From what I read in engineering circles a dowel is only supposed to locate things, and newer take any actual load, but it is recognized that they may help. Of course we know that they do make a difference. I guess our practice came from what Porsche did on the 356, since that had eight dowels, but what was Porsche trying to solve?
This is true, dowels are mostly used to align parts in the right position. Therefore, not much data is available on applications like this transferring shear force. Did however find something in the (German) car engineers bible, the 'Kraftfahrtechnisches Taschenbuch' from Bosch, 26th edition.
As earlier described, the dowels themselves are not an issue, manufactured from high tensile strength steel and maybe even hardened. It's the softer materials in crank and flywheel that suffer due to the edge pressure when shear forces are applied. The dowels have a sliding fit in crank and flywheel, the mentioned bible indicate allowable surface tension in the range of 10 N/mm2 in this case, the exact material condition of crank and flywheel being unknown to me. This means the dowels can take a torque of app. 8Nm provided all 4 take an equal share of the load. Double the number of dowels to 8 and you double to 16Nm, with even less chance of even distribution. Not really impressive...
Regarding the later Porsche 356 engines, they have 8 Ø6mm dowels which are 2mm shorter. They have the same sliding fit in crank and flyhweel. These can only cope with 8Nm. Don't know exactly why Porsche did this, but maybe to get some more crank end surface for the clever thing they did do; increase the torque of the center bolt to 450-500Nm on the 75, 90 and 95 DIN HP engines.
The situation is better if dowel pin holes in both flywheel and crank are precision drilled together and dowel pins then pressed in. Then you can transfer app. 200Nm with 8 dowels, but that's were you started anyway....
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