Engine cases

[dusty1]

I have heard of spun bearing races on the sprocket shaft side & cases becoming brittle with age,causing cracking.How many times can you safely re-use your cases before new cases are needed?Mine are 31 yrs old & will be needing a 2nd lower end in the not-so-distant future.I ride a lot & sometimes quite hard.Im just curious as to whether or not i should be looking into replacement cases at that time.

[easyricer]

The only cases that I've ever heard of getting brittle, were sitting in a shed for 40 years, in a box. Corrosion is what makes them brittle. As long as that engine actually get used you won't have to worry about that. 
NOW I've never had a bearing race come loose so, I'm sure Kirby or Scott can give you a little more insight on that. As far as cracked cases, they can be repaired. If you are running a monster engine that shakes parts off your bike, it would probably be worth the extra expense of Delkron or S&S cases.
I can say that when it starts to "spiderweb" around the bearing race insert, it's a real PITA to weld up. I've only done it once, it's still holding today (almost 10 years ago) as far as I know.
EASY

[Hillside Motorcycle]

We've machined up steel inserts in the past, to repair that area.
Earlier Delkron cases had some problems with those.
Scott

[gryphon]

Early STD cases had problems also.