*sigh*
the valve stems expand due to their captive heating from exhaust gases and
combustion(and their being shrouded by the guides). and the lifters and
push rods do expand to some degree under the warming up process to normal
operating temperature. these combined factors contribute to a valve to
rocker clearance deminishing as an engine heats up. the most expansion is
in the valve stem. some manufacturers give hot and cold lash settings.
cold is always more clearance than hot. experimentation on the dyno back
in the 70's yeilded little suprise to improved performance with reduced
valve lash. but! there is a point where under normal operating
temperatures some lash must exist to allow for extreme heat (excessive abuse
or sustained high RPM's for long periods) for aditional expansion! or
uh-oh! i think we burned a valve (or it's nasty relative..) i think we holed
a piston...it seems to have no compression..........yup! going fast has
some up's and down's...... that's why a daily driver is always best left
stock or near stock.....(it's a reliability thang....) :).
chuck.
been there.....broke that! :)
----- Original Message -----
From <ckotting at wideopenwest.com>
To: <spridgets@autox.team.net>
Sent: Thursday, March 20, 2003 7:13 AM
Subject: RE: Cold vs. Hot valve lash
> Crash -
>
> They do loosen when hot. As the hot metal expands, the
> pushrods stay relatively cooler than the block and head. As
> the distance between the camshaft and the rocker shaft
> increases, the length of the pushrods increases less, making
> the valves looser.
>
> However, that being said, I'd just set them at factory "hot"
> spec, even though the engine is cold. At the first start
> up, you've got to run it for at least 30 minutes to get the
> cam and tappets bedded in together (assuming you're using
> new ones), which is going to make the valve lash change
> anyway.
>
> You're not going to do any damage in 30 minutes of running,
> and you're going to have to readjust the valves after that
> initial bed-in anyway, and the engine will be at operating
> temp at that point. So just figure that things will be a
> bit rough at first, and do a complete valve lash - balance
> carbs - set mixture after that first run-in.
>
> At least that's how I've always done it.
>
> > I don't think so, do you think they will looser when hot?
> > Maybe add ..02?
> > Crash
> >
> > Chris:
> >
> > minus .02
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