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
Paul:
Do you remember what size rings you ordered for my engine? I think
you got them from McMaster Carr?
--
Perry Mick
Duckt N7XR
http://www.ductedfan.com
I'll have to look that up. The very first batch I got was from All O-rings.
Bad place as they were trying to obscure source and part numbers.
I think I did get some from McMaster Carr but not sure
if yours was among them. The last batch I got directly from Creavey
and I think those went into your engine.
The outer slot has an area of .13 by .070 or .0091 square inches.
so the radius squared would be .0028966.... working backwards.
The square root of that is .0538.
So the largest section diameter to fill the outer slot would be double
that or 0.107640. That lends credence to Bill's theory as a 3/32
section diameter ring is not .096 section diameter but is about
0.11 section diameter or about 14 thou larger (.014).
And then you have the O-ring tolerance on top of that. Some might be 0.005
under and some might be 0.005 over. Also any crude in the slot will not
help matters. Make sure the slots are spotlessly clean.
Perhaps Mazda themselves ran into Bill's contention back around 88 and
that is why some housings cracked in this area. Perhaps they reduced
the section dimensions on the stock O-rings slightly.
This is why we call them experimental airplanes.
Bill, do you happen to know what the coefficient of thermal expansion
is for red silicone?
The bottm line is measure your water O-rings and make sure they are
less than .093 +/- .005 section diameter. Particularly the outer
one.
Paul Lamar
Would I be correct in assuming this could be an infant mortality
problem. In other words, if it didn't show up soon after engine
assembly, one might be ok? My engine was running it's hottest
the first few flights after overhaul because of plugged rads.
Perry Mick
I would say that was a correct assumption. At least I hope so.
Our original theory on why some of the 88-89 O-ring lands cracked
was because of quality control... casting core shift. This may
still be a factor.
I know it is cold up there now Perry but what are you now seeing in the
way of oil temps and coolant temps?
Paul Lamar
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