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A week ago my Zodiac had an oil cooler failure which caused me to do an off-field landing. I wrote about the event in this blog post. That mishap turned out well enough with a bit of skill, luck and significant help from friends.
It has taken a week but I now know what went wrong and feel compelled to share the information. The short story is “builder error”. The longer story explains how subtle an issue can be.
When I installed my new engine I installed it exactly like the old one including using the same size and type oil cooler, mounted in the exact same way. I up-sized the oil lines running to the oil cooler. The new oil lines and fittings are substantially larger. The purpose of this was to reduce the pressure drop in the oil cooler circuit. That worked out very well. However the new larger oil line fitting on the passenger’s side was making contact with the cowling.
How did I miss that? It was all too easy. It has to do with the way I install my cowling. I mount the lower cowling with the screws loose, make necessary connections, verify clearances, look for stray tools loose wires etc, attached the top cowling with screws loose, center the entire cowling on the spinner and tighten all the screws. I have done this about a zillion times in the past 4 years. Centering the cowing on the spinner generally calls for lifting the entire thing a bit and then tightening the screws. This brought the larger oil line fitting into contact with the cowling but I could not see that because the top cowling was in place. I did not suspect it because it was never a problem before.
What the heck, it worked for 20 hours before the cooler fatigued and cracked open.