19-01-2019, 01:29 PM
I had always assumed that oil passing the rings and getting into a cylinder would be completely burned. However I once had to lift the head off an engine that had only done 50 miles after a rebuild, for reasons not relevant here.
The appearance was very similar to your photos, with sticky black deposits. I suspect that some oil and the heavier fractions of fuel soaks into the carbon deposits on the piston crown etc and the heatsink effect of the metal means that it never quite gets hot enough to burn off. This would be exacerbated by gentle usage during the running in period. The plug electrodes and exhaust valve heads run rather hotter and less prone to this.
If it's any consolation that same engine has now gone on to cover 10,000 trouble free miles, with oil consumption steadily falling as the rings bedded in. The ring bedding took some considerable mileage, and with hindsight I would have stuck to running in oil for rather longer before switching to an ordinary type. As others have recommended in the past, once run in get your engine properly hot and go for a good long fast run from time to time rather than always pootling !
The appearance was very similar to your photos, with sticky black deposits. I suspect that some oil and the heavier fractions of fuel soaks into the carbon deposits on the piston crown etc and the heatsink effect of the metal means that it never quite gets hot enough to burn off. This would be exacerbated by gentle usage during the running in period. The plug electrodes and exhaust valve heads run rather hotter and less prone to this.
If it's any consolation that same engine has now gone on to cover 10,000 trouble free miles, with oil consumption steadily falling as the rings bedded in. The ring bedding took some considerable mileage, and with hindsight I would have stuck to running in oil for rather longer before switching to an ordinary type. As others have recommended in the past, once run in get your engine properly hot and go for a good long fast run from time to time rather than always pootling !