Stem seals can give a big puff when starting first thing, and also at idle when there is enough vacum to pull the oil thru the seals

They're only shrapnel each, make sure you get the new 'top hat' shaped ones as they cant come loose

SP do a tool to let you change them without taking the head off - buy one from helfords, then clean it up and take it back for a refund when yer done

AND DONT DROP ANY COLLETS DOWN INTO THE SUMP!!!!!

Neil.