<VV> Engine Knock only at low RPMS

[Brian Goldin]

I've been pulling my hair out trying to diagnose an engine knock.  The sound
is loud and only exists at RPMs up to about 1200.  I'm confident that it's
not detonation or a problem with connecting rods, wrist pin, broken skirt
etc.  If interested I can give a number of things that I've done to rule all
of these out (see
http://www.vv.corvair.org/pipermail/virtualvairs/2004-April/003762.html).

Here's an interesting clue:  I pulled the blower housing off and after
another visual inspection and didn't see anything wrong.  I pulled the spark
plugs out and decided to crank the engine with the housing off get a
different perspective.  While cranking the engine with the starter I was
surprised that knock didn't exist.  I put a spark plug in #6 and I could
then hear the knock only when the #6 had its compression stroke.  I then
took the #6 plug out and put #5 in.  The exact same thing happened but on #5
compression stroke rather than #6; no knock on other cylinders.  This
pattern persisted on each cylinder.  If I put more than one plug in I would
get knocking on each compression stroke for those cylinders with plugs.

I'm wondering if it could be a cam or crank journal, the cam gear or
something in that area.  I've already swapped out the torque converter,
tranny and diff for another one that is known to work.

Any help would be appreciated.

Running out of ideas,
Brian 

1963 Monza Convert. 
#163 Travalon