This is exactly what my local Toyota Service manager said as to why HG jobs tend to fail the second timne around, that and warped heads, when I asked. A friend of mine owns his own shop and he told me the same thing. Before he had two V6's come back with bad HG w/in a year he started using new headbolts, and eating the labor/parts to do a job again. He also started replacing the knock sensor and wire, because he had rigs come back after he fixed HG with engine light on. He now turns away any customer who will not authorize new headbolts and knock senor/wire.

Thanks for the informative post.