Today I got back to making parts for the Wallaby, these are the tappets. They were a pretty straight forward lathe job, I used a 1/8″ ball end mill to make the pocket for the 1/8″ push rods. These are made from 5/16″ drill rod, the shank is turned down to 3/16″ for a smooth fit in the tappet guide. I have not decided whether I will harden them or not.
I have been distracted from the Wallaby by work in my new design, a 97 cu in Offenhauser Might Midget engine. I have been toying with the idea of creating a YouTube series on its construction. Below is a video of the machining the inside of the Wallaby piston on a CNC router.
Sorry, I misspoke. The tappets are made from 5/16″ drill rod, the shank is turned down to 3/16″ for a smooth fit in the tappet guide. I used a 1/8″ ball end mill to make the pocket for the 1/8″ push rods.
Charles, what you describe about the size of the tappet head is very interesting. Below is a cutaway of the engine:
And below is a close up of the cam lobe contacting the tappet as the lift begins/ends. I can see what you describe, the tappet head is too small and the cam lobe is pushing against the edge of the tappet, not the face.
And with the valve full open, the tappet needs to rise into the hole above it, which now is .406″. So your suggested 3/8″ should fit at full lift.
Now with your suggested 3/8″ top:
Closeup
Even the intake lobe with the smaller cam lobe nose radius pushes on the bottom of the tappet, not the side.
Below is a closeup cutaway on the other axis and it can be seen with the larger tappet face, we are still OK.