wow sounds like you know your metals! thank again

 

I look after Aluminium Die Casting Die Evaluation at work so one would hope so As mentioned above though I'd also have eassy access to die penetrant to confirm any cracking.

Looking at the photo's you've added above is that an oil leak at the joint between the aluminium casing or dust dirt/rust from the bolted on section?

I still reckon just heat cracks but worth having some close inspection around there.

 

no oil what so ever. its some sort of joint putty.

 

Given the Honda S2000 diff is from a Mazda RX8 (old style) you could take it to a Mazda garage and see what they think.