I have taken many well used waterboxer motors apart and most of the ones
that had amc heads on them had valves that have failed or showed noticeable
wear. These weren't heads that I installed, these heads have been installed
by various mechanics throughout the country. I haven't seen any similar
pattern of valve destruction on stock vw heads. Theres a big difference
between seeing a pattern develop in well used heads and not having any
customer feedback on brand new heads installed. it seems that the damage can
take 30 to 60,000 miles to occur. its been my experience that most owners
don't drive their vans that much and vans change hands every few years on
average. So, I think the pattern I've seen in well used heads is much more
reliable information than customer feedback from newly installed heads. The
first subaru conversion i did for someone now has about 65,000 miles on it.
i hadn't seen the customer in 5 years. she came in and I asked if shes had
any problems. she said the kennedy exhaust cracked after 2 years but didnt
contact me about it because it had been so long she figured it wasn't under
warranty anymore. Theres lots of reasons customers don't give feedback. they
move, the van gets sold, too much time has passed, etc, etc. theres a lot of
variables. whats concrete is the damage i've seen to the valves only on amc
heads.