Single ring piston?

[Racey Rider]

Is there any real benefit of a single ring piston in a two stroke?

Whats the down side?

Cheers.
Racey.

[pete376403]

1 ring = less friction. lighter piston & ring package (grams count). The pin could be higher, allowing a longer rod - night be an advantage
downside would be shorter life of both the piston and ring.
In a small race motor, this trade-off would be considered worth it.

[speedpro]

1 ring = less friction. lighter piston & ring package (grams count). The pin could be higher, allowing a longer rod - night be an advantage
downside would be shorter life of both the piston and ring.
In a small race motor, this trade-off would be considered worth it.

More or less what he said. BUT, even on some two strokes with one ring pistons the gudgeon pin isn't as high as it could be. Possibly something to do with piston rock/thrust forces/large holes in barrel. The longer rod would be an advantage with "peak" piston velocity, it'd be lower with a longer rod, but the engine revs seem to be limited by other problems, mainly to do with rolling element bearings. The longer rod would also benefit by increasing dwell time at BDC allowing increased time area angles without increasing port dimensions. I don't know that it would decrease piston life unless you let it get so worn that there was a lot of blow-by which could cause problems.

The benefit is increased power, mainly higher in the rev range. If a motor is to be revved hard you should be using as thin a ring as possible. A KT100 motor uses a 1mm ring i think whereas a typical road 2-stroke is more like 1.5mm.

[quallman1234]

Personally i would use trial and error.

Run it with one pistion ring one meeting and see if the increase in power is worth the decrease in realiblity

[Motu]

There have been engines running with no piston rings,usually ultra high speed engines,say over 20,000rpm.Close tolerances plus there just isn't time for blowby loses at those speeds.

[speedpro]

Personally i would use trial and error.

Run it with one pistion ring one meeting and see if the increase in power is worth the decrease in realiblity

I've been thrashing various poor 100cc engines for years and only ever used one ring with no ring related reliability problems. That includes one MB100 that made it's power between 9 & 13,000rpm. The failure when it finally happened was the big end bearing, after 6 years of racing every month.

[Racey Rider]

So would changing to a single ring piston alone make a difference, or would the con rod need to be changed also to the longer option?

Yep, try and see,,,, works for me.
just getting your input in case I'm totally Off Track. (wouldn't be the first time)

This would not be for a 'Streetstocks' bike in case somebody was wondering.

[speedpro]

So would changing to a single ring piston alone make a difference, or would the con rod need to be changed also to the longer option?

Yep, try and see,,,, works for me.
just getting your input in case I'm totally Off Track. (wouldn't be the first time)

This would not be for a 'Streetstocks' bike in case somebody was wondering.

Just the ring. It is really only an advantage on a fresh bore which is all straight and true with good clearances. With a worn engine the second ring will be better by helping to seal.

All the other bits are in addition if you want to get REALLY serious. My new motor is shorter stroke, longer rod, bigger bore, and every port and combustion chamber angle has been modified. The reed valve is now 6-petal(was 2) but there are internal chokes and the free length of the reeds has been reduced. Crankcase volume has been adjusted as well. You will be sorry if you start down this track of modifying 2-strokes. I'd been doing it for 20 odd years before I realised I don't know enough to make a REALLY good engine.

[F5 Dave]

Motu I believe those ultra high revving tether cars or planes are tapered bore to get the seal. General wisdom is one ring only gives advantage at higher revs with slight loss at lower, so if your bike peaks out at 10,000rpm then it prob isn't worth it. btw Tachos are often very optimistic.

[Motu]

What I was referring to was some research Honda was doing on ulta high rpm engines....they got as high as 36,000rpm.Pistons shaped to divert flow (vortices) to provide sealing.But yeah,model engines don't use rings.

[DEATH_INC.]

Years ago the Aussie Yammie team usta remove the second ring off their YZ250j's for a little more power, no worries for them about life though of course.....
The last KTM I owned ('85) only had one ring too, made by Mahle though.....and good for a season of mxing...

[Motu]

Also used to be common to remove the oil ring on 4 strokes for racing - the second ring is not a compression ring,it's called a scraper and used for oil control.

[Crasherfromwayback]

Also used to be common to remove the oil ring on 4 strokes for racing - the second ring is not a compression ring,it's called a scraper and used for oil control.

The 'scraper' is the thin flexible ring like object top and bottom of the oil control ring is it not? With the second ring from the top of most four strokes (race motors aside) indeed being a compression ring?

[DEATH_INC.]

The 'scraper' is the thin flexible ring like object top and bottom of the oil control ring is it not? With the second ring from the top of most four strokes (race motors aside) indeed being a compression ring?

You talking about the two either side of the usually wriggly ring? they are actually the oil rings, the funny shaped one is just a spreader to hold them apart...but the lower of the two solid rings is a scraper, it often has a step in it to aid this...

[DEATH_INC.]

Here ya go, from motor services website...

In general, we differentiate between compression rings, scraper rings and oil control rings. All of them have the primary function of sealing the combustion chamber against the crankcase.

The compression rings prevent leakage of combusting gases from the combusting chamber into the crankcase.

Scraper rings combine the function of the compression rings and the oil control rings, i.e. they seal combusting gases from the crankcase and additionally they scrape surplus oil off the cylinder wall.
Pistons ring sets of KS


The function of the oil control rings is to scrape excessive oil from the cylinder wall and to prevent the passage of lubrication oil to the combusting chamber. The objective is to achieve low oil combusting while at the same time providing sufficient lubrication of the sliding parts and minimising the gas blowby.

Another important function of the rings is to dissipate part of the piston heat to the cooled cylinder wall. Most of the heat transfer takes place via the top ring and less via the other rings. Inertia and gas loads as well as high temperatures impose high technical demands on the piston rings. It is only by an exact adjustment to the respective engine that optimum service life and compliance with the exhaust gas regulations are achieved.