Plenum Progress

Here’s a bird’s-eye view of the new plenum pieces painted and installed. I’m very happy with how they turned out. Now it’s just a matter of connecting the dots–joining the original plenum with the new pieces by fabricating a fiberglass lid.

Once again my good friend Phil came to my aid with my fiberglass work. He’s got more fiberglass experience and tools than any sane person ought to have. Using Phil’s equipment and expertise, we made a vacuum-bagged layup consisting of 2 layers of cloth, a 1/8″ layer of foam and another 2 layers of cloth. To complete the vacuum bagging package, we laid on a layer of peel ply and then a layer of breather cloth. The whole mess then goes into an airtight bag and a vacuum is drawn.

This little pump supplied the vacuum. We placed the whole package on the top of my engine with bags of lead shot to hold everything down nice and tight against the existing plenum and my new metal pieces. It took about 6 hours to cure to the point where I could remove the vacuum. I left the weights and everything else in place until the next morning.

I was back at it again first thing Saturday morning. This is what it looked like as I started pulling the peel ply and breather cloth off the cured part. The gold color is the foam core as the 2 layers of fiberglass are pretty much transparent once all the excess resin is sucked out. The piece is surprisingly light.

I laid the piece back on the engine to get a feel for just how much I’d have to trim. It fit surprisingly well, but will obviously need to be trimmed.

With the front-to-back trimming complete, I next marked strips along each side. I’ll use a Dremel tool with a small cut-off wheel to carefully remove the first two layers of fiberglass from the strips along the edge. This will allow the fiberglass, without the foam core, to rest on my new metal flanges.

Once the first two layers of fiberglass were removed, I just had to scrape the foam core out to leave the remaining 2 layers of glass exposed. This wasn’t a difficult process.

To finish the flange, I used a Dremel tool with a cone-shaped grinding tool to grind a 45 degree bevel in at the edge of the remaining foam core along the length of the flange. Even using the vacuum process, it’s easier to get fiberglass cloth to lay down over something less than a sharp 90 degree angle. After the bevel was cut, I applied another 2 layers of cloth tape along the flange so that the finished flange is 4 layers thick. 4 layers of cloth wouldn’t be enough for the entire plenum (without the foam core), but it’ll work well for the flange.

I drilled and clecoed the new fiberglass piece in place. I laid the blue tape on a flat surface and made my marks for the holes so they’d be equally spaced. It’s tough to determine the spacing on a curved surface so the tape works well. I drilled through the new plenum and into the flanges of my new baffle pieces. I also drilled and clecoed a few holes through the new plenum into my old plenum pieces.

I took the entire plenum off as a single piece. My plan is to remove 2 layers of cloth and the foam core where the new piece will mesh with the old plenum halves. This should make it easier to graft the new piece to the old pieces and create an invisible transition so that the entire plenum will be a single piece. Obviously there are some openings in the corners that I’ll have to address with additional wet layups.


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