Actually, I don't think I'm missing anything.
You should only be concerned with the clearances AT TEMPERATURE. Cast iron
has a Coefficient of Thermal Expansion of around 6 micro inches/inch per
degree F. Most bronze alloys are around 10, so the guides will grow quite a
bit more and cause the bore to shrink more compared to cast iron guides. Now
couple this with a change to 300 series stainless steel valves (CTE of 14),
and you have a real mess compared to the stock set up. You have to take these
differences into consideration when establishing the room temperature
clearances using materials other than stock.
I use bronze guides because those are readily available for my custom
application. I also use the recommended clearance provided by the guide
manufacturer for the valve material. Sure, I can make them a little tighter
to get better heat transfer, but I'm guessing the gain will be very small for
the potential tradeoff in reliability. I can get more performance out of my
car by eating salad for a few months (after all, salad does equal speed). To
be honest, I've used bronze guides now for over 15 years and couldn't really
tell you if the room temperature clearance is larger or smaller compared to
cast iron. My educated guess is that it is larger, simply because the
materials used have greater CTEs.
But feel free to tighten up the room temperature clearance with bronze guides
and stainless steel valves. I can almost guarantee you will get them to
stick.
Chuck
-----Original Message-----
From: Enquiries Road & Track <enquiries@roadandtrack.net.au>
To: fot <fot@autox.team.net>
Sent: Mon, May 14, 2012 4:56 pm
Subject: [Fot] RE -valve guides revisited
i think you guys are missing the point. bronze guides were intended to be
ble to run *smaller* clearances for better heat transfer. there
s absolutely no point in increasing clearances unless you have chromed
tem valves (very rare nowadays). you want the absolute minimum for best
eat transfer.
erry
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