Hey guys

Im going to be taking off my TH400's tail housing to replace the output shaft O ring. Im pretty sure theres not one on there right now cause when my step dad and I replaced my rear seal a while back there was an O ring loose on my yoke that just got overlooked and thrown away

Im wondering if any of you guys know where I can find the diameter of that O ring or where i can get one. I went to my local Napa and had them look through a parts catalog. I found the part in the catalot but when the guy went to the back room to get one for me he said it was unavailable so he couldnt measure one for me or order any....Should I just pull that housing off and measure the shaft for the o ring size? Im just kinda cautious cause i dont know if it is just a round O ring or if theres some special design to the o ring that makes it seal in a sertain way. anybody out there have any input on it.