[Fot] TR4 Valve Guide Clearance

[Michael Porter]

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....