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.

Ottawa Engine

The 2-1/2 HP Ottawa as purchased.

Photo By Peter Rooke

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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.

Hollowing the inside of the piston proved to be a lengthy job, particularly through the interference of the wrist pin supports and the projection for the oil drip to the connecting rod little end. I first used a 1-inch drill through the block of cast iron, stopping short to leave a projection in the middle of the crown around the center hole.

I removed the piston from the faceplate and clamped it on the milling table. Fortunately I had acquired a 6-inch-long 12 mm carbide milling cutter in a job lot of tools. This enabled me to square off the inside ends of the wrist pin supports so there was a parallel gap of 1.620 inches between them. Next, I removed any surplus iron from the inside wall of the piston between the pair of them as well as around the oil drip projection. After this roughing out, I again remounted the piston on the faceplate to run true.

I enlarged the central hole along its whole length of the piston to the diameter of the gap between the wrist pin supports, 1.620 inches — this was the easiest part of the hollowing. Then I opened out the section between the base of the skirt and the underside of the wrist pin supports to an undersize 3.500 inches, to be finish-bored later.

Next, I needed to hollow out the piston from the wrist pin supports to the inside of the crown. With the interference of the wrist pin supports, I needed extra-long cutters for the boring bar along with different styles of cutting tips. To prevent the cutting tool chattering, I used some 1-inch diameter steel rod for one of the boring bars. In view of its size I could only clamp it to the side of the tool holder, as it could not be held in it.

This was a time consuming operation and I was constantly changing tool lengths and boring angles to work around the wrist pins. When I completed this stage, I finish-bored the section between the base of the skirt and the wrist pin supports to 3.750-inch diameter apart from a small 0.250-inch lip left at the base of the piston where the diameter was 3.625 inches.

I clamped the piston on the rotary table, which had been fitted to the milling machine so that the inside cleaned up with the long milling cutter. I profiled the edges/sides of the wrist pin supports and drilled and tapped the hole for the 0.375-inch wrist pin clamp bolt.

I cleaned up the inside of the piston with rotary burrs, rounding the edges of the projections inside. Then I drilled the oil drip hole, 0.1875-inch diameter.

I set the piston on one side for a couple of months to rest and stabilize. In any event, I couldn’t do much more until the new piston rings arrived.

Some time later I started the final processes to finish the piston. I mounted the piston on the lathe using the 4-jaw chuck and then trimmed it to length, removing the final few thousandths of an inch of surplus metal left on the piston crown. Then I turned the piston to its overall diameter, 4.130 inches. This was 0.005-inch larger than the original, in part to compensate for uneven wear in the cylinder bore.

The final step was to cut the four piston ring grooves a width of 0.3125-inch, the new rings having 0.015-inch clearance built in. Once I cut them, I reduced the top three lands in diameter to give the piston extra clearance to allow for heat expansion. Tapering from the top I reduced the diameter on the first land by 0.017-inch, the second land 0.012-inch, and the third and fourth lands 0.009-inch. I took these measurements into account when cutting the depth of the ring grooves, being the thickness of the rings, 0.170-inch, plus a clearance of 0.005-inch under each ring.

I drilled a shallow depression on the outside of the oil drip hole through the piston with a countersink drill to catch the oil. The final step after machining for the piston rings was to cut rounded oil grooves, 0.0625-inch wide and deep, between the piston ring grooves and the depression in the piston for the oil drip.

Once I had completed all the machining I weighed the new piston and found it to be nearly 8 ounces heavier than the original. I used the rotary file to further shape the wrist pin supports to mirror the originals and thinned the skirt by 0.020-inch. The end result of the lightening was the new piston weighed 1 ounce more, a negligible amount.

Connecting rod

The piston end of the connecting rod was very rusty, with the head of the bearing lock bolt being badly eroded. After stripping it down it was clear that, as with the wrist pin, its bronze bearing was worn, and it had been fitted with a metal shim to stop it from turning.

In view of the poor state of these items, I made a new wrist pin from a piece of 0.875-inch drill rod and a new bearing from leaded bronze. I tightened a new clamp bolt against old feeler gauges to find the best thickness of shim needed, still allowing the wrist pin to turn freely in its bearing. Then I made a metal shim to this thickness.

To finish off the connecting rod, I made new shims after scraping the bearing to fit the crankshaft. The white metal bearings of the big end appeared thick, although they required scraping to get a good, even contact with the crankshaft.

The main bearings had plenty of white metal but closer examination showed the crankshaft was a poor fit in them.

Someone had fitted shims at some point, but they were made from cardboard and falling to pieces, so after scraping to adjust the bearings I cut new metal shims for a smooth-running fit.

Cylinder head 

At the outset I made attempts to remove the cylinder head, but it was stuck fast. As the piston was also stuck, the usual method of pushing a spar of wood down the cylinder would not work.

I had already removed the securing nuts and squirted penetrating fluid down the studs. I replaced two of the nuts so they stood slightly proud of their studs. Then I rested a piece of steel bar on these nuts and set up the gear puller to push against this and pull against the rim of the cylinder head. This was by far the safest course to remove the head, with even pressure being exerted on either side of the head.

Once I had removed the head from the engine, I took off the valve spring keepers along with the valve springs. The inlet valve came out easily, but this was after I cleaned the surface rust off the stem and soaked it in penetrant. The exhaust valve was another matter. There was no sign of it moving after I applied moderate pressure, so it was set up in the large vise to keep pressure against the stem. I applied penetrant over the next few days while increasing the pressure. I had to take care not to bend the valve stem.

Still nothing moved, so I applied some gentle heat and attempted to gently turn it, taking care to ensure there would be no damage to the vulnerable stem guide. The stem guides appeared similar to those on Amancos, and I had seen these broken when trying to remove stuck valves. The valve stem was still stuck fast and snapped off, so the only solution was to drill the stem out.

I used a center punch to mark the middle of the stem and drilled a pilot hole with a 0.250-inch drill. I opened this up using larger and larger drills. When the edge of the hole appeared to be getting close to the valve guide I used the letter drills, as they only increase in size by a couple of thousandths of an inch. This enabled me to watch closely for signs that the wall of the valve stem had collapsed, which would happen if the hole was off-center deeper down. The signs of a collapsed valve stem are flakes of steel and/or rust dust, rather than clean steel swarf. As soon as this evidence was seen, I inserted a punch and hit it with a hammer to drive out the remains of the valve.

Both valves were in a poor state and needed to be replaced. Looking at the valve guides, they appeared to show signs of wear and rust. A reamer for their nominal size of 0.375-inch failed to clean up the hole, so I used the next size up, plus 0.0156-inch. This cleaned the rust out but there were still ridges in the guide, so I went a further size up to 0.40625-inch, then made oversize stems for the new valves to fit this size.

I turned some drill rod to 0.405-inch, a close sliding fit in the cleaned up guides. I cut two 0.500-inch long washers of steel 1.750 inches in diameter, drilling to be a tight fit on the stems before I brazed them in place. I made the valve stems long so that 0.500-inch of the stem would protrude through the washer. I center-drilled this so that the end of the stem could be supported by the revolving center in the tailstock, yet still provide room for machining the head of the valve.

Next, I trued both sides of each new valve head and reduced the diameter to 1.685 inches to match the original. Then I adjusted the compound slide to cut a 45-degree edge to the inside face of the valve. I accurately set this using the dial gauge rather than the scale in the lathe slide. To get a true 45 degrees the dial gauge had to register a movement of 0.707 inches (sin of 45 degrees) for every 0.100-inch movement of the compound slide. This was fiddly to set up and tightening the locking screws seemed to upset the reading, but with a bit of patience I set the 45 degrees.

I cut the taper on the valve head, then profiled the head to the stem being shaped using a round profile tool until it appeared to match the original. Then I machined the second valve. I held the valve stems in a collet chuck on the lathe with a run out of 0.001-inch, so good accuracy was guaranteed every time I removed and replaced a valve.

Finally I replaced each valve in the collet chuck, as close to the head as possible with minimal overhang, so that I could cut off the surplus stem at the head and shape the gentle taper on the end of the valve.

The next step was to fit the valves to the cylinder head. They were not a good fit with a lot of daylight on one side. I covered the first new valve in engineers blue before fitting then rotating. I covered the high spots on the seat with blue and scraped the metal underneath. I repeated this process until the valve seat showed a full covering in blue. I fitted each valve to a particular seat and then stamped “EX” or “IN” to identify it.


I stripped the mixer and, apart from a missing choke plate, it only needed stripping and cleaning. I made a new choke plate from some 0.125-inch thick steel sheet, along with a new securing screw. The face of the mixer was a little misshapen, so I used a file to clean it up and allow better contact with the choke plate.

In Part 3, Peter tackles the fuel tank, magneto, crank guard, muffler, pushrod and drip-feed oil pipe.

Contact Peter Rooke at Hardigate House, Hardigate Rd., Cropwell Butler, Nottingham, NG12 3AH, England • peter@enginepeter.co.uk