Subject: Teflon O ring sizes.
From: ACRE
Date: 2/14/2005, 8:01 AM

Eamon was sent an engine to over haul that had some oversize
teflon encapsulated silicon O rings. Needless to say over size
TES O rings are a recipe for disaster as silicon expands with
considerable force when heated.  Powerful enough to crack
the cast iron end housings. 

I promised Eamon I would send the proper sizes.

Here are the proper sizes to use and some pictures of the proper
installation method. Also an old message from Bill Freeman on the subject.

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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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I have always thought of rubber, and that includes silicone, as being
"compressible". What has been brought home to me  after reading Tom's
experience is; rubber is no more "compressible" than hydraulic fluid.

Bill hit the nail on the head. The O-ring was oversize for the
slot and in the process of tightening down the tension bolts the hydraulic
force of the rubber broke the housing.

Bottom line is measure the O-rings you get and don't use
anything larger than .093" diameter in the outer slot. As Tracy points
out don't use RTV to hold the rings in place as it could harden
and add to your problems. Buy the O-rings slightly under size on the big
diameter (no more than 5% undersize) and stretch them on a plywood form for a
few days before installing them. They will then lie in the slot perfectly as
we discovered with Perry's installation. Don't try to stretch the ring
by hand or by another method as you could permanently neck them down.
Be careful when buying standard 3/32 O-rings sizes as they may be larger
in section diameter than .096. They could be as large as .115 or so which
is way too large for the outer slot. 

Best to buy the O-rings directly from Creavey the manufacturer.
In most case the prices are lower than what Mazda charges for lower
temp stock O-rings.

Here is a part of their catalog again showing the small increments in
section diameter that they sell.

http://www.creavey.com/


1.  Inner coolant oring -

	circumference  = 28.625
	diameter       =  9.111
	slot depth     =  0.062
	slot width     =  0.141
	section size   =  0.098



2.  outer coolant oring -

	diameter       = 11.340
	slot depth     =  0.062
	slot width     =  0.110
	section size   =  0.093


The proper oil O-ring sizes are:

0.093" section 4.68 inch ID
0.118" section 4.33 inch ID

Viton is good only to 400 degrees F. The center of the rotor runs
as high as 500 degrees F. I think 400 degrees F leaves's too
little of a safety margin when 500 degrees F is available
in TES O-rings at the same price probably.

I checked the installation of the oil O-rings with the new
sizes I got from Creavey. They worked perfectly and were
very easy to install. You will need a little O-ring lube
and a 2 by 8 to install them. They were also easy to remove
with no signs of distortion.

Paul Lamar


 
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