Needle roller bearing of approx 14mm dia x 10mm long.
This fits inside the first motion gear on the mainshaft. An absolute must to replace in our opinion as it is the second most weak area of a mini gearbox.
TIP: Polish the small diameter area of the mainshaft to see if the hardening is failing. If this area does fail usually 4th gear crunches and many metal filings spread throughout the engine.