If you are refering to the two set screws then use the square end of a 3/8th in socket.
You can go to the Toro site and get the service manual for most years of 521 models. Get the factory model and serial number off the back base of your machine. If the manual is not available you can go to the site and lookup on a pull down choice all the models and find one close to yours. The 521 ran for many years so had several sets of manuals but for the most part the overall body is standard between the models.
Go below to put in your factory number. If that does not work then use advance search and enter 521 to get a list. You can also view the 38xxxx part of the model list to find 521 models.
https://homeownersolutions.toro.com/portal/server.pt?space=CommunityPage&cached=true&parentname=CommunityPage&parentid=1&in_hi_userid=19462&control=SetCommunity&CommunityID=214&PageID=238#productDetailsPage
This message was modified Oct 18, 2009 by trouts2