Message text written by "Randall Young"
>> David Massey wrote :
>>
>> How about this: As the bronze heats up and expands it is restraned by
the
>> head (or the remnants of the old guide) and is compressed further onto
the
>> valve stem.
>
>Yes, but by how much? Coefficient of expansion for gray cast iron is
given
>as .000006, one variety of manganese bronze I found is given as .000010,
per
>degree F. The guide clearly exerts force on the head, which would deflect
>by some amount ...
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. And since the guide cross section (the
material between the OD and the ID) will increase at a greater rate than
the OD (as constrained by the head) the extra width is compressed into the
ID where the valve stem is. Besides, the valve stem is also increasing.
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). 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)
This, I believe, is the cause of valve sticking in bronze valve guides.
Cheers
Dave
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