I have been in touch with the Austin A90 / Atlantic club in the UK, and they confirm that on the A90 there WAS a chassis cross member right under the front of the engine, and thus the "step" in the front of the sump was to avoid that cross member.
As the Healey doesn't have that cross member, I cannot for the life of me imagine why no one has increased the overall total sump volume (even if they cut an inch off the sump) be removing that step.
I am not an engineer, or even a mechanic, so I am more than happy to accept it if someone can explain that the "step" is a vital component in the fluid dynamics of the oil flow in the sump, or something similar.
In the meantime, I intend to cut an inch out of my sump, and extend the full depth all the way forwards, and see what happens.
(BTW, all the alloy sumps I have seen still have that step at the front. Photo of the one from AH Spares attached demonstrating this. Why ???)
Watch this space........!!!!
100/4 Sump Guard
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