Subject: [Fot] RE -valve guides revisited

From: "Enquiries Road & Track" <[email-masked]>
Date: Tue, 15 May 2012 08:13:47 +1000

i think you guys are missing the point. bronze guides were intended to be
able to run *smaller* clearances for better heat transfer. there
is absolutely no point in increasing clearances unless you have chromed
stem valves (very rare nowadays). you want the absolute minimum for best
heat transfer.
Terry
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From: "Randall" <[email-masked]>
Date: Mon, 14 May 2012 17:14:49 -0700

> there
> stem valves (very rare nowadays).

Or don't want the exhaust valves to stick open when the guides get good and
hot, and expand inward (because they can't expand outward against the cast
iron head).

BTDT

-- Randall

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From: Michael Porter <[email-masked]>
Date: Mon, 14 May 2012 18:26:32 -0600

> stem valves (very rare nowadays). you want the absolute minimum for best

Umm, the issue is not the cold clearance, but the hot running clearance. 
Since the thermal coefficient of expansion of bronze is, depending upon 
type, about 2-1/2 to 3 times that of cast iron, their use in cast iron 
heads requires additional cold clearance, so that the hot running 
clearance is adequate.  When the guide grows with heat, the actual 
running bore gets smaller, because the mechanical strength of the cast 
iron is higher and the coefficient of expansion is lower than that of 
the bronze.

This is a matter rarely of concern when using bronze guides in an 
aluminum head, since the coefficients of expansion are closer to the 
same (a ratio of perhaps 3 to 4), but with cast iron, a much bigger deal 
because the ratio is more like 3 to 1.  This is why so many racers in 
this country have experienced valves sticking when using the recommended 
factory cold clearance (appropriate only for cast-iron guides).

Cheers.

-- 

Michael Porter
Roswell, NM

Never let anyone drive you crazy when you know it's within walking distance....
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From: Chuck Gee <[email-masked]>
Date: Mon, 14 May 2012 22:44:15 -0400 (EDT)

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 <[email-masked]>
To: fot <[email-masked]>
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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From: Tony Drews <[email-masked]>
Date: Mon, 14 May 2012 22:09:25 -0500

One of the FOT shared some calculations with me concerning the 
shrinking of clearances when hot...

With Bronze guides with 0.004" clearance and Stainless Steel valves 
with 5/16" stems, when the exhaust valve hits 1000 degrees there will 
be 0" of clearance.

I have no way to check the math and it's way above my pay grade, but 
what Michael is describing is exactly the issue in my mind.  You want 
to run a clearance just big enough to avoid sticking the valves.

I can't argue that Bronze is better than Iron for the guides, but a 
number of have found that if you don't run "enough" clearance, bad 
things can occur - and they get infinitely worse if you have enough 
lift that you're running an interference valve train.

The conventional wisdom seemed to be that the iron guides had higher 
wear or were somehow inferior to Bronze but I'm really unclear where 
that notion came from.

Tony

At 07:26 PM 5/14/2012, Michael Porter wrote:
>Umm, the issue is not the cold clearance, but the hot running 
>clearance. Since the thermal coefficient of expansion of bronze is, 
>depending upon type, about 2-1/2 to 3 times that of cast iron, their 
>use in cast iron heads requires additional cold clearance, so that 
>the hot running clearance is adequate.  When the guide grows with 
>heat, the actual running bore gets smaller, because the mechanical 
>strength of the cast iron is higher and the coefficient of expansion 
>is lower than that of the bronze.
>
>same (a ratio of perhaps 3 to 4), but with cast iron, a much bigger 
>deal because the ratio is more like 3 to 1.  This is why so many 
>racers in this country have experienced valves sticking when using 
>the recommended factory cold clearance (appropriate only for cast-iron guides).
>
>
>Cheers.
>
>--
>
>
>Michael Porter
>Roswell, NM
>
>

---

From: "Tim Murphy" <[email-masked]>
Date: Tue, 15 May 2012 22:03:56 -0500

I had sent Tony the spread sheet some time ago analyzing the clearance between
the guide and the valve at various temperatures.  This is based on the
coefficient of expansion of cast iron (cylinder head, 6.0X10b;b6 In / In /
Deg. F.), manganese bronze (guides, 11.8X10b;b6 In / In / Deg. F.) and
valves (21-4N stainless steel 9.9X10b;b6 In / In / Deg. F.).  I think, based
on an internet search, that 21-4N is the common stainless steel used for
exhaust valves.
	The model is based on the bore in the cylinder head being smaller than the
O.D. of the guide at operating temperature due to the differences in their
coefficient of expansion, just as others have recently mentioned.  This makes
the bore or I.D. of the guide smaller at operating temperature.  The biggest
factor is the coefficient of expansion of the valve being almost double that
of the cast iron, which controls the I.D. of the guide.
	I've since refined the model to incorporate different rates of temperature
rise in the head, guide and valve stem.  The next thing to do was to get
better data on the operating temperatures, particularly in a race
environment.
	I've been searching the internet trying to get an idea on temperatures of the
exhaust valve, guide and cylinder head.  So far I've found that the surface of
the combustion chamber is in the neighborhood of 266 to 496 degrees F.  	Then
I found that exhaust valves run in the area of 1000 degrees F.  The
interesting thing was that the point was made that the valve is only "exposed"
to the exhaust gases for 1 out of 4 of the cycles, more or less.  The point
being that there is a lot of time to transfer the heat to the seat (which
transfers the most heat) and the guide.

Question:  I am aware of the "downside" of not having enough clearance, i.e. a
stuck valve, but what is the "downside" of too much clearance?  I think we are
talking about a clearance of 0.004 versus 0.002 on the diameter of the exhaust
valve, cold.  That's only a difference of 0.001 on a side, cold!   And at
operating temperatures the difference will be quite a bit less.

	A stuck valve can cost a race weekend and more, what can a "loose" guide
cost?

Tim Murphy

-----Original Message-----
From: [email-masked] [mailto:[email-masked]] On Behalf
Of Tony Drews
Sent: Monday, May 14, 2012 10:09 PM
To: [email-masked]
Subject: Re: [Fot] RE -valve guides revisited

Tony

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