Conrods and Pistons

Restoring an Amanco 2-1/4 HP Hired Man

| February 2006

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.