motor that drives tape transport and opening/closing of cassette doors. Because it was made from semi-flexible material it also tends to crack during the time.

If you can hear ticking noise and tape transport and/or cassette door do not work correctly then it might be cause by cracked gear. Check service manual of your deck if there is REM0043. You can see it here:

It is marked “Reel Motor” because gear was not provided separately as a spare part but together with motor.
However, you can replace just gear with 3D printed part. You need to use screwdriver as a lever to detach the old gear from shaft and then press new one on the shaft. Disassembly of this type of drive mechanic is slightly complex because you need to use soldering iron to de-solder motor contacts from PCB board. But it is not so complex as it looks like. Just follow instructions in service manual.

The gear REM0043 is used in assembly together with RDG0206 gear that might tend to crack too. It also available as 3D printed replacement.

You can see demonstration of replaced gears at video bellow

The part is compatible with following devices:
RS-BX501, RS-CA1060, RS-CH404, RS-CH909, RS-DC8, RS-HD505, RS-HD505ES, RS-CH510, RS-HD515, RS-HD515PPS, RS-HD75, RS-HDA710, RS-HDA710PPN, RS-TR170, RS-TR270, RS-TR373, RS-TR474, RS-TR575, RX-DT75, RS-TR777, RS-TR97, RS-AZ7

It might work also with other devices – just check your service manual.

Available for purchase – go to shop.