[Fot] TR4 Valve Guide Clearance

[TR4 Tony]

That's a very good prC)cis Michael - thank you.

I'm actually running bronze guides in my rally engine and apart from one
initial sticky valve after the head build and ongoing oil consumption which is
a feature of them, they are mighty reliable (40000 miles so far at varying
revs 800 to well over 7000) so they do work and I would say enhance my
reliability rather than compromise it. I now understand a little more why and
why accuracy in machining is so crucial to this.

Regards

Tony

Sent from my iPhone

> On 30 Jun 2014, at 09:34, Michael Porter <mdporter at dfn.com> wrote:
>
>> On 6/30/2014 2:13 AM, TR4 Tony wrote:
>> Exactly ... I don't see it either, so are we missing something here ?
>>
>>
>
> When Ed Barnard sent out an email slightly disparaging bronze guides, I sent
this back to him privately which, I think, covers most of the bases:
>
> One must remember that even though bronze guides must be reamed oversize,
when they reach operating temps, the clearance has closed up a lot.  Getting
the right hot clearance depends greatly on getting the right cold
clearance--once that's done, the bronze guide will last longer than the
cast-iron one, if only because the natural lubricity of bronze is superior to
cast iron.  Bronze guides are typically able to run with valve stem seals,
while cast-iron guides cannot (some of the absolutely worst guide wear I've
ever seen has been in `70s Ford V-8s that used cast-in-place guides and
umbrella seals--typical to find slots, not holes, where the guides used to
be).
>
> I think the most important consideration is that bronze guides/cast iron
heads simply is not standard configuration, and the combination has to be
re-engineered to account for the big difference in expansion rates.  In the
typical configuration, with aluminum heads, the thermal expansion rate
differential is rather small, so the guides grow with the heads. With
cast-iron heads, the thermal expansion coefficient is small, and the bronze
guide's coefficient is large by comparison, and because the mechanical
strength of the cast iron is somewhat greater, the clearance shrinks with
increases in temperature.
>
> If, even after getting the clearances more or less correct, there is a
problem with sticking, there's a simple fix. Ream an extra thousandth, then
knurl the ID and then ream to the right cold clearance.  Adds a little space
for oil accumulation, and the bronze is just ductile enough to knurl--which
cast iron is decidedly not.  It chips rather than undergoing plastic
deformation.
>
> None of this is necessary or wise in a racing engine--the teardowns are
frequent enough that carbon accumulation on the valves isn't a problem, and
the additional oil consumption without guide seals probably isn't noticed.
But in a street engine, that's another matter. And in Triumphs, running over
4K and with no seals on the guides pumps a lot of oil down the guides, because
the level in the valve cover goes up.
>
> Sticking valves in engines with aluminum heads and bronze guides with stem
seals just isn't a problem.  Quite the opposite--it's a very reliable
combination, and usually results in quite a bit less oil consumption, and in
guides that don't have to be replaced at each teardown--bronze guides can be
knurled and reamed a couple of times before it's time to pitch `em, so they
are actually more economical than cast iron in the long run, especially when
adding in the difference in oil consumption.  So, using the bronze guide in a
head material for which it was not intended--in order to be able to use stem
seals (otherwise, why would one bother?)--requires doing the necessary
re-engineering work to get the operating clearances right.  If sticking valves
were an inherent property of bronze guides, they would have been dumped by the
manufacturers fifty years ago.  So, it must be the combination of materials
and the lack of proper clearancing, and not the guide itself.
> =======================================================================
>
> So, no, for racing, probably not necessary or advisable. Even so,
complaining about the amount of cold clearance required for bronze guides is a
red herring.  Once the engine is at operating temperature, the clearance is
much reduced--otherwise, would there be problems with valves sticking when
it's not a problem with aluminum heads?  Much of the bad press is a result of
improper machine shop practice when installing them, and is not due to the
guide itself.
>
>
> Cheers.
> --
>
>
> Michael Porter
> Roswell, NM
>
>
> Never let anyone drive you crazy when you know it's within walking
distance....