I have a bar and stem almost identical. This design was supposed to beat the fact that carbon-bar/stem combinations invariably break at the entrance to the stem and this provides a safety margin. But you cannot tighten carbon bars into carbon stems enough to keep them from rotating under heavy load. No carbon paste will improve it enough to matter.

Since it is possible to have a bar and stem combo of aluminum that weighs the same or less, that's the way I returned.