Rocker Arms, Rocker Boxes

The original plan calls for exposed rockers without rocker boxes. While this is authentic for the 1920's, I was desiring an engine with more of a late 1930's appearance. The design of the rocker boxes was carried out while the head design was prototyped so as to avoid complications later.

Like the heads, the rocker boxes were cast. The design of the head called for a rocker box with a width of .625". I made this pattern so as to produce a finished rocker box of .610, with the heads trimmed to present a flush mating. While I could have cored the rocker boxes, or cast the lid separately from the body, I elected for simplicity to cast them as one piece, with the plan being to slit off the lids. This pattern is shaped from a synthetic wood which has no grain and carves beautifully.

I wish I had photos of the many machining steps necessary to generate the final shap you see. The blank rb's were drilled and tapped 2-56 (the 3 small holes in the body) and attached to a fixture which placed the rb's every 2 inches exactly. With the left edge of the fixture located, I could drill/mill each rocker box using the mill DRO to increment. Tooling changes and setup are what slow an operation, and when you need 18 off, whatever fixtures you can design are sure to help!
A rather blurry but acceptable photo of the rocker box and lid assembled with scale hex bolts.
These are 2 of the very tiny rocker arms, with the bottom rocker having a built-up aluminum and steel pushrod inserted for clarity. The dark color is a mottled purple which is the result of the heat treatment: A2 tool steel, hardened and tempered to 700 degrees f. for toughness. The rocker pivot pins are of the same material. Note the tiny bronze bush, wall thickness .008"
Making 18+ tool-steel rockers this size called for some imaginative workholding. The lower jig is a "finger plate" which securely holds (and allows indexing by 4) of a blank rocker. The upper jig is designed to chuck a rocker in the lathe so the clearance cuts on the side can be made.

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