the main reason you see pistons with scratch and burn marks on new engines is a result in a quick that will do run in period. sure the newer model zenoahs now have a single ring piston were they used to have 2 piston rings , so naturally the run in period could be said to be double that of a single ring. it is wise to make up a batch of run in fuel which should contain abit more oil than what you will use after run in. my fuel mix is normally 50 mls per litre of petrol / gas and my run in fuel is 70 ml per litre. , i run 2 litres [ 2000mls ] of run in mixed fuel with also the main and secondary needles 1/4 turn rich, maximum throttle is just over half but never full . so for a longer lasting engine it is a good idea to run your engine in rich and slow for a few tank fulls then its ready for a long and speedy life. don,t expect to run in your engine in one day at the lake unless you have time to spare as a cooling down period is also needed before re starting plus all bolts etc should be checked for tension plus your shaft will require re oil / greasing often. mistycreek mentioned half a bag of fuel is enough but sorry i don,t think thats really enough for good seating of internal componants like the piston / ring unless the carbie is still set rich.