I've been told 2 stories about cutting the steerer tube of a Cannondale:

1. The included FSA compression plug/cap assembly should be completely flush with the top of the stem (tube cut 3mm below the top of the stem.

2. There should be a 5mm spacer on top.

I can't find it in my warranty book, any information on either of these stories?

If you check, the vast majority of fork manufacturers recommend spacers above the stem, to improve the clamping area and prevent pinching the top of steerer. Until very recently, it was absolutely common place for pro bikes to have a 5mm on top.

That said, it's very common now for the steerers to be cut a few mm below (but no lower that the centre of the top clamp bolt at steerer) to allow a cleaner look. If the Cannondale plug is the FSA type expander I'd have no issue cutting it slightly lower to allow a flush top cap, PROVIDED the top bolt centreline is below top of fork (actual steerer, not including the insert.

There are also a number of different top caps that have less insertion or are dead flat, making it easier to mount fork in this way without having to cut lower to allow the cap not to bottom on the insert. The older style Edge caps (use a 4mm chamfered bolt) are good for this and the logo's can be removed to plan carbon in 4s with some light sand paper.

I'm not worried about looks, just about my warranty and safety.

In my "owner's Manual Supplement" I got with my 2011 uerpSix says that you should have a maximum stack height of 55mm between the stem and the top of the head tube, including the headset top cap. I would think they would have mentioned it if you should have a 5mm spacer above the stem. Also, if you look on page 8 and look on the illustration underneath the text that says: "INSTALLED CORRECTLY" you'll see they don't show a spacer above the stem.

My carbon steerer tube it cut 3mm under the top of the stem and I use no spacers above the stem.

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Cannondale SuperSix 105 Team Replica: viewtopic.php?f=10&t=85941

I've been running my Cannondale carbon forks with no spacers above the stem for 7 years now with no issue.

It was done this way on my Six-13 and then my System 6 - both have full carbon steer tubes. And the Six-13 came from the shop that way.

As a side note, my wife's Spec. SL3 frameset came with a warning that putting a spacer above the stem WOULD violate the warranty. It kind of sucked because I had measured and cut with plans for a spacer above the stem.

It's covered here - no spacers on top:

http://forums.roadbikereview.com/cannondale/can-i-lower-my-bars-c-dale-slice-integration-system-stem-279919.html

The upper part of the compression assembly fits snugly into the steerer and helps support it against the clamping force of the stem.

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dangerous curves ahead

I ran a 3mm spacer on top of my supersix steerer. But, I now run a top cap right on top of my ritchey bars. The ritchey top cap is designed to fit right on top of ritchey bars, and they recommend no spacers above.

It kind of varies, plus, look at the Evo pro rigs, they typically have the 15mm cone spacer on the headset, then the bars with a 3mm spacer above the stem.