Baffle seals are evil—and they are sneaky! They sit under the cowling, directing airflow through the engine cooling fins for years and hundreds of hours. They start out new, fresh, and flexible, conforming to the shape of the cowling with a spring in their step. But as the years go by, they age, get less flexible, and begin to let that all-important cooling air escape. Gee…old, less nimble, and stiff—maybe they aren’t sneaky—maybe they are simply aging, like our pilot population! The good thing is that (unlike pilots) baffle seals can be replaced.
It’s not hard to tell that this seal is past its prime; it has taken a set and cracks are quite visible.
At any rate, aging baffle seals are becoming more of a problem for homebuilders who fly a lot and keep their airplanes for years. My RV-8 is a good example. First flown in 2005, it now has almost 1800 hours of hard flying on the airframe and engine. The baffle seals are original, and keeping the CHTs down in the reasonable range has been getting harder and harder. Noting that I have to take more care in climbs, I have thought about timing (the engine has electronic ignition, and advancing the spark generates more heat), air flow changes, and even wondered if my recent home base change to almost 5000 feet MSL has contributed to the problem. While the answer to those questions is probably “yes,” the main reason we have seen higher temps is probably the oldest one in the book—worn and stiff baffle seals that allow cooling air to escape from the upper plenum area of the cowl.
The fluting in this forward baffle might be causing a good bit of air to spill out the front of the upper plenum chamber.
A careful examination of the seals gave proof of the problem. With a piece of new baffle seal in one hand, fingering the old ones showed a marked stiffness compared to fresh material. It was also apparent that in several spots, the seals had taken a “set,” and were no longer conforming to the cowl. Finally, a close examination showed cracks and checking where the material was required to make a tight turn—sure signs that it was time for a replacement. The good thing about the Lycoming installation on an RV is that you can do all of the work with just the top cowl removed; very little disassembly is required. If you have a taildragger, putting the tail up will make the engine compartment much easier to reach, and a short step stool or work platform will make the work almost luxurious.
Tools and Materials
Before you start, make sure that you have plenty of new baffle material and fasteners appropriate to the job. Van’s design uses large-head pop rivets, which are cheap and easy to work with. Others use screws and backup plates. I have seen production airplanes that simply used staples. But using steel staples in aluminum baffles is a great way to ensure disposable baffles due to corrosion. If you find that to be your design, you might think about upgrading. You will need surprisingly few tools to do the job. In the case of pop-riveted attachment, I used a drill with a #30 bit, an angle grinder with a sanding disk, an automatic center punch, dikes, and a pop-rivet puller. Throw in a razor knife and a few Clecoes to help cut the new material, and that’s it.
There are, of course, numerous choices of baffle seal material. Most material comes in three- or four-inch wide rolls. I actually prefer to use sheet material because it allows more complex shapes and fewer segments. I have used orange silicone, blue silicone, and standard black material; it all seems to last about the same number of years, so long as it is the stuff that is fiber reinforced. Van’s sends a roll of the black material about 12 inches wide with their kits, and one roll is more than enough to do a standard four-cylinder Lycoming installation.
The rivet on the left has had the mandrel popped out with a center punch; the one on the right still has the mandrel in place. Trying to drill the head off with the mandrel in place is difficult.
A sharp automatic center punch is a great tool for punching out mandrels. It might take a couple of clicks before they pop, but they will eventually come out with vigor.
Out With the Old
Most baffle seals are installed in segments, and this makes it easy to simply work your way around the rim of the baffles, replacing one section at a time. Drilling out old pop rivets requires a few simple tricks. Start by popping the old mandrel out of the middle—use an automatic center punch with a small, sharp point inserted from the head side of the rivet. A few snaps and the remains of the mandrel should pop out the other side with great velocity. Safety glasses are suggested! Getting rid of the mandrel leaves only the soft aluminum head and collapsed shank. Drill from the head side with the #30 bit, and the head should come right off—the hole in the rivet will center the drill nicely. Once you have drilled off all of the heads holding a given segment of material, you can zip off the seal material, and only the shanks will be left in the baffles.
Remove the rivet shank using a sanding disk to grind the part that was on the baffle seal side. Grind it down flush to the metal baffle.
After grinding off the shank on one side, use diagonal cutters to grab the remaining portion and twist it out of the hole.
Removing the shanks is simple: Use the angle grinder with a coarse sanding disk on the head side of the baffle to grind them down flush to the aluminum. It’s OK if you “polish” the baffles a little—it will be covered by the new seal material, and you won’t see it. With the shank ground down flush, grab the tail of the rivet with a set of dikes from the other side, and it should pop right off, leaving a clean hole that has not been enlarged by drilling through.
Old versus new—one method of duplicating is to trace the old seal onto the new material and mark the rivet hole, then cut and drill the new piece using a razor knife and drill bit.
A precise method of duplication involves drilling the old seal onto a fresh new piece, then Clecoing the sandwich into a piece of plywood.
After you have put a Cleco in every hole, you can simply trim around the old seal and voila—the new one is ready to install!
In With the New
Duplicating the old segment of baffle seal can be tough if it is warped out of shape, but the task is made simple by using a piece of sacrificial plywood and some Clecoes. Lay the old piece on top of a new strip or sheet (making efficient use of the sheet—think ahead to make the best use of your material) and drill through one of the old rivet holes, through the new sheet, and into the plywood. Cleco the sandwich to the board. Now lay the material flat and drill the next hole—Cleco it as well. Continue until you have filled all of the holes with a Cleco, and the sandwich of old and new should be relatively flat. Now take a new razor knife and cut all the way around the piece, pushing it flat as you go around the edges. You should now have a perfectly duplicated segment—with holes ready to reattach. If you built the seals originally and remember how much time went into figuring out how each one should be shaped and fit, this is going to seem like magic—or cheating. All you have to do now is pop rivet the segment back into place and move on to the next one.
If you don’t have a large-head pop rivet, or need one that is longer than you have in stock, you can use a long rivet and a washer to make a substitute.
If you ever thought that you had left your seals a bit short, this is the time to lengthen them. Just trim a little outside the lines on the top edge when cutting the new pieces. When you get to overlaps with adjacent pieces, you might leave a couple of rivets out, then move on to the next segment and deal with those rivets when installing the next one. I recommend doing one segment at a time, rather than drilling off all of the old baffles and then trying to remember how the entire jigsaw puzzle fits back together. If you really want to do it that way (for instance, if you are thinking of painting the baffles), take lots of pictures before you remove everything so that you can go back and remind yourself how they fit together.
Note the large overlap between the forward and aft pieces of seal material. Overlapping in the direction of airflow will help to seal the joint.
You can join a corner to make it sit the way you’d like using a long pop rivet and two washers (one top, one bottom). The drilled-off heads of old large-head rivets make great washers.
Front baffle seals can be a mystery, but doing a replacement means you don’t have to figure them out—just duplicate what is there (assuming it works). This one is made from two pieces for each side.
Mind the Gaps
I find that after seals have been installed, there are gaps that need to be filled between the base of the seals and the baffles themselves. This is why they invented 500-degree silicone RTV! You can spend a lot of time trying to get seals to lay perfectly flat, and they will still gap when the baffles change shape as they heat up, so silicone is your friend. Use black or red (or blue) to match your seal material and look carefully for holes and gaps. Fill them all—it takes only a few small holes to lose a significant amount of cooling air. Also make sure that you have left generous overlaps between segments of seal. This is no place to skimp, or you’ll lose a lot more air out of these cracks and gaps.
New perimeter seals all around. These will make the cowling difficult to install until they shape to their new home—but this is a good thing because it means they are sealing.
Baffle seals are consumable items—they need to be replaced every so often based on condition. Good seal material will last more than five years, but probably less than ten if you use your airplane a lot. More severe temperature exposure will wear them out more quickly, and it pays to keep a fresh piece handy to compare to your installed seals during annual condition inspections. If you think your seals are getting stiff, they probably are. We lowered CHTs about 20 degrees across the board with a fresh set of seals on an 1800-hour installation. If your temperatures are getting frustrating, this might be a quick way to get those degrees back. Total time for a complete redo was about four hours in the shop—very little time for a very large gain.