[Fot] Valve Lash While Engine is Hot

[tylerpthompson@yahoo.com]

Thanks Michael.  Very informative. 

Ty. 

Sent from my Verizon Wireless 4GLTE smartphone

----- Reply message -----
From: "Michael Porter" <mdporter at dfn.com>
To: "Tylerpthompson at yahoo.com" <Tylerpthompson at yahoo.com>
Cc: "fot at autox.team.net" <fot at autox.team.net>
Subject: [Fot] Valve Lash While Engine is Hot
Date: Fri, Apr 13, 2012 7:27 pm
On 4/11/2012 8:04 PM, Tylerpthompson at yahoo.com wrote:

My engine has a Paeco TR-6600 cam.  The cam card calls for a .014 intake/.016
exhaust valve lash.  A little looser than stock, but not by much.  I know
folks running larger clearances on the hotter cams and wonder if these
clearances may be a bit too tight.

That said, In talking with a fellow racer tonight who will remain nameless (I
don't want to name him in case I get a lot of back "lash" from this question)
he thought I should ask the FOT group if anyone has measured lash on a hot
motor after a race.  We expect it to be tighter, and perhaps there is an
optimum clearance we should be shooting for.  Just a thought in case my valve
lash is too tight.






I'm kind of surprised Larry Young hasn't weighed in on this one. 
Here's his Tilden Tech page on valve lash:



http://www.tildentechnologies.com/Cams/Tip_Lash.html



Larry does make a point about the difference between opening quickly
and acceleration (i.e., the two are not equivalent).



As for lash hot/cold, it's a lot easier to get uniform settings
doing it cold, if only because one has to move like the bejesus to
get (in the case of a 6-cyl.) twelve valves adjusted while the
engine's still hot, and there's always the worry that the engine
will cool enough that some will end up being too loose. It's fine to
check them hot against the cold settings, and calculate the
best cold setting.  As for any uniform rules, engine to engine,
about them getting tighter or looser with temperature, or by how
much, I doubt it.  When I was doing a lot of air-cooled VW engines,
it occurred to me that a stock cold lash of 0.006" didn't seem like
much, given how hot the exhaust valves ran (thinking that the stems
would grow quite a bit).  Not so.  The heads got a good deal taller,
and the hot lash was ~ 0.016".  All-iron engines, on average,
probably grow less, because the coefficient of expansion is ~ 1/4
that of aluminum.  



Another tip on the lash is to have a really critical eye about the
rocker noses. Even though they're hardened, they have a tendency to
pound down after a while, and if there's even the slightest cupping,
you'll find it nearly impossible to get the lash right with an
ordinary shim stock gauge.



Cheers.





-- 


Michael Porter
Roswell, NM


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