First off, I am not trying to spread doom and gloom.

However, on a mailing list that I belong to read an interesting bit on Viton (which we frequently use in contra pistons) that I found interesting and sent me on hunt for more info on O Rings:

> Viton is brilliant stuff when used within its limits. If it gets too
hot, it starts to thermally decompose. Since it is a Fluoropolymer,
one of the products of this decomposition is Fluorine. Or Hydrogen
Fluoride - I can't remember the details. I do recall a good few years
ago working on industrial engines. There was also a military version
of the engine which used Viton seals instead of the Nitrile of the
industrial one. We were told never to open up an engine that had been
on fire unless we could verify from the full service history that it
only had the Nitrile seals in. If it had, or even may have had, Viton
in it, It went for scrap. We heard a few nasty stories, from people
not generally prone to exaggeration, about the effects of decomposed
Viton. None involved death. All involved amputation.
>
> The temperature is no problem for steam cylinders, but IC cylinders
may be a different story.
>
> Obviously, the final decision is yours, but do read the MSDS for
Viton. Go the extra step and see what you can find out about the
decomposition products, as well. Then make an informed decision. I
know a few people who have made spark plugs for miniature engines from
PTFE, also a Fluoropolymer, and used them every weekend for years
without problems, so it's not all doom and gloom.
>
> Personally, I would try to avoid Viton (or PTFE) for IC engine
piston rings simply because I would *have* to tear into an engine that
I built and that broke. It's not just money if you've built it, and
scrapping it without knowing why would really screw my head up.
>


That said, I wanted to find out more. I haven't really but I did make the connection between Viton and PTFE's (Teflon) which I do know decomposses under heat and produces some nasty stuff.

All of that not with standing, I did find some interesting information on o rings that I thought others might interesting seeing as the topic pops up now and again. A quick thumb through these handbooks has already answered a few of my questions.

The Seal Man's O ring handbook (lots of good info on the site in general as well)

http://www.thesealman.com/pages/orin...ook/index.html

Also, ran into a reference to the Parker O ring Handbook which you can find here:

www.parker.com/o-ring/Literature/ORD5700.pdf /Literature/ORD5700.pdf or just www.parker.com if you want to spend some time exploring much other related info.

So, whether to use or not use Viton in a contra? I really don't know, seems the normal temperatures our little engines reach shouldn't be an issue (you can get teflon coated frying pans to cook your food in after all). The choice is yours.

cheers, Graham

We've been using teflon in gaskets, head seals and other places since at least the 70's-and the only amputations I'm aware of have been done by propellors! Personally I would take these sort of scaremongering stories with a ladleful of salt!......and consign them to the bin of urban myth where they belong.


'ffkiwi'

BSc(Hons) x2 [chemistry,biochemistry], MSc[biochemistry], PhD[biochemistry] (in case anyone wants to dispute my qualifications to comment)

Have a look at the MSDS for Viton yourself. It's not so scary, unless you are a fire fighter at a blaze of thousands or millions of O-rings.

I was not trying to paint a gloom and dome type scenario. This was information that was new to me and I found it interesting which sent me on a search for more information. I am well aware of the risks of simply accepting something like that at face value without verification. I think you will agree that informed desicion making is important.

Urban myth, perhaps when pushed to the extreme or exagerated. However, the manufacturers themselves warn of the danger:

I forgot which site I copied this from otherwise I would post the reference:

<quote>
There is a remote risk of sustaining chemical burns from Viton® O-rings if they are burned or otherwise exposed to temperatures above 600ºF. Viton® begins to decompose above 600ºF, yielding some nasty compounds including hydrogen fluoride. In the presence of water vapor, this can produce hydrofluoric acid, an extremely corrosive mineral acid which causes severe burns to exposed skin. Proper protective gear should be warn when dealing with burned Viton®. There is no danger in dealing with cured Viton® used within the rated temperature range.
</quote>

For our use in our small engines the risks are minimal to non existent but it is nice to be aware of the risks so as not bury our heads in the sand.

I'll keep using Viton O Rings but now I am aware of any potential risk if used beyond their rated specs.

In any respect the other information on O Rings I found most useful and as the subject of O rings and specs turns on this forum from time to time I hoped that would be useful to others as well.

cheers, Graham

Sir, Could you tell me from which direction to remove the McCoy .049 contrapiston? Down from the top of the cylinder, or up from its bottom? Mine is frozen with age, and I want to be sure I don't damage the cylinder while punching out the contrapiston., and I'm surethe 60-year oldengine won't run without being able to adjust the compression.

Any help you could give me on this question would be appreciated.

William Hanshaw AMA 31631

I use the black o-rings from Ace Hardware and they do fine. No idea what they are. I have always removed contrapistons from the bottom, that is I beat on them from the bottom and they come out the top. Get a piece of hard motor mount and carve it to fit into the cylinder. Cut an appropriate size hole in a piece of 1/4 ply. Set the cylinder on it upside down and beat on the contra piston until it comes out.