The Thermoil Story

Thermoil Engine

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R.R. 2, Haubstadt, Indiana 47639

In the fall of 1973 while traveling down a remote southern Indiana highway, I noticed an old tractor sitting along a side road. I drove down to have a look and it turned out to be an F-20 with a V-8 motor. Not being interested in that, I drove on by without stopping and went a few hundred more yards to turn around at the next place. Just as I was ready to turn around, there appeared two flywheels sticking out of the weeds. No one answered the door there so I walked over to take a look. It was a 6 HP Thermoil on the remains of a horse drawn cart. The neighbors nearby said the owner worked for the county and usually would be home at 4:00. I went back later in the day and found him. He had four other rather common engines so I asked if he would sell any. He said the only one he would part with was the Thermoil. We talked it over and he agreed to take two sacks of seed corn for it. On December 26, 1973 we went over with a boom truck and picked it up.

The Thermoil sat in our shed a long time, but we did nothing with it. It wouldn't run as it was. We finally disassembled it, fit it with new rings and had the wrist pin refitted. Then it sat for a while again. Finally in the summer of 1978 we tackled it with enthusiasm. We had it rebored and sleeved, had new valves made, welded a cracked water jacket, re-poured half of the crank bearing and made a new fuel tank.

Four coats of Red Spot dark red were applied and it was striped with a 3/16' black stripe and 1?' in from the edges. Decals were made by recreating the design with felt tip markers on letter paper, covering with clear pressure sensitive tape and attaching to the sides of the water hopper with carpet tape.

Since the Thermoil fires by the heat of compression, it takes a more vigorous effort to start it than a flip of the flywheels. Once it is properly primed, the book says to spin it vigorously 8 or 10 times, remove the crank and push down the compression release lever and away it goes.

Our history on the Thermoil is sketchy, but I believe Sears must have started marketing them about 1915. At first sizes were 1?, 2?, 5 and 7 HP; then came the 1?, 3, 6 and 8; then finally there was the 1?, 3, 7 and 9 with production ceasing about 1927. They were made primarily by Hercules at Evansville, Indiana, but Cummins at Columbus, Indiana contracted to make 4500 of the 3 HP. That venture almost broke Cummins.

The Thermoil runs best on kerosene, but it is claimed that it will run on any fuel light enough to flow. Although it fires by compression, it is not a true diesel because the fuel falls in rather that being injected under pressure.

In addition to the Thermoil brand, there were a few made with the Cummins brand and toward last some green ones with the Hercules brand.

The one we have is a 6 HP model U. The serial number is 8475. It is said to have been purchased right after the owner got out of the army after World War I. We guess it to be a 1917 or 18. The Sears catalog lists the shipping weight as 1230 pounds.