Here is a good substitute piston/rings: Hoca Minarelli 47mm piston and rings $30 Why change out the stock one? 2 main reasons: 1) the stock rings are 2mm thick and have a lot of spring tension which wears the cylinder lining quicker than normal. 2) the stock rings have 3 times the normal ring end gap which allows too much blowby which reduces engine efficiency and reduces compression, thus reducing power. With the stock piston for the Dax/Speed-demon engines (the most common, the "low wrist pin" engines) the wrist pin is 21mm from crown edge to upper pin edge, and the piston for the Grubee engines (higher wrist pin version) is 16mm from piston crown edge to wrist pin edge. The Hoca Minarelli Piston has already been tested for this application. It has a 46.8mm diameter and has dual 1mm thick rings. When tested in two cylinders the chrome plating of the cylinder got scraped off by, we assume, the chrome rings so non-chrome rings are needed if the assumption is correct. The Hoca piston from the skirt to crown is 48mm, ours is 47mm. Once installed you need to double check that it doesnt hit the crank wheels at the bottom of the stroke. The 10mm wrist pin on the Hoca Minarelli was reported by the person testing to be 21mm from piston crown but the web site shows it as 20mm from the crown. If 20mm it will lower cylinder pressure which is already too low on the stock engine. You should buy a tester to verify the cranking pressure. Concerning the weight, which affects engine balance and vibration, the Minarelli piston is 112 grams (5 grams heavier than our piston) and the wrist pin is 5 grams heavier than our wrist pins (20g), so a lighter wrist pin such as a titanium one will need to be used although you may have to shorten it a little by grinding down one end. ($19 here) At 1977mopeds.com there is a 47mm non-chrome 1mm thick ring for $15. It is the Motobecane Airsal Piston Ring. When I emailed them the above info they replied: "That ring (Motobecane) is not chrome plated. Normally chrome plated rings are to be used in aluminum lined cylinders. Those motorized bicycle cylinders are normally lined with nikasil (a very hard very thin coating). Uncoated rings like the Motobecane are normally used with cast iron cylinders (uncoated). That is not to say that it will not work and is probably worth a shot. However it might not be the piston rings that are the issue in this situation but it might be a cheap coating on the cylinders that is the cause of the issues." Every China girl cylinder I have owned has lost some cylinder plating so I know that their coating process is not completely correct. But both of the two cylinders tested lost a large portion of chrome during the first tests so we have to suspect that a non-chrome ring needs to be used. If I had a 66cc engine I would definitely try the Minarelli piston and Motobecane rings. Also I recommend using a synthetic or semi-synthetic engine oil for better lubrication and less possibility of cylinder chrome flaking off due to friction.