Well, to some extent I'm just repeating what I've been told, and read in
books. Gray cast iron was frequently used in old machinery, and it's
self-lubricating qualities (or perhaps I should say, relative freedom from
galling) were one of the reasons it was preferred for things like the guide
ways on my mill and lathe. It was even used for engine piston rings up into
the 50's, when better alloys of chrome became available. See for example
http://www.ironcasting.org/Gray%20Iron%20-%20A%20Unique%20Material.htm
which states "Gray iron is also known for its resistance to galling and
seizing. Many explanations have been given for this behavior, such as the
lubricating effect of the graphite flakes and retention of oil in the
graphite areas. "
There are literally hundreds of standard bronzes, and countless more special
alloys. I don't know exactly which one is used for the valve guides, so I'm
not certain of the comparative specifications. But, almost everywhere you
find a bronze bushing, it's really sintered bronze, where the sintering
creates pores that are filled with oil. Thus the instructions (for example)
to soak TR2/3/4 starter bushings in oil for 24 hours before installing them.
Modern Oilite is 18% mineral oil.
I have seen unreamed bronze guides stick, and Ken G. says the solution is to
ream them. I suppose it could be due to the slightly higher thermal
coefficient of expansion of bronze ... but since the guide id would normally
get bigger as the bronze expands, the ME problem of what actually happens
when it's in a cast iron hole is beyond my skills. Anyone else care to take
a whack at it ?
Randall
>
> I believe, to the contrary, that the manganese-bronze valve guides ARE
> self-lubricating...I have never heard of cast iron being
> self-lubricating...
>
> I stand for correction...
>
> Larry M
> TR250
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