by peter anson » Fri May 12, 2017 9:21 am
Well I don't even have a VW engine, but my Jabiru does have aluminium cases, so does that qualify?
A couple of facts:
Aluminium has higher thermal conductivity than Magnesium so should spread the heat more evenly. (I read it on the internet, it must be true). Since the only way for the cases to get rid of heat is by radiation, which mostly depends on the colour of the cases (matte black good) and conduction, which is going to be mostly via the oil cooler, the difference in thermal conductivity is probably not going to matter.
Aluminium and magnesium have pretty similar coefficients of thermal expansion, but they both have much greater thermal expansion than the steel crankshaft, so the cases grow much more than the crank as the engine heats up. I'm not familiar with the arrangement of the thrust bearings in the VW, but it might be that your engine does not have sufficient clearance to allow for expansion of the cases relative to the crankshaft.
Peter
Sonex 894
303 hours