Joined: Feb 2012
Posts: 19,353
Likes: 5,471
From: Rochester, NY
Bikes: Stewart S&S coupled sport tourer, Stewart Sunday light, Stewart Commuting, Stewart Touring, Co Motion Tandem, Stewart 3-Spd, Stewart Track, Fuji Finest, Mongoose Tomac ATB, GT Bravado ATB, JCP Folder, Stewart 650B ATB
Several guide lines are in play. 2 1/2", insert so that the expander in below any threaded section (of the steerer), the more insertion the better. There are other guides. But I think you get the idea. If the stem expander is within the top threaded section it can cause a crack that will break off the top of your steerer. This is a problem on bikes that have had replacement forks installed that end up with most of the threaded section NOT cut down, due to the length of the head tube and headset.
So how far is your stem's insertion? How long is the threaded portion of the steerer? Andy