Substitute piston/rings: Hoca Minarelli 47mm piston and 1mm thick rings $30
It comes with 1mm thick rings with a normal ring end gap whereas the stock rings have 3 times the normal gap and allow too much blowby, thereby decreasing engine performance. The stock rings are 2mm thick, and although they create more friction they also spread out the outward pressure of the rings on the cylinder. This helps because the Chinese did a poor job of cylinder plating which easily flakes off. Spreading out the force means it is less likely to cause flaking off of the plating. It also means it will wear down the plating slower. But they made those rings with extra spring tension since they are made for a low compression engine. Once you increase the cranking pressure then the pressure which gets between the top ring and the piston becomes more than is safe for the weak cylinder plating.
So you have three choices: 1) Don't port your engine to rev higher than 7000RPM, don't increase cylinder compression, stick with the stock piston/rings, 2) Port your engine to rev higher than 7000RPM if you want to, increase compression, install a Performance CDI*, use a Minarelli piston and thinner Airsal rings (listed below) that seal better but be prepared to replace the cylinder every year, 3) port your engine and send it off to US Chrome for cylinder replating, increase engine compression, use Minarelli piston and rings.
* The Performance CDI is important because the stock CDI has overly advanced ignition at high RPM which causes the peak cylinder pressure to be too great because it happens closer to TDC than it should. That increased pressure causes more outward ring force on the cylinder. The Performance CDI retards the ignition at high RPM which allows the peak compression to happen around 15 degrees ATDC which is where it should occur. That reduces the peak cylinder pressure but gives the engine more power.
Unfortunately the substitute piston comes with chrome plated rings. Chrome rings are not to be used with chromed cylinders. The person that tried it had cylinder plating flake off the very first time he tried it out. The following is a quote from http://www.dansmc.com/pistons.htm "It seems all the rage now to have sleeveless plated cylinders. Sometimes, it's called Chrome Plating, Nikasil, Electrofusion, Boron Composite, SCEM Composite, Ceramic Composite, or who knows what. Cylinders are bored to the right size and then plated. The pistons use a softer metal than the plating for the rings. The idea being the rings will wear and the cylinder will not. On Chrome Plated cylinders you MUST use cast iron rings and NOT chrome rings. If you use chrome rings on a Chrome Plated cylinder, things do not wear so well. It will start pealing the chrome off. You can use chrome rings on Nikasil, Electrofusion, Boron Composite and some others.".
At 1977mopeds.com there is a 47mm non-chrome 1mm thick ring for $15. It is the Motobecane Airsal Piston Ring. Order two of them to go with the substitute piston if you are not having your cylinder replated at US Chrome.
Latest info: This Vespa piston has 2mm thick rings that should work fine on the China girl piston--> link
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 with the substitute piston and its chromed rings lost a large portion of chrome during the first tests. 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. Mix at 30:1 gas to oil.
Here is info about sizing:
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 from the skirt to crown is 48mm, ours is 47mm. Once installed you need to double check that it doesn't 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 and lathe off the cylinder top or the head so that pressure is around 135psi. 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)
Here is my experience: I have used for my 55cc engine (with 43mm bore) pistons from a Kawasaki KX60 and a Vespa. Both have dual 1mm thick non-chromed rings. I have put many miles on it with these pistons and the cylinder shows streaks of plating loss but it still runs really good although I'm sure it would be a bit better if all the plating was there. But the area available for blowby from this ultra-thin plating not being there (in streaks) is very small. There have been times I have ran it with 150psi cylinder cranking pressure when testing different cylinder heads so maybe that is when the plating loss happened. I say that because higher compression causes higher outward pressure on the rings. How that works is that some of the high cylinder combustion pressure leaks past the top of the first ring to pressurize the area between the piston and ring. That increases the pressure of the ring against the cylinder. But the pressure behind the ring is lessened because of the ring end gap which exposes that inner area to the reduced pressure between the two rings. I theorize that by increasing the ring end gap of the second ring we can reduce the inner pressure pushing the top ring outward. Increasing that gap lessens the pressure between the two rings which affects the top rings inner pressure. That is my current recommendation for anyone attempting this. Also use fine #400 sandpaper to round off the edges of both rings so that they scrape off less oil from the cylinder so that more oil winds up between the rings and cylinder to lubricate better and give less opportunity for plating loss. Also be sure to not let your cranking pressure be more than 135psi. Motocross engines have around 150psi but they have really good cylinder plating. Many aftermarket heads for these engines cause a cranking pressure more than 150psi. If you want to do things completely right then spend $190 to get your cylinder replated at US Chrome and sleep well at night.
US Chrome says that plain chrome plating (such as the Grubee cylinders has) has a temperature limit of 400 degrees Celsius so a higher cylinder temperature due to advanced ignition at high RPM, overly lean fuel-air mixture, or engine compression over 135psi can be a causative factor with cylinder plating flaking. Here is the info posted at the US Chrome site about the plating they can put on cylinders:
NiCom Nickel Silicon Carbide Dispersion Coating
NiCom is an electro-composite coating consisting of a nickel metal matrix with hard particulate dispersed uniformly within it. The process involves electroplating nickel in the presence of hard particulates, such as SiC, under conditions allowing the hard particulate to co-deposit with the nickel. The result is an electroplated composite material where engineering thickness can be achieved for both original equipment manufacturing (OEM) or overhaul and repair.
Electro-composite materials, such as NiCom, are viable alternatives to Hard Chromium Plating due to their excellent wear resistance. Superior sliding wear characteristics are achieved due to the hard particulates dispersed within the metal matrix. Mating components ride on the hard particles, which can vary from SiC to BN to diamond depending on the wear condition application, resulting in wear 5 to 10 times better than Chrome.
Once the NiCom coating has been applied, it can be diamond honed or ground to finish dimension to meet surface finish requirements. Common applications of NiCom are found in bores of internal combustion engines with uses ranging from weed trimmers to snowmobiles to high performance race cars. Most any cylinder bore/piston application where sliding wear resistance is important is a candidate for NiCom electro-composites.
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