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MMD > Archives > November 1999 > 1999.11.16 > 09Prev  Next

Clearance for Fluted Valve Stem
By Craig Brougher

After having rebuilt an old "Cardboard Stack" Amphion action again, it
came to mind that maybe some people would get a benefit knowing what
the maximum tolerances can be between a fluted valve stem and the sides
of the inside valve seat which guides it.

Ampico is probably the best known valve designed this way.  There were
two of these Amphion designs basically: the large one and the small
one.  The large hole was about .560 diam.  That part isn't so important.
Percentage clearances are.

So regardless what size hole the fluted stem fits in, if the clearance
is very much over 6%, the poppet valve will have too much slop.  The
large stemmed Amphion valve had about 4.8% difference in size and the
small stem had about 6% difference.

Percents are figured like this:

  Diam. seat - Diam. stem = diff.
  diff. / Diam. seat x 100  =  % diff. (not to exceed about 6%)

I am working on a stack having a 9% difference, and it creates far too
much valve play, causing uncertain seating of the valve.  Sometimes the
valves hiss, and sometimes they seat tightly, always on the upper seat,
though.  The sealing is fine on the inside seat.

The cause is incorrectly sized brass lower seats.  I am presently
modifying this valve to seal tightly on both seats, but while I was
doing it, thought that maybe someone who was having the same problem
would like to know what might cause it.  It isn't an easy problem to

My tactic may not work as a sure-fire fix for most rebuilders, so
if you are experiencing this, get in touch with me and we'll go over
it in detail.

Craig Brougher

(Message sent Tue 16 Nov 1999, 23:58:44 GMT, from time zone GMT-0600.)

Key Words in Subject:  Clearance, Fluted, Stem, Valve

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