Put the Grinders, Lathes, Dowels and Emery Paper away!

With a .6 mm variance between stem and h-bar you can install the bar easily.

Mount the stem in the bike. Remove the stem clamp bolt. Take a small, thin pry bar or flat stock and insert it into the slot for the clamp. Apply pressure on the pry bar until the sleeve of the h-bar slides through the stem, then release. Apply a dab of grease to the Stem Bolt, adjust to taste and tighten securely.

If some might think this is an archaic method, back in the day Cinelli, Sakae Ringyo (SR), TTT and VAR used to make a tool specifically for this purpose.

Hope this helps!

Alan