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Re: Cold vs. Hot valve lash

To: "Charles Christ" <cfchrist@earthlink.net>, <spridgets@autox.team.net>
Subject: Re: Cold vs. Hot valve lash
Date: Thu, 20 Mar 2003 08:00:35 -0500 (pop-3.dnv.wideopenwest.com)
Chuck -

D@%n.  You're right.  I should never post before coffee...

Chris K.

> *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@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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