Aermotor Redux

Unsticking a very stuck and rusty Aermotor 8-cycle water-pumping gas engine – Part 1 of 2


| August/September 2013



Broken Rocker Arm

The broken rocker arm and the partial wood patterns used to get the angle correct for recasting. 

Photo By Dave Irey

For five or six years I teased a fellow engine collector about his very stuck circa 1920 8-cycle Aermotor water-pumping gas engine. It had been used on a Texas ranch to pump water before it was taken out of service and allowed to sit outside for as many as 50 or 60  years. A flea market guy brought it up to Waukee, Iowa, in 2002, which is where my friend bought it. He tried for five or six years to free it up with no success. The engine was complete except for the rocker arm being broken off, it had no oiler, and the tin crank splash guard was gone. Finally, my teasing paid off, and I bought the Aermotor from him in 2008.

It had been soaking in penetrating oil for five or six years and, despite a very sincere effort, was still very stuck! I work in automotive repair and had made a slide hammer puller for pulling the front brake rotors off of front-wheel drive import cars. This was a big 8-pound slide hammer on a 7/8-inch shaft, 25 inches long. I took off the four-bolt head that fit to the brake rotors and made an adapter to fit the Aermotor connecting rod. This is a big hefty slide hammer and it proved too much for me to use on a long- term basis. I could only swing it a few minutes, so I went to a smaller one.

On an 8-cycle Aermotor there is no removable head, so the piston must come out the back end. This is why the slide hammer puller was needed. I used two brass blow torches and one acetylene torch for heat; a water hose was used to try to cool the piston. I would heat up the cylinder block and hammer a while until I got tired, oil it up again and start over. This took two full days; oftentimes once the piston breaks loose it will come out easily, but not this one! I fought all the way (it was stuck forward). WHAT A JOB, but once the piston was out and the cylinder was honed it looked good.

The cast iron piston, despite all the abuse it took, was OK. The ring grooves were square, so new rings from Starbolt were all that was needed. The Aermotor valves are in cages that unbolt from the main block and are small and easy to work on. The valve angle is 44 degrees and the seat is 45 degrees. There was nothing left of the valves; only rarely do you see valves this rotted. The guides weren’t much better, but from my automotive work I had saved old valves in a 5-gallon pail, ready to use again. In the pail I found two valves about the right size and with thicker valve stems. The 5/16-inch intake valves stem went to 11/32-inch stem size and the 11/32-inch exhaust valve stem went to 3/8-inch stem size. Then I bored the valve guides by drilling them out close to the correct size and carefully reamed the last few thousandths to a good snug fit. I then reground and narrowed the valve seats so the valves and seats fit perfectly with no leaks. The final part of the valve job was to drill holes in the valve stems at the correct height so the keepers and valve stem height are right. I carefully measured the valve stem height before I took them apart so I knew what height it had to be when finished. Despite my best precautions, I still ruined the babbitt in the connecting rod and had to repour it.

I just had to turn a new mandrel 0.03-inch undersize to go on my repour jig because of shrinkage when cooling. This left me some extra thickness to machine off when boring the rod beating to fit the crankshaft journal.

At swap meets and farm shows I scrounge old babbitt bearing to repour and reuse. This is an economical way of recycling old bearings.