These are 5mm O-rings designed to be used in many of Tamiya’s oil-filled sealed gear differentials like the ones’ found on the TA-05 option sealed gear set as well as the included sets found on the TA-06, XV-01 and TRF417 series of cars. The inner diameter of the O-ring is slightly smaller than the standard kit black O-ring. The smaller inner diameter O-ring better prevents oil from leaking out of the gear differential.