After doing a pretty much top to bottom rebuild on my 87Xr250 I struck a slight problem. After starting up from cold (after the bike has been sitting a day or two) I get white smoke for a minute or so till it warms up. About the only thing I didn't do was fit new valve guides (I fitted new kibblewhite valves and springs) Since the valve guide stem seals were replaced I'm thinking it is either a worn guide or one of the O-rings seals on the EX guides have burnt out letting oil dribble through to the cylinder (as per pic below which I stole from an old thread on here). I am thinking to fix this I need to 1. buy a new head gasket, oring seals, stem seals, valve guides. 2. replace the old guides/seals 3. reassemble My questions are: If I buy new valve guides what diameter do I buy? I see in the kibblewhite catalogue above there are a range of diameters avrying in 0.001 inch increments (dimension G). I am pretty sure the guides in the bike are stock and are not loose as I recall. I know it is not wise to lap factory honda valves as you can wear off the thin hardened coating on the back of the valves. When the bike was rebuilt it has a 3 angle job but I am guessing it would be wise to give it a quick lap before I reassemble it? Is it OK to lap the kibblewhite black diamond valves?