After years of running Mogas in both the O-200 in my KR, and the O-320 in my SuperCub, I have run into a particularly nasty bunch of Mogas after moving to Arkansas.  In NM, I had a good source for alcohol free premium Mogas and burned it for years with no issues at all.  But after moving to a new area, other than testing for alcohol, how does one verify that the mogas being sold is free of any detrimental chemicals?
In April I started running a new brand of premium Mogas (everything where I moved is new to me).  Testing for Alcohol showed it to be alcohol free.  This summer I was away from the planes for a few weeks.  When I got back to them, the flow meter on my fueling rig had failed (plastic gear that runs submerged in fuel failed), there were several leaks in my fueling rig, including the fuel nozzle and the hoses.  At the same time, both tanks in my supercub developed leaks and the O-rings in my primer failed.  I'm not sure what kind of additive is in this fuel, but whatever it is dissolves epoxy, pipe dopes, and damages neoprene rubber.    Impressive.  I was in a real panic as I had been running this stuff in both planes. 
I have spent the last 2 weeks working on the fueling rig and the planes, and trying to assess the damage to the planes.  What I found is, the epoxy tanks in the SuperCub are both compromised and leaking at points where there were penetrations through the tanks for various plumbing parts like the fuel drains and fuel pickups.  I will have to cut the tanks open and repair from the inside, which will give me a chance to better assess the damage.  Additionally, the O-rings in my 2 year old primer failed.  New O-rings and some fuel lube fixed the primer problem.
The fueling rig in my truck (12V fuel pump, flow meter, filter assy, and fuel nozzle all mounted to a 110G steel tank) had to have new hoses, the 1-1/2 and 2 inch piping had to be dismantled and put back together with new pipe dope.  The fuel meter had to be replaced.  And the fuel nozzle needs new O-rings. 
I drained the tanks on the KR, which are also epoxy, but were sloshed with an alcohol resistant slosh compound 22 years ago.  Upon inspection I can see that the fuel etched the surface of the slosh compound, but I can't find any peeling or flaking compound; and I have no leaks.  So at least the KR is OK for now. 
Not looking for advice, but am putting this out there for discussion.  I'd love to hear ideas about how one verifies that the fuel is good other than testing for alcohol.  I don't need to hear about the wonders of vinylester as that was an unknown at the time my KR was built, and the tanks in the Cub were already built when I bought the project. 
For now, the KR is still flyable.  But it is unlikely I'll complete the fuel tank and wing repairs to the SuperCub before late fall.
-Jeff Scott
Cherokee Village, AR
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