Lower Baffle: How tightly wrapped?

How tightly should the lower baffle parts be pulled against the bottom of the cylinder cooling fins?
I'm fabbing the threaded rods that pull the lower parts of the baffle around the cylinders, and I can't find any guidance on how tight they should end up. Something tells me they shouldn't actually make contact with the fins, but I've been wrong before. Any specific gap to strive for?

The way to go there is to form them (bend them) where they naturally fit tightly and evenly against the cylinders. Then, you use the tie-rods (or your method of choice) to secure them in that position so air pressure doesn't push them outta place. If they are properly formed, the tie-rods don't apply much pressure at all.

Tight. The point is to maintain flow all the way to the exit at the fins in the spark plug and CHT sensor vicinity, and not leak out the sides of the wrap.

Tie rods are common, but they directly tie two independently vibrating cylinders, in addition to being in close proximity to (or contact with) the oil drain tubing. Safety wire (0.041") works well, although it does require the addition of a curved flange in a few spots.
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Impregnated fiberglass

A wrap of Ultra Black RTV impregnated fiberglass around the cylinder fins helps keep the baffle from leaking and they don't have to be quite as tight. I marked where the baffles cover the fins and applied the fiberglass to those areas.

What DanH said.

DanH said:

Tight. The point is to maintain flow all the way to the exit at the fins in the spark plug and CHT sensor vicinity, and not leak out the sides of the wrap.

Tie rods are common, but they directly tie two independently vibrating cylinders, in addition to being in close proximity to (or contact with) the oil drain tubing. Safety wire (0.041") works well, although it does require the addition of a curved flange in a few spots.
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Like Dan says - otherwise the cylinder fins will wear right through your baffles. Using safety wire is a good idea too.

wirejock said:

A wrap of Ultra Black RTV impregnated fiberglass around the cylinder fins helps keep the baffle from leaking and they don't have to be quite as tight.

Click to expand...

They kinda do, although gasketing seems to help with wear as well as leakage.
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I cut the thin baffling material supplied by vans to fit, then contact cement to the baffle. I tighten up to seal the material against the cylinder fins. This not only seals to direct airflow through the fins, it also protects against the cylinder fins wearing through the aluminum baffle due to vibration. I also use a small compression spring on the baffle tie that allows a bit of expansion/contraction of the cylinders while still maintaining tension of the baffle.

Ooh, like the idea of using a spring

I'm seeing some silicone/glass wraps which appear pretty rough. Here's the technique...

Lay out some 4 or 6 mil (i.e. heavy duty) plastic sheet. Lay a ply of 9 oz plain weave fabric, then squirt Permatex Ultra Black (aka Loctite 598) on the fabric. Lay another sheet of plastic on top. Now roll back and forth over the sandwich with a rolling pin, chunk of pipe, whatever you have, working the silicone down through the fabric. Flip the sheet to see how you're doing; it's clear plastic sheet, right?

When the fabric is fully saturated, roll hard to work excess silicone out to the perimeter of the sandwich, off the glass.

Clean the edges of the fins with MEK or acetone. Measure the exact size of the fin area to be covered. Draw the shape, exact size, right on the top plastic sheet. Cut the shape with your shop scissors, the end result being a plastic/glass/plastic sandwich which exactly fits where you want it. It's not sticky or messy, so hold it in place to check fit. Not perfect? No problem, trim to suit.

When the fit is good, peel the plastic off the side which will stick to the fins and smooth it into place. Leave the plastic sheet in place on the outside until the silicone cures a day or two, then peel it off. The result will be neat and smooth, with no strings or blobs.

Don't want air sneaking between the silicone/glass wrap and the aluminum wrap. If they are not pressed firmly together, a tiny bead of sealant will block the path.
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Howdy everyone. I am ready to wrap the baffles around the cylinder fins. I like Dan H's idea, but it is not clear to me how the safety wire is twisted to draw the aluminum tight against the fins. I don't understand how to twist the wire, draw the baffles tight to the fins and then secure it in place all together.

I have also seen people use short pieces of welding rod at the ends of the safety wire to span the width of the baffle flange. Again, not sure how to install these rods and twist/draw the baffles to the fins and secure.

Much obliged.

Mike, you hand form the baffles to be tight. The wire just keeps the pressure differential from spreading the lower baffles in flight.

You use safety wire pliers to twist the wires.

Thanks Kyle. I have been building this plane for 6 years come December; I never thought about bending the baffles to fit first, then wire to hold in place! Funny how the mind works...and doesn't!