I run a plenum as well, and have had my share of cracks. I used an RV-10 baffle kit as a starting point, but had to modify or fabricate some of the parts to fit my engine (and the stretched -6 cowl). The plenum is a modified RV Bits -10 plenum, with added carbon fiber layers inside and out.
My cracks have been around the aft-mounted oil cooler, though they are different than what Bill and others have shown. I beefed up the heck out of that corner at Cyl #6, including a doubler on the face of the #6 baffle (you can see it under the standard angle stiffener), a doubler on the aft face of the aft corner baffle, and a very robust corner build-up.
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I did buy correct-length bolts and got rid of all those washers…this was in the baffle re-build and fit phase
I also have an inner brace running from the aft baffle, just above the cooler, to an upper case-half bolt.
With all that strengthening, my cracks are, IMHO, evidence of transfer of fatigue/failure to weaker points. In my case that has been to the curved baffle pieces that go around the #6 cylinder barrel. Failure 1 occurred where the outer curved piece intersects the oil cooler mount. Sheared right off.
It was either make that entire corner over, and risk a repeat, or work out a doubler and alternate mounting method. I chose the latter, and did this:
It mounts to the bottom of the oil cooler standoff bottom flange, with a little rubber layer as a shock absorber, and a thin coating of RTV on the inner face of the curved section (a Paul Dye suggestion). I also had a chronic loosening of the two 1/4-20 baffle bolts on #6. I added the fillister head bolt seen in the last pic above and safety wire it. Both of these fixes have held up well.
However, this Condition Inspection revealed the curved inner (lower) baffle piece on #6 just broke as well…likely another slight shift in failure point…and very frustrating! Especially since this particular baffle piece is a real bugger to remove and install.
I scabbed this together…rivet locations dictated by crack location...but it's (as Cheryl Crow would say) "plain ugly to me"…
...so am ordering new copies of those two aft-left baffle parts…in fact two of each, to see if I can double up the places where these have failed. I am also thinking a shock-mount for the cooler would be a big help here. Next engine gets a firewall mounted oil cooler for me!
All that being said, I would think the plenum would be a strengthening measure, as it would, I'd think, keep the baffles from stretching/flexing outward under the pressure that Dan described. Perhaps the rubber baffle material in a stock setup would absorb that pressure. At any rate, I think that many baffle cracks, if not most, are from vibration…especially if the parts are under any preload when installed (forced to fit…another lesson from Paul!)
If anyone has good fixes for my failure modes, I'm all ears! Thx!!
Cheers,
Bob