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Holman Moody Cowl Plenum buck and initial metalwork

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  • Holman Moody Cowl Plenum buck and initial metalwork

    This is another repost of an ongoing project. As long as it's mine & already public knowledge, figured it should be here as well to hopefully help our traditional methods-

    Long story shortened- I like mid 60’s big Fords, especially 1967 Ford Galaxies. When I had my first 67 decades ago, actively trading musclecar parts led to a NOS factory 1966-67 Chevelle Cowl Plenum air cleaner assembly. That got me thinking. Along with many old pics and an article about John Vermeersch’s 1961 Ford Starliner showing a Holman Moody Cowl Plenum (some called them “batwing” but to me that’s a tri-five chevy part term) air cleaner on his car, I soon had an unused Holman Moody unit from the Thackston Brothers in Woodruff, SC- a local NASCAR privateer shop- hanging on my wall next to the GM unit. Both breather and then car were sold before coming together.

    John Vermeersch's 1961 Starliner with 427 SOHC and original HM Cowl Plenum unit

    After buying an original 428 1967 Galaxie in 2012, I decided that this one would have a real cowl plenum breather. Used original cowl plenum units basically are not available and current NOS asking prices are more than I paid for my Galaxie. So I would make an accurate one.

    There are a number of variations to the original versions. I’ve been collecting images for years. Accurate profile measurements from NOS and used cowl plenum units came from the remains of nearby NC racing legend Banjo Matthews’ parts inventory. Since my 428 is on the stand beside the car right now, another friend provided engine vs engine compartment measurements.

    NOS units from Banjo Matthews' inventory

    These dimensions facilitated full size 1:1 scale pencil & paper drawings of the cowl plenum assembly parts. Drawing included three cross sectional profiles- one for lid, top half and bottom half- across the common midsection of each part. Renderings with elevations clarify how they fit each other and allow accurate rotation parts/drawings as needed while keeping all properly justified. No CAD- I enjoy old fashioned drafting by hand, using old equipment I've collected for years.

    The original versions use a now-expensive & harder to source 16” diameter pleated filter measuring approx 3 1/4” tall with a unique wider top rubber to help locate and secure the filter in use. A nearly identical filter is available for approx $10 to fit a 1959 Cadillac air cleaner, which is popular in the custom car scene. It measures only 2 7/8” tall, so the design was amended to accommodate this difference and couple of filters to help guide the project. The rear dimensions were also narrowed slightly. The widely flared versions can interfere with both the wiper motor and VIN tag on firewall. Not cool.

    After an experiment with a radial hammerform (coming in separate build thread..) for the lid effort worked, it was time to make the buck. New 1” maple plywood quickly turned into a top, bottom and height spacers before being screwed together. #10-24 Hanger Bolts area used for the center pivot on all pieces.

    Using a plunge router with trammel that I modified for a past sculpture project, parts were quick to make accurately and entirely without a lot of additional finish work.

    To achieve the proper height measurement and leveling in studio, I use wedges alone or in pairs. For this job, I cut matching pairs of long thin wedges to adjust simultaneously. Once I had the right height, make sure they are both the same to assure the top and bottom are parallel. Without moving them, take a pair, pinch them tight and use them to adjust your table saw fence. Then cut a stack of extra long spacers. Measure, mark, pre-drill and screw everything together with the spacers protruding.

    Once together, excess was removed with band saw followed by a japanese-style pull saw to accurately cut a faired edge in two planes- both top to bottom and front to back- on each spacer. Very fast, very simple. These pull saws have no outboard kerf, so can be laid flat against the work. For <$20 at Lowes, they’re hard to beat.Minor sanding with a quick coat or two of urethane helps keep the buck clean and manage slivers.

    I didn’t open up the center of the top & bottom pieces of the buck (yet). The bottom of the originals were expected to be modified to fit whatever carb/intake is used. But I did produce each indexed to a common center pivot and installed the same size hanger bolt. This allows some transfer of parts between hammerforms and buck during production to monitor fit and I didn't want to lose that center pivot.

    The rear portion of the bottom half has some taper, so additional work was needed to develop the contours. Plywood is not as agreeable to shape and carve as real wood but the plies do allow an easy visual guide to balance and symmetry of shape.

    The trailing edge of the bottom piece was left long to better allow the shaping in plywood. At the end, I trimmed off the excess flush. The buck is approx 1" longer than needed but figured it should be simpler to trim the part to fit the car than add to it...

    While paint was drying, I made a quick basic spacer to fit upon the intake in place of the carb to check the fit the buck on the engine. The distributor was a little crowded on the shorter original stock cast iron “S” intake on my 428, but the scheduled C7 428PI intake is approx 1 1/4”- 1 1/2” taller so all is good. Very cool, especially with original Ford 427 pentroof valve covers. It will look even better with the correct long 427 cast iron exhaust manifolds waiting on the shelf…

    The project is ongoing and well under way. The buck was the second stage of the project, done after the radial hammerform-lid experiment worked. Another pending thread or two will cover the hammerforms and overall build.

    This is my first buck and major metal shaping project. It comes after literally 30 years of reading, studying and following this work while building my shop & studio doing other work. Lots of the big names, as well as many here who have generously shared information and threads about their projects, had a part in motivating me. No matter, I give the most credit to both RockHillWill and MP&C Robert McCartney for lighting the proverbial fire under my feet. Otherwise, this project would still be being planned instead of started & well under way.

    Thank you, sirs.