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VW-derived aero engine | Selecting an Engine Case

The standard VW Type-1 engine case is a magnesium casting.
I heard and read a number of reports about magnesium engine cases not always holding up very well to the rigors of transplanting them into homebuilt aircraft. For this reason I initially was in favour of using an aluminium case.
However, an aluminium engine case is some 6.35 kgs (14 lbs) heavier than a magnesium job is. So, as always the extra longevity of an aluminium engine case comes at a price. Like in real life, there's no free lunch...

Engine case

VW Type-1 engine case

During my visit to the 2007 RSA Fly-in (Vichy, Fance) I talked to Stéphane Malandain, who since its first flight in 1995 had put over 680 hours on his IBIS (F-PBSM), which incidentally is still equipped with it's original 1870cc VW engine.
Stéphane told me that he had done a minor overhaul at around 635 hours. He also mentioned that his magnesium engine case didn't need any attention. Although I made a note to myself about this fact, at the time it didn't change my mind about wanting an aluminium case.
Re-reading some of my communications with Steve Bennet and with the friendly and patient help of Brian Smith of ACRO Engines and Airframes Ltd., I since learned that magnesium cases that develop cracks (most notably behind cylinder #3) do so for a number of reasons, each of which can be handled if you know them:

It's all about symmetry

It's all about symmetry, really. It's about ensuring that equal loads are put on equal parts, fore and aft, left and right.
Equal thermodynamic loads are a result of precise fuel/air metering per cylinder and by equal cooling per cylinder.
Symmetric mechanical loads are result of all of the above, PLUS using equal/accurate torqueing during assembly and by making sure that identical parts have identical weights and/or are balanced.

Of course it is very helpful if your propeller is not adding to any residual vibration mode, so I suggest you keep a keen eye on that part as well... :)

Decision in favour of an magnesium case

Now, whilst we're battling a known enemy here, it might also be of interest to note that most new magnesium cases are shipped with a reinforcement patch welded in behind piston #3 anyway, which addresses most concerns as well...
All of which leads up to my decision to not use an aluminium case after all, but use the lighter magnesium engine case instead.

RJ.01 IBIS VW Aero engine pages