Smallest Diameter Piston with Ring(s)

[Carlr]

Hi,
have an engineering application (essential a high temperature plunger) and am looking for the smallest dia. commercially available piston and ring(s) that I can easily source.

So far the smallest I have found is 1.25" (32mm) from 25cc 2-stroke garden products such as weed wackers and hedge trimmers.

I know that the pistons in many smaller models don't use rings, I assume do the excessive friction.

Does any one know of any engines with pistons that have 1 (or preferably 2) rings, that are < 1.25" (32mm) dia, and places to source them? I'd be appreciate some ideas.

 

[WOB]

https://www.deboltmachine.com/products/piston-ring-051-wall
There may be others: http://www.antiquengines.com/Davis_Model_Suppliers_List_.htm

WOB

 

[Dr Jo]

There are plenty of small pistons and rings around

For Steam engines 3/4" are common: https://www.stuartmodels.com/search?qs=ring

For model aircraft engines eg: https://www.rcjapan.net/index.php?main_page=index&cPath=80_413_462 companies like OS do pistons as well.

Really you need to define better what you need, especially wrt the piston as there are many different variations in the distance between the gudgeon pin &crown and shape of the piston crown. Both Pistons and rings are extremely easy to make your own.

Why do you think you need rings? Very small diameter model aircraft engines normally don't use rings and have a honed piston running directly in the bore of the cylinder making them suitable for higher pressures & temperatures.

Jo

 

[werowance]

for the Webster that I am in the middle of building, I purchased my 7/8 o.d. rings from here:
http://www.pistonrings.net/ his name is Dave Reed. they were 14.50 shipped to my door for 2 of them. I have his email address to but this is the guy the plans recommended buying from, he just changed his domain name since the plans were done.

 

[werowance]

here is what the plans call for, and the old web site address which will redirect you


PISTON RINGS - 3/32" X .875"

(2) REQD.

SOURCE: OTTO GAS ENGINE WORKS

(410)-398-7340

2167 Blue Ball Rd

Elkton MD 21921-3330

http://www.dol.net/~dave.reed/otto.html

 

[Carlr]

Thanks for the help everyone. It seems that 3/4" is the most common small dia.

To answer a few of the questions posed:
- I can tolerate most piston crown forms, and the crown to gudgeon distances too.
- I would like to use rings as I believe it will make machining to bore easier/cheaper for my application. I think that I can get an acceptable seal with lower tolerances on the bore and I can tolerate a relatively high level of friction in my application. I need a seal (although does not have to be perfect) in a high temperature application, but the piston velocity will be very low and driven by a high powered actuator.

 

[enfieldbullet]

How many assemblies are you making?

Also, what would be the working fluid?

How high temp?

What is the stroke length and preferred diameter?

It's an awkward question that you posed because if you are only doing a "one of" you could surely get a piston machined to size. And if it's a production run you can coordinate with a piston manufacturer to produce your specific size.

Plenty of machines with simple small diameter pistons around...if you don't require low mass as in a ic engine application then it could be just a billet of material with a crosshole...

Maany ways to skin this cat that i can think of without further constraints...

 

[Tim Wescott]

If this is for a production application, check out OS and Magnum engines. There are still model helicopter engines that are ringed, and rings need to be replaced, so they ought to be available.

Keep in mind that such small rings don't even approach the reliability of rings in automotive applications -- a typical model helicopter probably doesn't get more than a couple of hundred hours on it in a year.

 

[Chris Murphy]

The smallest commercial ringed engine I recall was the 'McCoy 9' which appeared in 1950...this had a bore and stroke of 0.5"....but even so-they soon dropped the ringed version in favour of a lapped piston. The engine review may be found here: http://sceptreflight.com/Model Engine Tests/McCoy 9.html

As for ringed pistons-well CO2 motors have been using O-ringed pistons since the 80s....it may be that a high temperature 0-ring could suit your purposes-depending on how high the planned operating temperature is: http://www.animatedengines.com/co2.html The particular engines in question are those developed by Stefan Gasparin in the Czech Republic, and sold either under his own name or under the 'G-Mot' brand....the important point in this discussion is that these engines are tiny....even the largest (GM500) being about 0.5cc displacement-so the system works perfectly well even in these tiny sizes at reasonably high pressures-several hundred psi...what the upper temperature limit would be is hard to say....the cylinders are metal-but the piston is normally an engineering plastic fitted with a rubber O-ring for seal, and a ball and socket little end on the conrod. A possible alternative if your 'high temp' is not too high.....