My psychic powers say your allen head caliper securing bolts are quite possibly 7mm and thus you'll need a 7mm allen key to undo them.
Many allen bit kits exclude the 7mm bit, so you might need to nip to GSF or similar and buy a dedicated allen key with the rubber sheathed handle to undo the bolts, or buy a 'proper' bit kit.
I only really posted this to use the word 'sheathed'