Midget gas engine 1

I first became aware of this engine when one popped up on YouTube and I quite liked the look of it. The video said that the design was in a 1936 issue of Popular Mechanics which detailed making the patterns and a second issue had the working drawings and machining description. After a bit of Googling I found the first part on the web but not the second. However, the two parts were also published in Popular Mechanics ‘Shop Notes’ from 1938 which I managed to buy an old copy of.

The parts that needed the most work and that would make or break the project were the two crankcase halves so I started with them by exporting my design from Alibre to F360 where I used the CAM to create the codes for cutting the crankcases.

Rather than make nesting jaws to mill the inside I decided to use the lathe, firstly bringing the case down to thickness with the tailstock giving support as there was not much to hold during the interrupted cuts.

Followed by boring out the space for the crank and an undersize bearing housing. You can just see that I also included a small locating recess for a spigot on the other half, the original did not have this but I felt it would help keep the two halves lined up.

The rear half was milled out with a few changes to the CAM to take into account what I had learnt doing the front half, it is basically a mirror image with the addition of a boss for the vent.

This was then turned in much the same way except a spigot was left rather than the recess.

I also started cleaning up the machining marks with needle and riffler files, and Emery cloth.

The screw holes were reamed to 4mm and some dowels made with M3 tapped holes each end. These also help keep the two halves lined up and are again another feature of my own. It was now possible to hold both halves in the lathe to bore both bearing housings at one setting to ensure perfect alignment.

The next job on the crankcases was to mill off the last 0.5mm that I had left on the cylinder flange. To get things lined up I bolted a 20-40-80 block along the X-axis and clocked that true and with the ends of the main bearing and cam bearing housings against that the case was true in X&Y. A couple of 15-30-60 blocks supported the two mounting lugs.

The top was then opened up by drilling and then taken to final diameter and depth with the boring head. The four cylinder screw holes were also drilled and tapped M3 at the same setting.

Part one here Part two Part three Part four Part five Part six Part seven