You would need the shim if it were the other way 'round, that is, an oversize stem with a bar that's too small for it. If that were the case, the allen bolts would "bottom ou" but the stem would not be tight over the bar. As it stands, the stem isn't even all the way 'round the bar, so it can't be too loose/big for it. As has been stated, just tighten the allen bolts some more, until the bar stops rotating in the stem.