Tuesday 27 April 2021

The AJS V-Twin Crankshaft: Part 2 – Connecting Rods & Pistons

Design of the crankshaft and, in particular, the calculations for crankshaft balance, requires the weights of various reciprocating and rotating components. Part of the input to these calculations is the weight of the connecting rod small ends but before these could be measured some work was needed to make them fit the pistons. Apart from the small-end diameters being different the connecting rod was too wide to even fit between the small-end bosses of the piston, illustrated below.

As can be seen, the connecting rod small-end width is 0 .027” wider than the available space.

The connecting rods were reduced in width to 0.896” by removing equal amounts of metal from both sides to give a total clearance of 0.14”. The small-end bushes were pressed out before machining.


New phosphor-bronze small-end bushes were made to replace the HD bushes and to fit the Ducati piston gudgeon pin – the new ones and originals shown below

These were then pressed into the HD connecting rod small-ends and which now fitted properly to the pistons.

Weighing could now begin….

Weights are required for the piston(s) (which are identical), the crankpin and both small and big-ends of both connecting rods. A couple of pictures below illustrate this.

The question arises as to how accurate are the scales? I don’t have any precise weights to calibrate them and so I machined a short length of EN24T round bar as a test piece and measured the weight of that. The weight (more strictly, the mass) can also be calculated from the product of the volume and the density, which is quoted as 7.84 g/cm3. It turned out that the 2 values of mass – ie that measured on the scales and the calculated value, were identical. It could, of course, be possible that both are wrong and have compensated exactly to give the correct result but that is highly unlikely.

After making a number of repeat measurements, the following table gives a summary of the results. All masses are in grams (g)


   “Knife” Rod + Bearing

    Big-Eng         Small-End


        “Fork” Rod + Bearing

      Big-End           Small-End











Additionally, the connecting rods + bearings were weighed individually and the masses compared. These were: “Knife” Rod + Bearing = 603 g; “Fork” Rod + Bearing = 758 g. The correcting differences (+ and -) were distributed in the ratio 0.6 to big-end and 0.4 to small-end.

Other masses are:

Big-End pin = 515.7 g

Nuts (both) = 97.6 g

Piston + rings + circlips = 373.2 g

All of the information was now available to start detailed design of the crankshaft.

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