After purchasing some parts with small fatigue cracks from sellers who I believe were unaware of the cracks, and various off list discussions with CR members, it seems like I should share this in case you are unaware.
Small cracks develop in materials due to fatigue. Some materials are more susceptible to fatigue cracking than others - aluminum is more susceptible than steel, but steel can and does fail by fatigue. Regardless of the material, obviously it is a major advantage if you can detect the cracks before they lead to failure. With vintage bikes and parts this is even more important than to other riding communities.
Some have argued that they do not have access to x-ray equipment or other sophisticated NDT equipment. OK, but dye penetrant is readily available and effective at detecting cracks too small to see - although once detected by dye penetrant, one can usually then see it upon close inspection.
You can buy the dye penetrant components individually, but they are usually sold in kits. The kits are sold with three spray cans, one is a powerful degreaser to preclean, the second is a strong red dye penetrant which is wiped or incompletely washed off. After it is dry, a white powdered developer covers red on the surface and the red dye 'develops' by capillary action out of the crack leaving a bright red on white signature.
The only potential issue is the degreaser is so strong it can destroy paint. The dye can be used and removed with denatured alcohol without using the degreaser on painted items, but paint can hide developing cracks anyway.
Wikipedia has a concise description:
http://en.wikipedia.org/
And a Google search produced two kits under $50:
http://www.beautomotivemachine.com/
These kits should allow the inspection of several hundred parts.
I have not used either of these brands, but I'm sure they would work well. We have a welding supply place near here that I buy another brand from.
Safe riding to all!
Jon M. Crate
Marietta, Georgia USA