Chosen Solution
I have an early 2013 iMac and the tilt mechanism is no longer functioning properly and will not ‘lock-in’ at any point.
Apple offered an extended warranty on this issue. Basically, a plastic tab which holds a counter spring snapped off so the spring is not counter balancing the weight of the display Apple Offering Refunds to Customers Who Paid for iMac Hinge Repairs. Hopefully its still available. Otherwise, it’s a bit of work as you need to pull the logic board to get to the hinge.
For a jury rigged solution you can use the iMac Service wedge. This will hold it in the back position. If it is to far back for you, carve it down so it’s in the position you desire: [linked product missing or disabled: IF145-224-2]
