Piston sealing devices, iron rings vs. everything else

[lohring]

Piston friction in small engines makes a huge difference in power. When we were working on an RC boat to set the first record over 100 mpg, our engine developer found that the hone pattern could make a 1/4 hp difference in a 7.5 to 8 hp 26 cc engine.

The seal is also critical. Several engine builders including us found that twice the recommended oil mix (8 oz per gallon in place of 4 oz per gallon) helped power. Up to 10 oz per gallon helped big end bearing life at over 20,000 rpm. More oil than that started to hurt power.

Lohring Miller

 

[Tin Falcon]

What is PTFE?

the abbreviation for Polytetrafluoroethylene most commonly known by the Dupont trade name Teflon.
great stuff but like most plastics needs to be used under 450 degrees Fahrenheit. But not a problem for most applications. Not recommended for super heated steam.
Extremely low friction Teflon rings are standard in PM research model engines.
Tin

 

[robcas631]

I like the idea of using cast iron piston rings. I understand that internal combustion engines require more compression, and have a completely different configuration then air powered ones. So my question is what is the proper ratio and gapping sizes recommended for air powered engines?

 

[DianneB]

I had only ever used cast iron rings in a steam engine up until last year but some of the boys in our model engineering club have been running O-rings for a long time so when I needed new rings of a non-standard size from my 1-1/2" American I made new pistons fitted for O-rings.

It is a bit of fiddling to figure out the right ring groove dimensions and proper lubrication is critical but once I figured that out, it ran like a top. The biggest difference I have found is how easy rolling the engine is now! You sure don't want to park it on a grade without brakes on!

 

[Brian Rupnow]

I now have six i.c. engines running using Viton O-rings. They work extremely well, and are very inexpensive. I find that you only need one ring per piston. I have engines with bores ranging from 5/8" (16mm) up to 1" (25.4mm) and they all use O-rings of 1/16" diameter cross section. The groove width in the piston is made .093" wide because that is the width of my parting off tool, and is very close to what the o-ring spec. book calls for. The true diameter of a 1/16" o-ring is actually 0.070", and I find that the best groove depth is about 0.060". any less, and the friction in the piston to cylinder is too high. any deeper (more) and I am afraid that the seal wouldn't be as good. Viton is a special heat resistant compound and I have never melted one in any of my engines. They have no special requirements for the cylinder bore, other than that it should be quite smooth to keep from quickly wearing the o-ring out. I give my cylinders a #600 lapping compound finish. They are not used in commercial applications, because they probably wouldn't stand up to long hard hours of running, but model engines never see that anyways. I have had some people warn me of dire consequences if the Viton o-ring does melt--apparently it releases a very powerful acid and should not be handled with bare hands if it does melt, although I have never experienced this issue myself. I run a bout a 40:1 mix of Naptha gas and two cycle engine oil in my engines, and that seems to provide all the lubrication that is needed. ---Brian Rupnow

 

[majorm]

I dont know how I missed this post but I have thought about using these teflon backup rings for my engine. http://www.theoringstore.com/index.php?main_page=index&cPath=14_552 The ones that I have bought before had the cut overlapping so it wasn't "open" like what the picture looks like. There are also multi turn ones that almost seem solid since they have an extra coil to cover the joint. I haven't found a good source for those yet though.

They make solid ones as well but I think you would have to make a two piece piston and pin it together after adding the ring.

 

[kquiggle]

In preparation for building a webster engine, I have been collecting information on making piston rings. Perhaps others will also find these useful (click on the link and scroll down to "piston rings"):

https://sites.google.com/site/lagadoacademy/useful-links#misctools

For information on calculating dimensions of o-ring grooves, and other o-ring information:

https://sites.google.com/site/lagadoacademy/useful-links#info-other

 

[gus]

Over the years I have read many threads about modelers having trouble getting good compression with their I.C. engines. Usually the biggest problem is valve/seat machining and sealing. In my personal opinion if a model engine builder can machine to tolerances good enough to solve these problems then making piston rings should be a piece-of-cake.
I have used several different methods and have settled on the Trimble method of making them.
All that's required is some fine grained iron, (Durabar, Schedule 40, continuous cast to name but a few.)
I fixture needs to be made to expand the ring and hold it while it's brought up to annealing temperature.
The machining is straight forward. Turn the I.D. to size. Then turn the O.D. to size or +.0005. Never undersize.
Part off up to .002 wider than nominal then hone it down to the proper thickness. It's too hard to part it off dead nuts.
Each of these steps are much easier than turning valves and seats and making them seal.
Give it a try! You'll never know until you do.
By the way, here's a link to a great article on making rings.
http://modelenginenews.org/techniques/piston_rings.html
gbritnell

Hi gbbritnell,
Thanks for the article which I have downloaded into my ic engine folder for future reference. About to begin building the Nemett-Lynx IC Engine. Article is j.i.t..