Valve Guide Wear

[Darrel Kline <darrelk@domain.elided>]

Here's the message on valve guide wear Eldon was kind enough to type out
for someone he had never even met:


From: <EldonMcf@domain.elided>
Date: Mon, 8 Jun 1998 16:52:07 EDT
To: darrelk@domain.elided
Subject: Valve guide wear
X-Mailer: AOL 4.0 for Windows 95 sub 170

In a message dated 6/5/98 1:11:45 AM Central Daylight Time,
darrelk@domain.elided writes:

>  I think the first thing is to check the valve guides
>  with my dial indicator (maybe I can do this without removing the valve
>  springs?)

  Darrel,

  Yes valve guide wear has alot to do with it, and putting seals on with worn
guides is only a band-aid approach, or a short term fix that likely is only
gonna solve part of the problem.
   
   It is really difficult to get an accurate indication of guide wear with the
springs installed. It is also difficult with the spring removed because the
movement at the tip of the stem is greater than the actual wear, so a precise
measurement is nearly impossible with this method.  An experienced mechanic,
or machinist will develop a feel for guide wear, but this also vary's with
each individual. most service publications reccommend using small hole gauges
to measure guide id, then a micrometer to measure stem wear. The stem-to-guide
clearance is calculated from these measurements.  These measurements are time
consuming, and can be inaccurate because the guides wear more at each end, and
very little in the center.  Also engine manufacturers seem to allow quit a bit
of wear in their alowable wear limits.  Some allow as much as .006-.008".  The
problem there is that lets the valve head move quite a bit on the seat, which
affects the ability of the valve to seal against the seat.  I would usually
replace guides that have .001-.002" wear.  Most engines that I work on have
replaceable guides, so replacement is just a matter of pressing them in &out.

   I don't know if you are familiar with valve seat equipment, or not.  To
grind, or cut the valve seat a pilot is used in the guide to center the
cutter, or grinding wheel.  These pilots have a long, gradual taper to center
them & lock them in place.  Pilots are available in .001" increments.  The
method I use is to try to insert a pilot the next size over the standard size
into the guide.  If the pilot won't enter the guide it's ok, if the oversized
pilot enters the guide, or is loose the guide gets replaced. From experience
any guide wear over about .002" will shorten the life of the
valves/seats/overhaul job, etc.

   Another potential problem with worn guides is one that will show up under
certain conditions such as descending a long mountain grade.  the engine rpm
is at a medium to high speed, but the throttle plates are closed.  This
condition exposes the intake vaves/guides to maximum vacuum. this will suck
oil past the valve stems & allow deposits to form on vales & seats, keeping
the valves from sealing properly, and in extreme cases fouling the spark
plugs. Using positive type valve seals will help eliminate this, as well as
eliminating air leaks that can cause poor performance. (forgot to mention this
before)

   For your 150K engine it is hard to guess at the condition because these
engines have been known to last for many more miles with reasonable care, and
alot less if abused.  You can just remove a valve cover, and try to move the
valves with the springs assembled.  If you can detect any movement the guides
are probably worn enough were you should consider a cylinder head overhaul.
If you can detect little, or no movement they are probably in good enough
shape to try just the valve seals.


   Eldon
----------------

Thanks again to Eldon for this priceless information.

Darrel Kline
Colorado Springs, CO