Actually, grey cast iron is usually used instead of steel, and it's
usually chilled. The choice of materials has more to do with production
cost than performance. Cast iron is easier to ream and machine (it's
generally done dry) and it's cheaper. Phosphor bronze is challenging since
it grabs the cutter and often overheats. It's hard to get a bronze guide
to have parallel sides since it heats up the reamer and the guide as it
goes. They should be reamed undersized with lots of coolant and then honed
to fit--in the head. People often have bad experiences with bronze guides
pumping oil and assume it's early wear, when it might be inadequate
cooling during reaming.
You can set up phosphor bronze (or better yet, an alloy of bronze that
includes a little nickel to increase the hardness and wear resistance) to
have tighter clearance than cast iron since it is less likely to seize,
conducts heat better, and beds quickly. That means the seat can be thinner
to flow better without wandering and losing seal. It also means you're
less likely to burn your exhaust valve.
All that is fairly philosophical and my comments are all based on building
motorcycle racing engines, not tractor motors, where we always used fancy
bronze alloys and were real, real careful about getting heat out of the
exhaust valve and keeping the intake seat very thin. But then we were
getting more than 200HP/liter with carbs just to be in the ballpark of
competitive.
I doubt the guide material matters in these engines, but you're looking
for the "best" meaning superior performance and not ease of use, it's
unquestionably bronze.
-----Original Message-----
From: Brad Kahler [mailto:brad.kahler@141.com]
Sent: Monday, June 09, 2003 12:24 PM
To: fot@autox.team.net
Subject: bronze or steel
Amici,
What is considered best for racing, bronze valve guides or
steel valve guides?
Thanks
Brad
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