Here is the beginning of the cutout in the side of the frame.
I have a little more grinding to do to get it symmetrical.
The wall of the frame is a little thin in spots, but still workable, and the beading that goes around the cutout in the frame will stiffen it considerably.
Here are some tips if you are considering making a scratch bottle engine:
1. Steel tubing can be purchased from Speedy Metals (or others) in various diameters and wall thicknesses up to 1" thick. Layout your stacked disks carefully, leaving enough extra thickness between disks. Use the thinest wall section necessary for any given disk.
2. "V" the disks to allow a full penetration weld, but leave a small lip to allow the disks to be accurately aligned.
3. Turn the curved surfaces on the insid of the disks before you weld them together to allow easy access to the interior. Minor smoothing of the interior faces can be done with a long boring bar after each section is welded on, but before the next section is added.
4. I used multiple passes for the welded joints, and I chipped the slag and wire brushed between welding passes.
5. You can use a hand held grider to smoothe up the welded surfaces a little before you even then up in the lathe.
6. I neglected to do this, but weld a round rod on the bottom plate of the frame so that you can chuck the bottom of the frame in the 3-jaw chuck. The square bottom of the frame can be chucked into the 4-jaw, but then can be tricky to get aligned.
7. I cut a round recess into the top of the bottom plate before you weld it to the frame, in order to get the frame exactly in the center of the bottom plate.
8. I should have grooved the bottom plate to make it easy to open up a hole in it, since the bottom of the engine will be open, and needs to be open in order to weld the frame from the inside at the bottom.
9. Make a top flange and weld it in place, and then make a temporary top plate the same size as the bottom plate, and bolt it to the top flange so that you have two flat surfaces to bolt to the mill table. Add a round rod to the top plate so you can chuck the frame up in the lathe. Use a centerdrill in each round rod at either end of the engine, so either rod can go in the live center.
10. The bottom of the engine will eventually be turned round, but don't turn it until all the milling work on the side of the frame is done.