2-1/2 HP Ottawa

Peter Rooke makes headway on his 2-1/2 HP Ottawa, focusing on the piston, cylinder head and connecting rod – Part 2 of 4.

| October/November 2013

Ottawa Engine

The 2-1/2 HP Ottawa as purchased.

Photo By Peter Rooke


Rust damage to the piston on the 2-1/2 HP Ottawa was minor, but after cleaning the rust off with fine emery cloth I noticed a hairline crack around the wrist pin area. The crack looked fairly old and had not been caused when I removed the piston, as this part of the piston had not been put under any pressure. Looking at the inside of the piston casting, the holes for the wrist pin had not been bored centrally and one side of the wrist pin support was extremely thin at the point where the crack started.

I did not consider repairing the piston as a long term solution because the crack was close to the wrist pin. I also didn’t think it was worth the risk of a repair failure that might result in major damage to other components of the engine. This left my options to finding another Ottawa piston, locating any piston that would be a good enough fit or making a new one. I’ve made pistons for model engines in the past, so this was a good time to try something new and make a full-size one. To get a casting to reduce machining would involve using the old piston as a pattern by sawing it in half, then making a pattern core for the inside. The alternative was to machine one from solid, the cost of a cast iron bar probably being cheaper than the cost of getting one cast.

A week later a block of cast iron arrived from my usual steel supplier and my goal of producing a bucket of swarf was about to commence. Before starting “between center” turning on the lathe (when complete accuracy is essential), I checked the alignment of the tailstock. I used a length of 1-inch diameter steel for the test bar with two thick washers 2 inches in diameter fixed about 9 inches apart. I put the test bar in the 4-jaw chuck, making sure it was accurately centered, then centered the tailstock before taking a light cut off each washer. I then measured both with a micrometer. If there was any difference in their diameters I moved the tailstock to compensate and repeated the test until both washers were turned to exactly the same diameter, indicating that the tailstock was correctly aligned.

Taking the early cuts on cast iron can be difficult, as inevitably there will be grit and other impurities cast in the skin layer that soon ruin the lathe tool, even dulling a carbide composite tip. First, I set up the block of cast iron in the 4-jaw chuck to run true and drilled a center hole in one end so the tailstock center could be used. Then I faced off the block at this end before turning it to an oversize diameter of 4.300 inches, up to the point where it could be held in the jaws of the chuck. After I reversed it in the 4-jaw chuck and set it to run true again, I drilled another center hole before trimming the piston to length plus 0.100-inch. Then I turned the remainder of the outside down to 4.300 inches. I made a start to bore out the skirt area, taking light cuts as the piston was only held by the 4-jaw chuck.

Next, I measured the old piston and used these measurements to drill a 0.875-inch hole through the side of the cast iron for the wrist pin. I took measurements from the skirt with the additional allowance of 0.100-inch left at the crown end in case it was damaged during the frequent changeovers from the mill to the lathe when machining.

I then cut two 2-inch lengths of 0.875-inch steel to fit in the wrist pin hole and cross-drilled a hole in each one for mounting bolts. I clamped the crown of the piston to the faceplate using these fittings and took a little time to align the piston so it ran true.