Hugh Fader writes:
> On the way home from work, I stopped at a machine shop that was highly
> recommended to me. The owner told me that he did not recommend bronze
> guides. Said that cast iron was harder and would last longer under normal
> road use. This is not what I thought. Any opinions?
This may be a personal opinion of his. It's not exactly borne out by
experience. Some of the worst guide wear I've ever seen is on the old Ford
iron-guide engines.
As for hardness, it depends upon the process used to make the guide. But,
hardness is not the only quality necessary for a valve guide. If it were,
they could be made from anything hard. In fact, the phosphor bronze and
manganese bronze used in valve guides today is about as hard as cast iron,
is tougher (anyone who's tried to knurl a cast-iron guide knows this), and
has better natural lubricity. Natural lubricity is important--that's why
one can run a bronze guide with a valve stem seal, but an iron guide, with
iron-iron contact, has to have a fair amount oil going past the guide.
And, regarding wear, it's more or less impossible to refurbish a cast-iron
valve guide by knurling. They chip, rather than deform, during knurling,
while a bronze guide has just enough malleability to deform sufficiently to
be knurled and reamed to size, so it can be re-used at least once.
Look at it this way. Manufacturers today have to meet much more stringent
emissions than when Triumph engines were being built, and that means they
can't have oil going down the guides. And, some manufacturers, such as
Ford, are now warranting some engines for 100,000 miles. What sort of
guides are they using in such engines? Not cast iron. <smile> If cast-iron
were a superior material for valve guide use, they probably would be using
it.
Cast iron guides were suitable and cheap when these engines were being
made, but there are better materials in use today.
Cheers.
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