David Massey wrote :
>
> That doesn't take into account the hardness of the two materials. Since
> the cast iron head is a harder material and since it is much more massive
> than the bronze the hole in the cast iron will expand at a slower
> rate than
> the natural OD of the bronze guide.
Then why does freezing water break concrete sidewalks and cast iron engine
blocks ? Ice isn't nearly as hard as concrete or cast iron ...
> Since we engineers like to assume a horse is a cube and a dog is a sphere
> to keep the math simple, lets assume that a valve guide has an OD of .5
> inches and an ID of .25 inches. If the temperature of the guide were to
> increase 100 degrees it would grow to .5005 inches OD and .25025 ID. This
> leaves a wall thickness of .125125 (from .125).
Actually, I get .125025, but that's ok ... could be this Pentium I'm using
<g>
> But if this guide were
> constrained in a cast iron head the OD would only grow to .5003. If you
> subtract two wall thicknesses from this you are left with .25005 ID.
>
> Now if the valve stem were also .25 it will grow to .25015 which is now a
> .0001 interference where as before it was a just fit. (In other words if
> you had a .0001 clearance before you now have a binding situation)
Except that valve stems are not cast iron, and probably have a different
Cof.
> This, I believe, is the cause of valve sticking in bronze valve guides.
Except that even the factory minimum clearance is .001", so your example has
only reduced this by about 1/8 ...
Randall
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