I am trying to figure out some of the details with regard to crank design and looking through various threads there seems to be a difference in opinion.
My main interest is 66mm stroke 2.0 litre cranks and particularly 906 Types.
Looking at a standard 66mm 911S/E crank the counterweights appear to be relatively small and not a full counter weight.
The latest 66mm Supertech crank seems to have full counterweights that are larger than the 2.0 litres and the 906.
As we only want the crank for 2.0 litre engines is there any reason to increase the counterweight size compared to the 2.0 litre?
Rod bearing diameters are also interesting. The large rod bearing of the 66mm crank will, of course have more losses than the smaller diameter used on the 70.4mm crank and it may be interesting to use a smaller diameter.
I have read that the 70.4mm crank has a wider journal?? And also that web thickness was increased but these comments seem to contradict each other.
I thought that bearing widths and hence web thicknesses would be the same for both cranks.
If this is the case then the reduction in stiffness in using a smaller diameter rod bearing is unlikely to cause edge problems as the crank would still be stiffer than the 70.4mm stroke.
As we will be needing to make rods we can use the NASCAR Bearings recommended by Steve Weiner and get away for Glycos.
The final question is concerned with cross drilling. I was considering grooving both end journals and cross drilling and grooving No4 main.
I realise Henry’s crank has all mains cross drilled but this is quite costly. Is just cross drilling No4 good enough?
The crank will be made in EN40B and gas nitrided so I think it will be relatively strong.
We have just modelled an H Beam Rod
We are planning to model an I beam and then compare weight to stiffness ratios before deciding on which design to adopt.
All comments gratefully received.