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<x-flowed>Out of curiousity, was the case you did this test on an AS21 or AS41 alloy?
On 3 Jun 2006 at 18:50, Keith Park wrote:
I believe the reason they did that is if you have an engine case that has
been through a traumatic event, like really bad bearings or worse yet,
overheating, then align boring may be a short lived solution with stresses
and warpage still present in the case. When given an random case you may
have no indication what its really been through with overheating. When I
got mine bored I knew it was a solid 87Kmi case that hadn't been overheated,
so I felt it worth while, I wouldn't have just grabbed a random case...
I think this is exactly right.
I once oven annealed a case, just to see what the effect would be. I started by breaking off bits of a broken case and annealing them, then testing the bits on a hardness tester before and after. I found that they came back to OEM hardness after about 4-6 weeks. This is called "precipitation hardening" or "solution hardening" and it only happens in certain alloys.
Then I annealed a whole case, in order to remove some warpage. Once this case was annealed, aged, machined, and reassembled it ran fine for over a year, but that's as long as I kept track of that car.
My theory is that VW chose an alloy that would harden in the time it normally took for the castings to move from the foundry where they were cast to the shop where they were machined. Hard aluminum machines very nicely, while soft alloys are gummy and really difficult to do a nice job on. I assume that magnesium alloys are the same.
So you could get in trouble if you took a badly overheated case, machined it and put it right back in service.
-- ******************************* Jim Adney, jadney@vwtype3.org Madison, Wisconsin, USA *******************************
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