Bl$$dy hell!... the got the 33mm all the way in the port!? On the head in the guide I think a long shank burr... but these days I use a course big flute burr... it chops through aluminium like you wouldnt believe!... use them with caution! You can infact thin down the boss to not much thicker than the guide for very little flow difference.. it's just easier to chop it out.