Well, you and I will have to respectfully disagree on that as I have been using them for my high performance heads for 45 years, lol. So in my view, they are more than just ok. I just ran a 261 I built in 2015 at the trifive nationals. Ran the motor to 5800 rpm multiple times, multiple runs, as I have at every trifive nationals since 2015. In fact, I won the street tire class this year. That motor has to have near 100 runs on it. Yep, bronze liners.
I happen to believe that the bonze helps the lubrication of the valve stem, using less oil (and teflon seals) then just teflon seals and cast iron guides. I have no empirical data to support my belief other than I have built dozens of these motors and all used the bronze liners and not one single issue.
That being said, I don't think there is anything whatsoever wrong about using new cast iron guides. I just prefer the bronze liners.
My friend owns the machine shop that I use. This is his son drilling out and installing a set of liners for one of my 848 heads. This is from a couple of years ago but you can get the jist of the procedure.
