Using anything other than the stock oil coolers requires workable, but what I consider to be a hokey setup of hoses to a remote oil cooler.  I ran a stock folded fin in my 2700 and it borderline was acceptable.  I've bought a 3100, and there is no way I'd be able to get away with that again.  But, what if I could use TWO of those folded fin coolers. 
Found one on Ebay for $45 shipped.  It is on its way.  In the meantime, I used a holesaw to cut a couple disk from a 1/2" aluminum plate, and turned a couple fittings.  The through hole is 3/8".  The outside of the shaft is 1/2".  The shaft stands 0.08" tall on the side for the outside cooler. The O-ring is 0.1" thick, so I left room for it to be compressed between the fitting and the recess of the oil cooler.  The disk is 1" in diameter and 0.1" thick.  There is a 5 degree slant toward the inside to capture the O-ring.  The O-ring is the viton ones used on the head bolts.   
I will cut two 1/2" holes in the back of the inside cooler,  push the long end of the fittings in with an O-ring to seal, then bolt the second cooler up to the first using a longer bolt.
Double the cooling for practically nothing (in airplane dollars).

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I'm trying to add a picture of the fittings, with no luck.

You say you found your "new" cooler on Ebay. Is it a virgin NOS unit or is it serviceable used? If used do you know how many miles of use are on it? Oil coolers are kind of the liver of the engine - lots of junk gets stuck in there and no amount of rinsing it with solvents will get rid of it all yet a hunk can break loose at any time and circulate in the oil to your bearings. If it is brand new, wonderful. If it is used, it would make me nervous.

That is one reason I like the external hose and Niagara cooler solution - a brand new aircraft engine should have a brand new aircraft oil cooler in my not so humble opinion. Lycoming uses external hoses to a Niagara cooler, I don't see that as hokey in a Lycoming powered plane or a Corvair powered plane.

Let us know how your experiment works out.

That is a concern that requires a valid response, Bob. 

The reason it doesn't make me nervous is that most of the Corvairs still running are using the stock coolers, and there a lot of airplanes that have flown with the stock coolers.  As stated, the engine I'm retiring ran with a stock cooler for 60 hours.  The only horror story I've heard of from using old coolers is from a guy on another list that serviced VW engines that were used to make snow on ski resorts. . . a high intensity, unsupervised use.  The engines that would blow then be put back in service without changing the cooler would blow again shortly thereafter because of gunk in the cooler.  Not really applicable, since the VW doesn't have a filter.  For there to be damaging metallic FOD in our coolers, there would have to have been a devastating engine event that allowed material to pass through the oil pump, through the oil filter, and then get lodged in the cooler.  I just don't find that to be a likely event.

I'll definitely post updates, and I also plan to cut up a 12-plate cooler that I cleaned a while back with paint remover.  I should have everything running "any day now".  :-)

I'm not picturing this set-up. Is there an outside wall for the o-ring to stop against? If not, won't it extrude right on thru the 5 degree taper?

On the side with the stock holes, the cooler has a raised "wall" to stop against.

On the other side...hmm?  ma-a-ay-be?  My expectation would be that the Viton ring gets wedged into the area between the fitting and the back of the inside cooler.  A few minutes of ground running would show if it leaked or not, but a groove would have been a much better solution.

It's an easy fix.  I'll make up a new set of fittings this weekend.  Thanks, Marlin.

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