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They're mounted underneath each engines on the JU-52. I saw them up close at the museum. Each one consists of bundled half-inch pipes running through the whole length, open at both ends.

Ju-52

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The three cylinders hanging below each engine are the oil coolers - this page has a much closer shot which I've reproduced below.
JU-52 oil coolers
Oil enters and exits the cooler through the two vertical lines and swirls around the hollow air channels. The cold air in flight moving through the tubes cools the oil, which is then returned to the engine.

Courtesy of eBay, here's a photo of an old, rusted-out cooler where you can clearly see the air channel tubes inside. (Using an oil cooler in this condition would promptly empty the engine of oil...) Almost-Cutaway oil cooler

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Why is the oil cooled separately from the engine blocks? The Townend rings around the engine focus the airflow around the blocks - that's where the oil cooling should take place, it seems. –  Sebastian Henckel Jul 10 at 20:43
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@SebastianHenckel On most air-cooled aircraft engines you'll find some kind of oil cooler assembly. While we call these engines "air-cooled" that's really only true of the cylinders and cylinder heads: the rest of the engine is really liquid (oil) cooled. Splash lubrication draws heat off the pistons into the oil, and the oil cooler dumps that heat to the outside air. –  voretaq7 Jul 10 at 21:58

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