A while ago the discussion came up about if you burn waste oil you need more space between the crucible and furnace wall to burn oil to get the hottest flame. With propane you can get away with less space between the crucible wall and the furnace wall. With my furnace there is a 75 mm (3") gap to burn waste oil and it works really well. So I watched a video from a former AA member and his furnace has a 25mm (1") gap, so is that too small and what is the best gap people should use ? He melts iron and pours castings so maybe that gap is enough. It is great to see that others are putting their iron melting adventures on youtube. To see the furnace gap have a look at 5:51. If you are interested in iron melting the whole 40 minute video is good viewing.