With the used 550 heads, I have about 50% luck with the valve guides being good, or not. The ones that have been worn were also the ones in the heads with the locking rocker shafts. I suspect Honda hedged its $$ costs with the later heads by using less-expensive valve guides to help offset the $$ of the added lock shafts. This was similar in the 750: up until about 3/72 Honda used Stellite valve guides in this engine, which can last a LONG time (mine went 130k+ miles and were still not fully worn out). When the injection-molded cases and (in 1974) the upper end parts came out, the guides were changed to simple cast-iron, which last about 10k miles to the wear limit: those machines were EXPENSIVE production machinery in the 1970s, so something had to 'give'. In the 750F0 they switched back to Stellite for that model only, then back to cast iron in the F1 and later heads. The wear pattern is similar to the early engines, and Ofreen here has 150k+ miles on his original Stellite guides, beating my record!
