[Fot] RE -valve guides revisited

[Michael Porter]

On 5/14/2012 4:13 PM, Enquiries Road & Track wrote:
> 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.
>

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....