Got some honing " stones" which I use in all guides that I make for lots of cylinderheads. Some of them contain diamond pieces that are glued/sintered in metal, others are made with siliciumcarbide. I hone them before inserting in the head to slide just over the valve stem. Depending on the ratio between outer diameter and valve stem, many valve guides shrink where they are sitting in the head as the hot head contracts the guide a bit when cooling down so they end up a bit like an hourglass shape. (DonĀ“t overheat the aluminium heads - above 200 centigrades the aluminium loses a lot of strength)So far only the Norton 850 with the silly 16mm guides does not contract measurably.
Once the head has cooled down completely a quick session on the sunnen hone brings the perfect shape and correct diameter back. I tried (expensive) reamers many years ago but found several problems that put me off: good bronce guide materials are quite difficult to ream properly, valve stem diameters vary in each batch that I can buy - sometimes more than 3/100 mm, and finally I repair so many different heads that I would have to buy dozens of reamers to have the correct size at hand.
cheers, Harty