Larry,
Let me give your technical question a try:
The silicon bronze guides are made of a softer material than the cast iron
guides. This results in a few differences. Although the use of silicon
bronze guides is common when converting to the harder valves used in a
"no-lead" head, some machinists still prefer the cast iron ones. The silicon
bronze guides are prone to spalling and a different expansion rate under
heat. This means that, in use, small bits of the silicon bronze can lift and
cause the valve to hang up in the guide. Therefore, the common practice is
to ream the guide a little (.003" or so) larger than would be the case with
the cast iron ones. This results in an earlier onset of oil being sucked
down the guide - thus the belief that they don't last as long.
If your machinist prefers cast iron guides, then I would both stand behind
that belief and strongly recommend you believe him. It is, after all, your
machinist to whom you will go back should things not work out as well as you
would like. Allow him to stand behind his work.
Regards,
Lew Palmer
=========================================
What are the relative advantages of cast iron vs. bronze valve
guides? Both Dave at BMS and my machinist prefer cast iron because
they last a lot longer, but can't quantify how much longer. Dave does
have bronze guides in stock, but I very much want to do things right.
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