Rick,
True, the last one won't be quite as warm as the first one but it'll be a
darn sight closer to its operating temperature - and thus it's needed
operating clearance - than it would if the engine were stone cold.
I agree that the co-efficient of expansion is related to the metallurgy of
the part, not its amount of wear. However, a worn pushrod end, when it
expands with heat, will likely present a different profile to the ball on
the rocker arm than an unworn one. In fact, it presents the surface that is
going to be in use when the engine is running. Therefore, that's the time to
make the adjustments, in my opinion.
I agree with you on one point - this discussion doesn't deserve any more
bandwidth spent on it. You are free to believe whatever you like. I speak
from many years as a practical - and conscientious - mechanic but you are
free to prefer theory to experience if you so desire.
Lawrie
British Sportscar Center
-----Original Message-----
From: REwald9535@aol.com <REwald9535@aol.com>
To: Lawrie@britcars.com <Lawrie@britcars.com>; mgs@autox.team.net
<mgs@autox.team.net>
Date: Tuesday, August 17, 1999 10:56 PM
Subject: Re: Re-Torquing
>Lawrie,
>A few points I would like to discuss with you.
>First if you drive a car into the shop and turn it off and remove the valve
>cover and start adjusting valves, by the time you get to valve #8 the
engine
>has cooled somewhat. Maybe only 10 or 20 degrees but you have to admit the
>engine is no longer at operating temp. (don't forget not everyone who is on
>this list, can adjust valves as fast as you can, they don't get the
practice
>you do :-) If the engine has cooled by the time you have reached valve #8
>then not all the valves were adjusted under the same conditions. If on the
>other hand you are working at room temp. the valves will all be adjusted
>under the same conditions.
>Secondly, as far as coefficients go, wouldn't you agree that differences in
>expansion coefficients is a function of differences is materials, not wear
of
>those items? (regardless of how worn it is, aluminum expands at a different
>rate than iron.)
>BTW one of the engines I teach on has a new valve adjustment spec that
>SPECIFIES 20 degrees C (68F) and has corrections for each 1 degree C
variance
>from that temp! (The clearance changes by 0.01mm for each degree C)
>The differences we are discussing are small and perhaps, not worth the
>bandwidth, but I like doing my valves cold.
>Rick Ewald
>
>In a message dated 8/17/99 7:40:34 PM Pacific Daylight Time,
>Lawrie@britcars.com writes:
>
>> In my experienced (and not-so-humble) opinion, valves should always be
>> adjusted when they are hot. After all, they do their work when they are
hot
>> and that's when the clearance is critical. Who cares what the clearances
>are
>> when the engine is cold and at a standstill? And, yes, I know a lot of
shop
>> manuals state that clearances should be set when the engine is cold, but
>> they were written when all the valve train components were new and
>> co-efficients of expansion throughout the valvetrain could be relied
upon
>to
>> be fairly uniform. We are mostly dealing with old, worn engines where
some
>> things may expand at different rates than others So, if the book gives a
>> "Cold" valve clearance, just subtract 2 thousandths from it and set the
>> clearances when the engine is at running temperature.
>
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