Originally Posted by Branko D

If we want to talk about structural integrity, the area where the bolts are must be supported adequately from inside by a compression plug for everything to be ideal. The reason why it's safer to recommend everyone leave a 5mm spacer are compression plugs like this one:


Not ideal

You'll notice the top (black) part of the compression plug doesn't actually expand, so without a spacer on top, the top bolt of the stem will, depending on location, clamp a partially unsupported bit. This type of steerer plug is also why a stack of spacers above is unwise if you haven't checked what sort of compression plug you've got; it's really short, so if you put 20mm of spacers above, the bottom bolt will clamp an unsupported bit. It's not a very good design, but it gets used.

This one is completely fine structurally without a spacer on top, and is longer meaning that with a spacer or two on top you'll still be fine nevertheless:

A much better design.