HOME and how to join Forum Open Area General Scott topics Decarbonize pistons ,why???? Reply To: Decarbonize pistons ,why????


Difference? Well no, there’s not really, when you really come down to it any piston is just there to bung up the ‘ole its in!

More seriously, with modern lubricant technology coupled with the improvements in the design of oil delivery systems and leaner running engines it is to be expected that carbon formation will be much reduced. Indeed it is more common to see a biscuit colour on the top of a piston rather than black.

A properly made piston has the correct clearances designed in so that when at working temperature it is a proper fit and therefore should not need protecting. In any event insulation of any sort only slows down the flow of heat it never stops it, run any engine for long enough and a heat balance will be reached no matter what or how much insulation there is. In any event the Scott has a pretty low compression ratio by current standards.

There is a school of thought that says slip an old ring into the bore atop the piston and only clean up to that. The thinking it seems is that the carbon formed at the edge of the piston and down to the top ring helps with sealing. I have to say that I am not entirely convinced by this as it is the combustion pressure getting behind the rings and forcing them out against the bore that does most of the sealing. Therefore reduced access for the gasses equals less pressure. As with a lot of such things we have a compromise.

There is a good case for saying that gas flow will be better if all the surfaces it comes into contact with are clean and smooth, I know we’re supposed to love our bikes but surely all that smoothing and polishing where no one can see it is for more than just the pleasure of caressing the brute!

If there is a noticable drop off in performance then a decoke might be in order otherwise “If it ‘aint busted don’t fix it”!

My Moto Rumi Formachino a “mini Scott” in so many ways, would benefit from an occasional decoke when it started to get sluggish. Carbon build has been known to reduce the exhaust port from a designed 1/2″ x 1″ down to a 1/4in hole, but then it did run on petroil SAE30 at 12:1! Good for hiding the number plate from the following fuzz though!