Conrods and Pistons

By Staff
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2 / 9
Starting to turn the leaded bronze for the little end bearing.
3 / 9
Piston cleaned.
4 / 9
Piston showing how shimstock is used to remove rings.
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Connecting rod cap showing finished bearing and grease groove.
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Connecting rod showing slight kink.
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Original gudgeon pin showing wear in the center section.
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Completed connecting rod with bearings, the braising joints visible.
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Scraping a main bearing cap.

The connecting rod presented a major problem as it was badly twisted and there was also a crack in the casting, which ruled out heating and straightening. I searched for a replacement but had no luck so after six months I decided to try and make a new one.

To simplify the process and work within the capabilities of my workshop I decided to fabricate a new connecting rod from steel in three parts, rather than from solid. The connecting rod itself is generally under compression, so I expected pegged, braised joints to hold, as my engine had a poor bore and would not be working under heavy load. Some were of the opinion that this was a dangerous step, but to date, after 50 hours of running, there are no signs of any problems.

Measurements were taken from the original, making allowances for the kinks. The small end was turned from 1-3/4-inch round stock, with an undersized hole of 3/4-inch bored in it to be finished off later.

The big end was fashioned from a 3-1/2-inch length of 2-1/2-inch by 3-1/2-inch steel, roughly shaped on the mill and finished by filing. Again, an undersized hole was bored in the block for the big end bearing and 7/16-inch holes were drilled for the clamping bolts, which were then turned on the lathe. The two nuts and their jam nuts were made earlier. The partially finished big end block was then cut in half on the milling table with a slitting saw and a center punch was used to mark both blocks at one end for correct re-assembly.

The central section of the connecting rod was made from 10-1/2 inches of 1-inch-by-1-1/4-inch steel and was again rough-shaped on the milling machine to provide the taper and create the “H” profile with two 1/2-inch round pegs machined at each end on the lathe. These two pegs were to locate in two similar sized holes drilled in the big and little ends with flats machined on these end pieces to provide a close-mating surface. These holes were dual purpose: to provide a positive location point and also increase the surface area for brazing.

The hole for the grease nipple was drilled in the top of the connecting rod and threaded 1/8-inch BSP (rather than the correct American thread) with a 1/4-inch hole drilled at 90 degrees from the inside of the bearing shell to join up with it to create the passageway for grease.

The big and little ends were then cleaned and fluxed, clamped to the center section ensuring everything was square and level, then brazed together. After cleaning off the old flux and braze, the rough connecting rod was set flat on its side on the milling machine table. The big and little ends were finish-bored without disturbing the settings to ensure both holes were parallel and the correct distance apart. Using a combination of a grinding wheel on a Dremel and hand files, the connecting rod was finished to its final shape.

In keeping with the original, a leaded bronze bearing was machined to be a friction fit in the little end. When fitted, an oil hole was drilled through the top with a 1/8-inch groove scraped in either side to within 1/8-inch of the edge to distribute the oil. 

Rather than change to a leaded bronze bearing, white metal was used again for the big end. While the correct method was to pour white metal into a mold created around a heated component, I elected to machine a cast bearing and to use a modern engineering adhesive to set it in place.

To make a mold for the cast bearing, I drilled a 2-inch oversized hole in a block of wood and made a 1-inch peg, smaller than the internal diameter of the bearing. A thin coat of fireclay was wiped inside the hole in the wood and the block was placed next to the stovetop in the kitchen to thoroughly dry overnight. The center peg was set in the middle of the hole, and wearing protective glasses, gloves and a thick overall I melted the white metal in a cast pot. While this was heating on the stove, I warmed the mold with a torch. I then poured the metal into the mold and left it to cool.

A hammer blow on a wood chisel on top of the mold split it open and the central peg was drilled out of the block of metal. Apart from some surface blemishes, it was perfectly formed.

The rough casting was then set up centrally in the 4-jaw chuck on the lathe and bored to 0.002-inch below the size of the big end journal and at the same setting one of the end faces was turned square. Next, the bearing was mounted on a mandrel and the external diameter turned to match the hole bored in the connecting rod, and finally the second end-face finished so that the bearing was the correct size to fit the width of the connecting rod. The bearing was finished by drilling a 1/4-inch hole for the brass tube that carries grease into the bearing.

Since I used a 1/16-inch slitting saw to cut the big end cap in two, I made a pair of temporary shims from 1/16-inch brass for when the bearing shell would be assembled. The bearing shell and big end were degreased using a commercial degreaser and the shims and bolts lightly oiled in case any adhesive was spilled on them. Adhesive was applied to the bearing, which was then put in position and the clamp bolts were finger tightened to hold it in place. 

The clamp bolts and shims were removed when the adhesive had set, and a thin blade hacksaw was used to cut the bearing in half, which was filed flush with the steel of the connecting rod and was then ready for fitting. 

I applied Engineer’s Blue to the big end bearing on the crankshaft, and the connecting rod fitted around it so that on removal the high spots were marked. These high spots were then removed with a scraper and the process was repeated until most of the bearing was colored blue. Most of the scraping was required around the edges of the bearing where the journal was contoured. An equal amount of shims of different thickness were cut for each side of the bearing with tin snips and 7/16-inch holes were punched for the mounting bolts. The shims were made thick enough to allow the bolted bearing to move freely on the big end without any play.

An X was cut in the top of the bearing around the grease hole and a single groove was cut in the bottom to allow the lubricating grease to reach most of the bearing.

As the main bearings for the crankshaft were in good condition they were not re-poured but were scraped after refitting the shims removed during stripping. To get a good fit it was necessary to reduce the shim thickness by a few thousandths of an inch.


The original piston pin showed some signs of wear, and while this would have been just about acceptable, I decided to make a replacement as new bearings were being fitted. 

Silver steel is generally accurate to a thousandth of an inch so a 7/18-inch stock piece was used for a replacement and no turning was necessary. The ends were rounded and flats were cut for the retaining bolts to match the original. Both the original square retaining bolts were badly rusted, so replacements and lock nuts were made.

The skirt of the piston was badly pitted where it had been exposed to the elements, but after considering the options of metal spraying, welding or sleeving, I decided to do nothing so there would be no risk of damaging the piston. The pitting would in any event be covered by black oil after a few minutes running. The old piston rings were removed using four pieces of shim. Two were already broken, and as the bore of the cylinder was badly pitted I decided to order and fit four new piston rings. I ordered piston rings and a name badge from Hit & Miss Enterprises.

The piston rings appeared to be the correct size but were checked by sliding one into the cylinder, using the piston to ensure that it was square and measuring the gap with feeler gauges. The gap was 0.006-inch, which gives adequate clearance when hot, but the rings needed 1/8-inch holes drilled in one end for the location pins on the piston. The ring grooves were scraped clean with a piece of broken ring and the burr in one of the walls was removed with a fine needle file and stone.

The cylinder itself was badly pitted, and for a technically perfect job needed boring and lining, but this could not be achieved with my modest equipment. I therefore decided to leave it alone and see how the engine worked before incurring any additional expense. 

Using the tin strips again, the rings were fitted to the piston and later, after liberally coating the piston and rings with oil, a spring steel piston compressor was used to slide the piston into the cylinder.

Tune in to next issue for part three, which will cover the making of the cylinder head, pushrod, igniter, exhaust and muffler.

Contact engine enthusiast Peter Rooke at: Hardigate House, Hardigate Road, Cropwell Butler, Notts, England, NG12 3AH;

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