[Fot] TR4 Valve Guide Clearance

[Bill Tobin]

Great dissertation.
Thanks, Bill
----- Original Message ----- 
From: "Michael Porter" <mdporter at dfn.com>
To: "TR4 Tony" <tr4.tony at virgin.net>; "MadMarx" <tr4racing at googlemail.com>
Cc: <fot at autox.team.net>
Sent: Monday, June 30, 2014 4:34 AM
Subject: Re: [Fot] TR4 Valve Guide Clearance


> 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....
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