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

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Deleted member 12676

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.
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 :poop:

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 :poop: 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 :giggle:

(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 :giggle:
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".
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... :unsure: 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 :rolleyes: ...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... :D 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 :giggle:
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
why not make new patterns for the cylinder and get them made PROPERLY? thats why i was getting quotes

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.
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...

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.
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. :giggle:
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.
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.
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