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A little voice in my head insisted that I should pull the cowling off to have a look. It wouldn’t go away, so I gave in and removed the cowling late Wednesday afternoon. I’m very glad I did. I discovered that the aft outboard edge of the air inlet on the left side of the upper cowling was cracked, and almost broken off.

I’ve got an enclosed plenum chamber on top of the engine, and to minimize the air leakage between the cowling and the plenum I fabricated fiberglass and rubber transition pieces. Things are not symmetrical between left and right sides up front, as the engine itself is not symmetrical (the cylinders on the right side are ahead of the ones on the left) and the engine is mounted so the thrust line is offset to one side to help counteract the effect of the prop wash. The transition piece on the right side turned out very nicely, but the one on the left proved more troublesome. It ended up with a bit of overlap where the aft edge of the transition piece overhung the outer edge of the baffle side piece. This raised the possibility of interference when the engine rocked on the mounts during start and shutdown. I watched this area closely over the first few flights, and concluded that there were no signs of any such issue.

A few weeks ago I noted that the outboard rivet that secured the metal mounting flange where the transition piece attached to the cowling had pulled out. I diagnosed the cause as a combination of the engine heat softening the fibreglass and air loads. I replaced the rivet with a #8 screw and nut, using a Tinnerman washer under the screw head to spread the loads, expecting that this would be much stronger than the original rivet.

Well, it turns out that I misdiagnosed the cause of the earlier failure, and my repair was strong enough to transfer the loads to the next weakest point, with was the aft edge of the inlet on the cowling, which almost broke off.

Looking closely at the lower edges of the rubber on the transition, I can clearly see marks from multiple pinches where the rubber was jammed between the lower edge of the transition piece and the upper edge of the baffle side piece. So, work to do - repair the cowling and modify the transition piece so there is good clearance to the baffle side.

I repaired the cracked cowling on Saturday afternoon, and plan to modify the transition piece during evenings this week. I’ll cut off the outboard portion and graft on a new part to make it narrower so it no longer overhangs the baffle side, allowing much more room for engine movement during startup and shutdown.

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