tom kimsey wrote:
please see the attached photo of the teflon encapslated silocone o-ring
damage to my engine that i was discusing with you at the gathering at
Tracy's house. I asked you in a later e-mail if you remembered the part
number of the ring that you sent to me( outer coolent) and you told me that
you could not remember or had lost that information. I believe that the TES
o-rings are a worth while improvement to this engine and ended up using the
TES o-rings at the inner position and using the stock Mazda rings at the
outer after i discussed this with Bruce Turrentine. I have some more jpgs of
the damage to my front and rear side housings if you would like to see them.
Tom kimsey
That is real strange looking Tom. Did the silicone part extrude itself out
of the teflon cover and into the crack? I can't for the life of me
understand
how this happened. Perhaps there was a core shift in this casting that left
a very thin amount of metal under the O-ring groove. Can you take another
close up picture with the O-ring removed and the two cast iron parts
removed so we can see the floor of the O-ring groove?
Paul Lamar
How can they stretch that much? That looks like about an inch worth.
How was the engine running when this happened? On a test stand or
in a car or plane? How many hours? I would like to see more photos.
Looks like it had to be some kind of flaw in the housing. What is that
circular area in the "floor" of the groove above the break?
I've got 55 hours on my new engine with these rings, everything is
perfect. I think Ed Anderson has them too.
--
Perry Mick
Duckt N7XR
http://www.ductedfan.com
The installation trick for the new guys is to get them slightly undersize
and stretch them over a plywood form for a few days so they lie in the slot
perfectly when assembly the engine. They tend to take a set once they
sit around in a given position. This was discovered after Don installed
his O-rings and his o-rings might have been slightly oversize.
Paul Lamar
Try this on for a theory, guys. Looks to me like the o-ring is too big for
the groove, not necessarily "won't fit" but, fills the groove TOO FULLl. The
silicon and the teflon both increase in size very substantially when heated,
and the teflon appears to have expanded to perfectly fit the groove in the
photo. If you wind up with the groove 100% filled with teflon and silicon
rubber
before it is fully expanded due to thermal expansion it will generate
essentially
UNLIMITED forces, it WILL get the expansion. This is what this looks like
to me.
This is a phenomenon that I have seen to AMAZING damage, and
breaking a chunk of cast iron is well within the capabilities. I do a lot of
analytical work on electronics and mechanical stuff for thermal expansion
damage possibilities and we have seen this many times. I have seen
trapped silicone rubber bend a 2" thick steel lid between 1" dia. studs on
3" centers.
The key is to ensure that the fully expanded o-ring takes up LESS
volume than the groove. I would like someone to drop a teflon encapsulated
o-ring in boiling water and measure the section diameter. If this hot diameter
converted into cross sectional area isn't some significant number smaller than
the section area of the groove, you are in trouble. This doesn't account for
the axial growth of the o-ring, too.
I suspect that you need one step smaller section diameter for these rings.
Bill Freeman
Long EZ builder & pilot
BSME, MSME
EAA Technical Counselor
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