Substitute piston/rings for 66cc/80cc China engine (47mm bore)

[HeadSmess]

Because in Australia things are horribly expensive to manufacture - there is no real competition to reduce prices, unlike Asian countries.

Just to plate a Chinese 47mm cylinder bore with Nicasil will cost me $600 at my local plating shop.

^))????

hmm. you cant shout in numerals, so...SIX HUNDRED DOLLARS???? cant remember who i spoke to, but they said 180... that was for a BARE cylinder, but...so maybe they charge lots because of the chrome removal?


anyway...leakage.

the piston is 46.8mm
the bore is 47mm.

the clearance between piston and bore is then 0.1mm

theres 5mm before the first ring... so 5X0.1= 0.5 square millimetres as an orifice before it hits the first ring. (double this because usually the top land has slightly more clearance than the skirt etc...) (edit...theres one major flaw in this part, that i just realised.... circumference. so... erm... (PIx23.5x23.5)-(PIx23.4x23.4) = 1735-1720 = 15 square mm is the area of the gap between piston and bore. thats still only an orifice of 3mmx5mm say...)

the gap, ok...huge, at 1.9mm. cant say i have ever worried about measuring the gap myself. due to piston and bore clearance, maximum area of this gap is still... 1.9mmx0.1mm= 0.19 square millimetres. (so now we have a 15mm orifice restricted down to a 0.2mm orifice)

the gas then has to travel from this gap, through the passage way between the two rings from one locating pin to the other... still restricted to 0.1mm x the distance between rings. say 5mm. so, once again... an orifice of half a square millimetre, approximately an inch long (love mixing my measurements) then another gap in the second ring, of that previously calculated figure... 0.19 square mm.

not terribly large, is it? and the pressure is substantially reduced just by the first land, before it even hits a ring or gap!

i agree, new rings make an engine sit up and behave. the bore should be rehoned, the rings bed in nice and properly, and hey...its just like a new engine again. even if the bore was oversized, meaning the end gap may have been larger than specified when new...

an engine with worn out rings will be hard to start...its revolving rather slowly, so theres a lot of leakage.

but when spun fast enough they still start, and run...albeit, woefully.

usually, by this point...the bore is glazed, the rings arent sealing the same, you get leakage around the whole circumference...



no, i always liked the idea of thin rings, but now i had a good think about it...id prefer the wide dykes ring before using thin rings, if flutter was my major concern. which it is

experience on one or two engines, none of them being full racebred things such as a YZ250, NSR500 or the rotax 125 in a gokart, too mention a few... just a good old HT. like reading "spot goes to the zoo" and then being an expert on pachyderms and dromedaries...

and one more rant!!!!

you tell everyone that the piston and gudgeon pin are too heavy in the first place, use a hollow gudgeon pin, then tell us this minarelli piston and gudgeon combo weighs an extra 10 grams!

titanium just isnt going to make that much difference, i suspect... unless it weighs only 10 grams?

be better to drill the crank, wouldnt we? and live with the other (secondary) vibration caused by doing so?

regards making new cylinders altogether...we are all familiar with the lousy ports as designed on the 66. (isnt that what half this site is based on? http://www.dragonfly75.com/motorbike/index.html ) such as the piston never opening the exhaust completely. the tiny 16-17mm cross section intake port. the woefully finished and angled transfer ports, the lack of material in the stud area, yarda yarda, blah de blah blah... rhubarb rhubarb

patternmaker... roughly $5000 dollars to make a useful pattern. to quote..."how many do you want to make, and how many will you sell?"

foundry...about $20 to 150 depending on how many required, and reject ratio.

most foundries in australia are scared by anything less than 3mm thick! despite hobby machinists doing the same thing in their backyards for YEARS!

then the machining work... plating. stocking, advertising, shipping... its amazing these engines can be bought for under $150 at all!


gee im rude

 

[Fabian]

cant remember who i spoke to, but they said 180... that was for a BARE cylinder, but...so maybe they charge lots because of the chrome removal?

Can you please list the name of the plating company that replates cylinders for $180, because i have 2 chainsaw cylinders that need replating

 

[Fabian]

roughly $5000 dollars to make a useful pattern. to quote..."how many do you want to make, and how many will you sell?"

foundry...about $20 to 150 depending on how many required, and reject ratio.

most foundries in australia are scared by anything less than 3mm thick! despite hobby machinists doing the same thing in their backyards for YEARS!

then the machining work... plating. stocking, advertising, shipping... its amazing these engines can be bought for under $150 at all!

I agree.


I have consistently maintained the line that these engines should be worth around $600 (as a kit); allowing scope for better quality control and design improvements.
I can't see how they can make money selling bicycle engine kits for $230 in Australia when the things need to be shipped half way around the world, plus payment of wharf fees and warehouse fees, and that's before it gets to the middle man.

 

[Deleted member 12676]

Fabian, I am pretty sure the cost of the engine kits is only $50 before shipping and other costs (when you buy a lot of them)

HS, I never recommended people use the heavier Minarelli wrist pin. I recommend the light titanium pin to make up for the heavier Minarelli piston (which will be lighter after shortening the skirts and possibly adding some lightening holes to it).

 

[Deleted member 12676]

thanks for challenging my idea HS. You always cause me to dig deeper to know more and sometimes to correct my suggestions.

My 55cc has a 43mm bore and the dual ring Kawasaki piston in it is 42.8mm wide at the top. So it is a good comparison to the suggested Minarelli piston for the 66cc. The top ring gap is now 1.3mm which is twice the ideal ring gap. The engine has been unused since I moved and the last time I worked on it I broke the second ring so I cant measure its gap. The top ring has .03mm groove clearance. (the Engine Builder's Handbook by Tom Monroe says "Ideal ring groove side clearance is .0025" (.06mm) and absolute maximum is .006" (.15mm)").
What we want with a quality piston and rings is some ring push but not too much because the added pressure causes added friction and added cylinder lining wear. My cylinder now shows loss of chrome at the very beginning of the downward piston movement. Either that is because the piston to cylinder clearance was too little or because I had significantly increased that ring pressure above what the cheap plating could withstand. The guy that tested this Minarelli piston with chromed rings got cyl lining loss at the top also.
So now the question is-- What causes increased ring pressure and how do you reduce it for extended cylinder liner life?
The obvious answer to the first section is "increased combustion pressure" due to either excessively advanced ignition or excessive compression ratio such as what most of the high compression cylinder heads provide. If you have more than 135psi you need to reduce it. period.
And there is a possibility that increasing the second rings end gap may reduce the outward pressure on the top ring. Both end gaps serve as pressure relief valves. Look at my drawing:

It is obvious physics that by increasing the second end gap you reduce the intermediary pressure between the two rings (the "orange" area). And the pressure pushing outward on the ring (which slipped in thru the ring/groove clearance) is also reduced because of that, I believe.

So my final advice would be to make sure cranking pressure is no more than 135psi, use a file or rotary tool to increase the second rings gap, and use a synthetic or semi-synthetic oil at 30 to 1 to counteract any increased friction. I always use the best and most costly oil I can find just as a point of habit.


the following is from http://courses.washington.edu/engr100/Section_Wei/engine/UofWindsorManual/Piston and Piston Rings.htm

A piston is a cylindrical engine component that slides back and forth in the cylinder bore by forces produced during the combustion process. The piston acts as a movable end of the combustion chamber. The stationary end of the combustion chamber is the cylinder head. Pistons are commonly made of a cast aluminum alloy for excellent and lightweight thermal conductivity. Thermal conductivity is the ability of a material to conduct and transfer heat. Aluminum expands when heated, and proper clearance must be provided to maintain free piston movement in the cylinder bore. Insufficient clearance can cause the piston to seize in the cylinder. Excessive clearance can cause a loss of compression and an increase in piston noise.

A ring groove is a recessed area located around the perimeter of the piston that is used to retain a piston ring. Ring lands are the two parallel surfaces of the ring groove which function as the sealing surface for the piston ring. A piston ring is an expandable split ring used to provide a seal between the piston an the cylinder wall. Piston rings are commonly made from cast iron. Cast iron retains the integrity of its original shape under heat, load, and other dynamic forces. Piston rings seal the combustion chamber, conduct heat from the piston to the cylinder wall, and return oil to the crankcase. Piston ring size and configuration vary depending on engine design and cylinder material.

A compression ring is the piston ring located in the ring groove closest to the piston head. The compression ring seals the combustion chamber from any leakage during the combustion process. When the air-fuel mixture is ignited, pressure from combustion gases is applied to the piston head, forcing the piston toward the crankshaft. The pressurized gases travel through the gap between the cylinder wall and the piston and into the piston ring groove. Combustion gas pressure forces the piston ring against the cylinder wall to form a seal. Pressure applied to the piston ring is approximately proportional to the combustion gas pressure.

Piston rings seal the combustion chamber, transferring heat to the cylinder wall and controlling oil consumption. A piston ring seals the combustion chamber through inherent and applied pressure. Inherent pressure is the internal spring force that expands a piston ring based on the design and properties of the material used. Inherent pressure requires a significant force needed to compress a piston ring to a smaller diameter. Inherent pressure is determined by the uncompressed or free piston ring gap. Free piston ring gap is the distance between the two ends of a piston ring in an uncompressed state. Typically, the greater the free piston ring gap, the more force the piston ring applies when compressed in the cylinder bore.

A piston ring must provide a predictable and positive radial fit between the cylinder wall and the running surface of the piston ring for an efficient seal. The radial fit is achieved by the inherent pressure of the piston ring. The piston ring must also maintain a seal on the piston ring lands.

In addition to inherent pressure, a piston ring seals the combustion chamber through applied pressure. Applied pressure is pressure applied from combustion gases to the piston ring, causing it to expand. Some piston rings have a chamfered edge opposite the running surface. This chamfered edge causes the piston ring to twist when not affected by combustion gas pressures.

Another piston ring design consideration is cylinder wall contact pressure. This pressure is usually dependent on the elasticity of the piston ring material, free piston ring gap, and exposure to combustion gases. All piston rings used by Briggs & Stratton engines are made of cast iron. Cast iron easily conforms to the cylinder wall. In addition, cast iron is easily coated with other materials to enhance its durability. Care must be exercised when handling piston rings, as cast iron is easily distorted. Piston rings commonly used on small engines include the compression ring, wiper ring, and oil ring.

Compression Ring

The compression ring is the top or closest ring to combustion gases and is exposed to the greatest amount of chemical corrosion and the highest operating temperature. The compression ring transfers 70% of the combustion chamber heat from the piston to the cylinder wall. Most Briggs & Stratton engines use either taper-faced or barrel-faced compression rings. A taper faced compression ring is a piston ring that has approximately a 1° taper angle on the running surface. This taper provides a mild wiping action to prevent any excess oil from reaching the combustion chamber.

A barrel faced compression ring is a piston ring that has a curved running surface to provide consistent lubrication of the piston ring and cylinder wall. This also provides a wedge effect to optimize oil distribution throughout the full stroke of the piston. In addition, the curved running surface reduced the possibility of an oil film breakdown due to excess pressure at the ring edge or excessive piston tilt during operation.

 

[Deleted member 12676]

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. These 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."

Pro-X Pistons says .05mm is the normal piston/cylinder clearance for a small cylinder, so the .2mm clearance of the Minarelli and the Kawasaki piston is not too small and not the reason for plating loss. (www.pro-x.com/media/downloads/Piston Kit Instructions.pdf‎)

 

[Fabian]

A piston ring must provide a predictable and positive radial fit between the cylinder wall and the running surface of the piston ring for an efficient seal. The radial fit is achieved by the inherent pressure of the piston ring. The piston ring must also maintain a seal on the piston ring lands.

If there is an excessive level of cylinder bore distortion from poor metallurgy, the ductility of a piston ring won't ensure a conformed fit between cylinder and the face of the piston ring. This is where higher oil/fuel ratios come into play, and perform three functions over and above lubrication: hydrodynamic barrier seal, which helps prevent excessive blow-by, improved thermal transfer between piston ring and cylinder wall and reduction in hot spots on the cylinder wall due to bore distortion.

A Chinese 2-stroke bicycle engine requires oil as lubricant, but lubrication of piston ring to cylinder wall is by far the least important aspect of oil in these engines.

 

[Deleted member 12676]

Motorcycle Tuning Two-Stroke by John Robinson also says that chromed rings cant be used in chrome lined cylinders

 

[Deleted member 12676]

I think the core problem is the cheap chinese cylinder plating. For $189 you can have your cylinder replated with NiCom at USChrome.

One last suggestion for those with the original plating: use fine sandpaper to take off the sharp edges of the piston rings. Make the edges roounded so that the rings dont scrape the oil off the cylinder but rather ride over it. That way there is more oil between cylinder and rings to lessen possibilities of wearing down or flaking off the original cylinder plating.

 

[HeadSmess]

thanks for being polite jag sorry to be an a-hole at times... thats just me. i overthink, sometimes contradict myself, but yeah... knowledge is power and the more we know, the more we can achieve (but by the time you know everything youre too old to do anything as my old mate says )

totally agreed, the lousy china plating is very suspect.

i like the little mention about the 70% of heat is transferred through the top ring...regards contact area, im still inclined to go with thicker rings for max heat transfer... on a side note...how are the 1mm rings located? im assuming on a ring that thin that they use the ring overlapping the pin, that also prevents it from ejecting itself into the cylinder wall, unlike the HT standard notched ring with the pin exposed? in that case, i prefer the thin rings!

flutter on a cylinder this size shouldnt be an issue until 10,000rpm or so, which takes me back to thick rings... see what i mean about contradicting myself?



fabian.

i was quoted from electrosil, who i believe is down in victoria anyway. for a KX60 size cylinder, looking at (after retrieving the email...) $270 for a single cylinder, $180 for 5, and lower costs again for larger batches. they also have various other plating services available. i believe the largest cost is setup and surface prep, the material itself is "relatively" cheap.... the first result from google, the only company i contacted. i mentioned they will be bare, but not sure if the quote was taking that into account or not

if enough people were willing to pay $200 or so for a nikasil cylinder, it might be worthwhile trying to get hold of bare unbored cylinders directly from the manufacturer, and plate 100 or so at a time... but you still have all those port design issues, regardless, taking me back to patternmaking and foundry work...(think integral reed valve block like on a puch cylinder ) who has a few thousand in capital theyre willing to spend, and possibly lose? (the guy that feeds several $50 notes every ten minutes into the pokies at the local club...thats who!)

taking this further... reading some of my metallurgy books...chrome plating is usually actually just a very thin coating on top of NICKEL plating, which, i believe, is plated onto copper plating! (though the inside of my sachs km48 rotary is hard chromed to about a 2mm thickness, but i have no idea how much of that is actually chrome, and i cant see any obvious signs of copper... and theres some nasty big chips in there, why its a shelf ornament rather than a going concern)

the reason a lot of cheap chrome (fenders etc) rusts, is because chrome itself doesnt resist corrosion, hence the layer of nickel on expensive work, that im sure is ignored on the cheap stuff.
chrome is extremely brittle, nickel is quite ductile. not really a problem inside a cylinder, but...the compounds created from burning oil can be quite corrosive... i know this isnt the issue here, but on RC engines, the fuel burns and produces NITRIC ACID as one of the byproducts, hence the need for after-run oils when storing them for any period of time. all engines produce nitrous oxides that create nitric acid when combined with water... side effect of air being approximately 80% nitrogen.


you can buy chrome plating solution for hobby use, its just that unfortunately...you need a rather high current, variable power supply... an inverter TIG welder springs to mind. you can remove chrome plating by reversing the current BTW, which saves needing superhard (and expensive) cutting tools to remove the chrome. its also primarily intended for plating brass cylinders.

its been years since i read into chromeplating alloy...i just recall some nasty acids like hydroflouric acid being mentioned, along with quite a few processes before it enters the plating bath... aluminium oxidises instantly when in contact with oxygen. the oxide layer does prevent further oxidisation. only certain acids will dissolve aluminium oxide. i know concentrated h2so4 is cheap and easy to obtain, but nitric and hydroflouric... not quite so easy, for the home user.

possibly the main reason the china plating has issues. i really wonder just how many get done at a time, and how well theyre prepared? any grease, oil, oxidisation or similar on the surface, and the chrome just wont stick... and even a new china cylinder has small pinholes in the chrome...