Bill Babcock wrote:
>
> Yup. I don't exactly recall the alloy I used most often but it was
> something like AM435 (but don't take that to the bank--my neurons are
> notoriously toasted). I'd be happily tapering the nose on a guide when it
> would suddenly grab the toolbit and rip itself to shreds. Or you'd ream
> one with a perfectly straight ream and get .002" taper top to bottom in a
> 2 inch guide. Maddening.
What I've found with many street engines over the years is that a lot of the
sizing problems with bronze guides can be diminished by boring the guide a few
thousandths oversize, single-knurling the
guide bore and then reaming. This has the additional advantage of minimizing
the amount of oil pumped down the guide if valve stem seals are not used, and
reduces drag and sticking, because some oil
is retained between the knurled ridges. While the guides don't last quite as
long with this method, they can be re-knurled and reamed a couple of times
before replacement.
Also, bronze guides were really made for aluminum heads. The coefficient of
expansion of most bronzes and aluminum are relatively close. When bronze guides
are installed in a cast-iron head, however,
the difference in mechanical strength and coefficient of expansion means that
the bore of the guide closes up slightly at operating temperature. If the
optimum stem-to-guide clearance for iron guides
is 0.0008-0.0012", bronze guides should be reamed to about 150% of that value
to minimize sticking when hot.
As for the machining problems, many bronzes are difficult to cut because they
behave a lot like copper, which is very smeary and sticky. Some of the cutting
problems can be minimized by using
TiN-coated tools and lots of lubricant. Even so, bronzes are a snap compared to
any alloy containing significant amounts of nickel. One of the worst materials
I ever had to machine was Hastalloy 556
(55% nickel), which is commonly used in jet engine hot sections. Hard as can
be, grows with heat to unbelievable proportions and is so tough that it's
difficult to make a decent chip. And, if you get
a splinter of it under the skin, you know exactly where it is--it feels like
it's burning.
I'll take bronze any time over that stuff. *smile*
Cheers.
--
Michael D. Porter
Roswell, NM
[mailto:mporter@zianet.com]
Never let anyone drive you crazy when you know it's within walking distance.
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