Originally Posted by
cpach
Forks with aluminum or steerers are generally designed for relatively modest maximum spacer stack heights (often around 2-3cm) , though that is taking into account liability protection against failure for the largest, strongest rider riding the roughest terrain expected--obviously, the actual mechanical stresses relate heavily to rider weight, position, technique, and terrain.
The only forks I've seen with recommended spacer stack limitations are carbon steerers which are usually specified as a maximum of 25 mm for a 1" steerer and 40 to 50 mm for a 1-1/8" steerer. I've never seen a published limit for aluminum or steel steerers.
BTW, if you are going to cut the steerer long enough to use a spacer above the stem (which you should), the steerer cut does not have to be accurately flat since the spacer provides the bearing surface for the top cap.