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Re: [TR] adjusting valves hot

To: Gary Nafziger <nafzigerg@yahoo.com>
Subject: Re: [TR] adjusting valves hot
From: Michael Porter <mdporter@dfn.com>
Date: Thu, 17 Sep 2009 19:22:23 -0600
Gary Nafziger wrote:
> I sometimes see mention made of setting valves when hot.  I'm curious about
> why this might be preferable to setting them cold.  And what is the differance
> between measurements........that is...............how much would metal expand
> or contract when hot or cold.  Or how would my valve lash of ..013
> cold......compare to valve lash of .016 hot?
>
> just wondering
>
>   

M'self, I think this practice persists from the early days of hot rods 
and hydraulic lifters.  Then, it was fairly common to see V-8 owners 
cutting a big slot in a spare valve cover to minimize oil splash while 
adjusting lash when hot.  I suspect this was necessary because cam 
grinding wasn't quite as precise as it is today, nor was the manufacture 
of the lifters, so the lashes necessary varied.


For a solid lifter engine, the practice of doing valve lash when hot is 
going to unnecessarily introduce error, since the engine is cooling 
while the job is being done (as Randall previously suggested). 


As for the change in settings between hot and cold, it depends a great 
deal on the construction of the engine and the geometry of the valve 
train (particularly with regard to the size of the rocker and the length 
of the pushrod, and the variation in temperature of the various 
components).  For example, in a typical all-cast iron/steel engine such 
as a Triumph, lash might only change a few thousandths.  In an 
air-cooled VW, however, a cold lash of  0.006" translates to 0.016" when 
fully warm. 


Cheers.

-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....
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