>Be careful about bronze valev guides. The inside diminsional specs are for
>steel
>guides that expand less than bronze at a given temperature. Bronze guides set
>up to steel guide deminsions may try to seize valves on a newly rebuilt head.
>This is quite common in MGBs. So if you get bronze guides have the internal
>dia. enlarged one or two thousands larger diameter than the manual calls for.
I'd like to expand on this. The *silicon* bronze valve guides are a big
improvement over cast iron. The phosphor bronze are a marginal improvement.
I can't find the passage right now, but David Vizard says that the cast
iron guides tend to wear in a couple hundred miles, the phosphor in a
thousand, and the silicon last much longer.
Two things are critical when installing bronze guides:
One is that they be honed to size. The bronze guides have a tendency to
"squish" into an hourglass shape when pressed into the head, causing a
tight spot on the valve stem. The exhaust guide actually wants to be a bit
looser than the intake, to allow some oil down the stem to help heat
transfer from the valve to the head (oil can't be sucked down this
direction). The spec I have is 0.0022 - 0.0026" clearance on the exhausts,
0.0011 - 0.0015" on the intakes. Since you're going through the trouble,
install seals on the intakes (never on the exhausts, since you want a
little oil there). There are some very trick Teflon seals available now
that have an extremely long life. Among others, David Anton at APT can
supply these and will machine down guides to accept them (it's an easy job
if you have the guides out and a lathe). It adds a very small amount to the
total cost of the head work, and is well worth it.
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