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

Discussion in 'Performance Mods' started by jaguar, Jan 26, 2014.

  1. jaguar

    jaguar Well-Known Member

    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 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.
    elgallo and Fabian like this.

  2. HeadSmess

    HeadSmess Well-Known Member

    im going to raise an issue with point 1. sorry, i always do this, dont i? :)

    the spring pressure of a ring has no bearing on its sealing efficiency.

    the combustion pressure forces the ring DOWN onto the bottom of the groove, and gets in between the piston and rings ID and forces it OUT into the cylinder wall, with a pressure that far exceeds the pressure created by the rings natural springiness. this phenomena is well documented. so if the rings tension is wearing the bore out... see what im getting at? ;)

    any extra pressure on the wall due to the larger surface area on the ID due to the thickness, is counteracted by the reduced pressure on the OD a camels foot, its all about surface area. vice versa... a thin ring you would expect to "cut in" more as it has less surface area, just like sharpening a knife....but the pressure on the ID is also reduced... force cancelled!

    the pressure from the combustion process and ring design still remains, forcing that ring out so hard it would be bent straight if not confined!

    maybe the 1mm rings wear the cylinder out because its very thin chrome and the alloy behind it isnt terribly rigid? which would render my last comment about the ID and OD pressures/areas as :icon_bs:

    rings upside down? chrome side is normally DOWN. if chromed on a flat face. erm...they have a sharp edge? ie a "scraper" design.

    does the chrome wear from a particular place?

    (its important that the ring is made correctly and is perfectly cylindrical to match the bore when compressed into the cylinder... been reading a lot on various methods of making rings lately, for another project... heres just one sample of some of the information... in a way, the cam turned ones are very similar shape to internal circlips, that are designed to contract uniformly around the circumference...they dont come out easily if they bend elliptically. the curve just isnt so apparent on a ring, because its a different thing entirely!)

    otherwise, all the information is nice to have, i like 1mm rings and a suitable piston are available simply because flutter is far less likely :) shame i refuse to use the 66 huh?

    and who said these motorised bikes are NIKASIL? thats complete and utter :icon_bs: maybe in a morini, aftermarket puch or similar, sure... aluminium lined cylinder? huh? used on the lowest end and cheapest available model rc engines circa 1940, guaranteed to not run for long :) its either plated with something or its cast iron liner. or brass (thats still chrome plated) if we really must stay in RC land now ive mentioned it :jester:

    (ok, there is a way of heat treating certain aluminium alloys to precipitate an integral "liner" consisting of silicon carbide, but that is super high tech stuff!)

    id ignore anything those guys say! or that guy at least.

    i do agree with a non chrome ring. just plain steel. a 1mm cast iron ring will just shatter, chromed is too hard.

    erm, i was quoted around $180 for nikasil on a 48 bore... 150 for a batch of 50... and lower for a huge yes. still at least $100 a cylinder + manufacturing costs (AU)... could you get them super cheap in bulk from china, unplated? at which point...why not make new patterns for the cylinder and get them made PROPERLY? thats why i was getting quotes ;) dont even ASK what a pattern maker said, let alone the non ferrous foundryman when i showed them a cylinder maybe a year ago :jester:
  3. jaguar

    jaguar Well-Known Member

    My point concerning the rings outward pressure onto the cylinder had nothing to do with its sealing efficiency. I know how rings work. Excess outward pressure contributes to cylinder wear since it only adds to the normal pressure due to cylinder pressure pushing the rings outward.
    And even though the 2mm thick stock rings help to spread out the force more, they still contribute to increased friction.

    One person commented that he had only seen steel rings on chrome lined cylinders, or chromed rings on steel cylinders. That makes sense because if both were chromed then how would they wear for a perfect seal?

    In my 55cc I use a Kawasaki KX piston with dual 1mm thick non-plated rings. No problems.

    When he said "aluminum lined cylinder" I think he meant "aluminum cylinder with lining".
  4. HeadSmess

    HeadSmess Well-Known Member

    yeah, but the extra pressure of the rings spring added to the combustion sealing pressure...its like saying the straw that broke the camels back. hmmm.

    i was thinking more a drop in the ocean...

    maybe it is a camels back but i dont see any problem in bore longevity using the stock rings...(on 48's mind you) other than the (possible) ring flutter! and yeah, blow by... yet... whats the standard gap? do you know what material stock rings are made of and can confidently say exactly how much they will expand when heated to operating temperature?

    ill agree with the extra friction.

    maybe you should get rings made from invar, then you can keep the gap so small it would be almost unmeasurable... in fact, it would INCREASE! theoretically, an alloy ring would require no clearance, if it was the same alloy as the cylinder itself... its simply to allow the ring to expand, and take up wear.

    actually...the gap of a steel ring in an aluminium bore should increase anyway... ive never even thought about that before! but no. thinking further, i take all that back, because the rings get FAR hotter than the cylinder does! where does the piston heat escape?


    and now i want my thick rings back, because what gives better thermal transfer?????!?!?!?!?!?

    (i seriously need to stop thinking about this now... but what follows i already wrote. cant just walk away now! once again, i do apologise!)

    on chromed rings... now the bore is chromed, and is very hard, and so is the ring...
    which shouldnt really affect the bore, but it wont ever bed in properly, either, thats true.

    also i dont like the idea of running similar metals like that. galling, microwelding...stainless in particular is notorious for it, but the golden rule with (plain) bearings is to use different metals. ring to cylinder is a sliding surface. other than cast iron, which makes wonderful sliding surfaces...go figure... :thinking: machine tools, cylinder liners, yeah... hard is the chinese chrome plating? theres various grades of chrome plating and im sure the chinese one isnt much better than a mere cosmetic like on a bumper bar! hit that with high quality hard flashed chrome ring.... yeah...i can see see cheap chrome just being scraped right off... surface prep plays a large role in the tenacity of the plating too, and, as suzuki found out with the re5... chrome has a habit of flaking off at times anyway. and being hard, it does a lot of damage... so thats why the plain kx rings work well in a standard bore.

    i hope thats what he meant! but i still scratch my chin over the nikasil part...was that a direct quote? but then, on rereading...he meant the minarelli mb engines are the ones that are nikasil. ok. and even more, just so i can look like a real :dunce: ...he even blamed the cheap cylinder lining, just like i did! cool, thats agreed, the cheap china chrome is useless :) get a cylinder nikasil professionally, run chrome, last forever ;)

    and the plain steel ring will work perfectly, in the stock bore, of course :)

    guy that quoted me nikasil says it doesnt matter what ring material you use... so hard nothing affects it. of course, the honing would tend to break down and smooth out faster with chrome, and plain rings would tend to produce more "debris" during the break in period, but would also break in a lot better, with a better seal, simply because theyre basically being wiped across super fine sandpaper...

    now i did some more backtracking, i argue point two as well.... i knew id read this recently...

    "The idea that the ring gap was a serious source of leakage was exploded over half a century ago. it is, in fact, ludicrously small. Fig. 4 shows an idealized model of the gas flow through a pair of ring gaps. "A" represents the top land, a short passage with high friction loss. "B" is the orifice formed by the top of the first gap; typically of area 2/1000,000 for a piston with 2 thou diametrical clearance and a 2 thou gap. The gas (or steam) expands through this, losing more pressure into cavity "C", the space between the ends of the ring, and that behind the ring. It then enters the second orifice at "D", and expands through this, (with pressure drop) into "E", which is the second land; a long and frictional passage round the piston to the next gap, which it enters at the orifice "F". The same procedure is followed here, into "G", through "H", and finally, to the back pressure via"I", the tortuous passage past the skirt of the piston. The point is that it is the pressure drop at the final orifice which governs the rate of leakage, and NOT the high pressure drop at "B". The minimum gap - for steam or IC should be 0.002" and a guide might well be an installed gap equal to 0.001" + 0.001"/inch of cylinder bore."

    this is fig4. yep, took me a while to see it. its stylised, not a cross section.


    theres leakage when turned by hand, the time factor is huge. different story at 6000 rpm. air can only flow so fast through a given size orifice at any given pressure differential... and flow is severely reduced as the shape deviates from the perfect circular shape with lead in and lead out...

    ring flutter. by the above quote... the blowby isnt the issue. (that thread fabian started about that ringless piston...even that makes a bit more sense now) its the hammering it receives as it gets slammed around and the loss of contact with the cooling surface, ie...the cylinder, quote jennings..."the ring is bathed in fire!"

    (i have to say it, its been lurking... the contact area of a ring is also underneath, so thats the point the piston transfers the majority of heat, through the bottom of the ring... now, when a ring wears in the bore, the gap increases, yes...but the contact patch on the piston is reduced? see where im heading with this train of thought? the ring will transfer less heat from the piston... maybe, in real engines where the manufacturers have actually done destructive tests, they figured out at what point the heat transfer is so reduced that the piston overheats? then measure the end gap, and well, add a fair percentage for safety, and require a specific size gap in the first place for the measurement to be accurate! thus explaining the "max/min" limits... just a thought... just thinking of one of those pics of a typical engine on a stand in a lab, pipes glowing, being revved to death... :biggrin: they would have measured absolutely everything possible!)

    thats my only interest now. as before. flutter-byes! and the thoughts on heat transfer have me thinking ill have to work out what rpm flutter starts at before i even worry about it.

    what rpm does a minarelli get up to?

    im shutting up now... always so freaking negative :(

    "post quick reply" is a bit of a joke :jester:
  5. jaguar

    jaguar Well-Known Member

    sure, ring leakage is minimal when the gap is around .002"
    ha! measure yours. Your jaw will drop!

    any racer will tell you the bike runs stronger when wore out rings (with excessive ring gap) are replaced with new rings. very basic
  6. Fabian

    Fabian Well-Known Member

    Now that idea has me jumping for joy, but if you were to get a cylinder pattern made up, the possibility exists to increase the bore size from 47mm to 52mm if using 6mm head studs, then boring out the top of case halves to accept the larger piston.
  7. Fabian

    Fabian Well-Known Member

    Leakage past the rings is a non issue when running a higher oil/fuel ratio like 25:1 or 20:1 as the oil forms a hydrodynamic barrier, manifesting itself as improved torque at low rpm.
    Higher rpms give an ever reducing amount of time for gas flow to move past the piston ring gap.

    Higher oil/fuel ratio also allows better piston ring to cylinder wall seal in an air cooled engine suffering from excessive bore distortion due to poor metalurgy.
  8. jaguar

    jaguar Well-Known Member

    Fabian, why are you always waiting on others to do all the work?
    You want it done right? Just find a motorcycle piston of the right size (making sure the distance from wrist pin to piston top is about the same), Bore out the cylinder to match, and then send it off to get plated.
    Forget changing the transfer port design because it is adequate for engines reving up to 7500. All you need is a bigger bore, motorcycle piston, and good cylinder plating.

    Do that and report back to us. :jester:
  9. Fabian

    Fabian Well-Known Member

    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.
  10. jaguar

    jaguar Well-Known Member

    the small amount of oil available to seal that ring end gap is not enough when the gap is .075" (1.9mm) which is 3 times the normal maximum.

    I know from experience that using a motorcycle piston with thinner rings extends cylinder life and increases compression, therefore increasing engine power.
  11. HeadSmess

    HeadSmess Well-Known Member


    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, maybe they charge lots because of the chrome removal?


    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 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? ) 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 :)
    Last edited: Jan 30, 2014
  12. Fabian

    Fabian Well-Known Member

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

    Fabian Well-Known Member


    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.
  14. jaguar

    jaguar Well-Known Member

    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).
  15. jaguar

    jaguar Well-Known Member

    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 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.
    Last edited by a moderator: Dec 18, 2015
  16. jaguar

    jaguar Well-Known Member

    "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. ( Kit Instructions.pdf‎)
  17. Fabian

    Fabian Well-Known Member

    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.
  18. jaguar

    jaguar Well-Known Member

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

    jaguar Well-Known Member

    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.
    Last edited: Feb 4, 2014
  20. HeadSmess

    HeadSmess Well-Known Member

    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 :joker:)

    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 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? :jester:


    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 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 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...
    Fabian likes this.