Armstrong lever arm shocks are available in a variety of damping grades.
The variations are made primarily in the valving.  The way these devices
work is that there are two pistons pushing oil back and forth through an
orifice.  This orifice is small, so there is resistance to flow due
to the viscosity of the oil in the piston chambers.  If the size of the
connecting hole was not allowed to vary, the damping would be VERY strong
for sudden jolts. It would be so strongly damped that it would be rigid
for all practical purposes. In order to accomodate sudden jolts, the oil
is allowed to force its way past the small orifice through a larger
spring-loaded valve.  There are actually two separate valves in the valve
assembly.  One to control the upward jolts (bounce)(#13 in the diagram) and
another to control the downward return of the suspension to its rest
position (#15).

The springs (#17,18) adjust the damping.  When these units were new, there
were alternate compression springs available.  Kastner said that certain shocks
had as standard a steel (grey) colored compression spring, but other shocks
had a bright copper colored spring as standard.  Mine have the bright copper springs.
The "competition" spring has a dull copper or bronze color.  I have seen none
of these competition springs available nor do I know the various spring
strengths to try to get new ones.  There are "heavy duty" valve assemblies
for certain cars, but none that I know of for the TR's, and I doubt that they are
completely interchangeable.

In order to adjust damping today, we can tighten the nut holding the
rebound spring and insert spacers (washers)under the compression spring.
I have two sources that pretty much agree on what modifications to make.
First use 30wt oil for the hydraulic fluid (Kastner recommended 40wt).
Next adjust the springs.  I have been told to use Harley-Davidson fork oil
because engine oil has detergents which will damage the rubber seals, and the
fork oil has anti-foaming additives to cut down foam when stressed to the max.  You
have to use H-D oils because the others use stuff that is maximum 20wt.

My first source says to tighten the nut on the rebound spring "all the way down",
and install a 0.070" spacer under the compression spring at the bottom of the bore.

My other source says to tighten the nut on the rebound spring 2 to 4 full turns,
and a spacer of 0.040" to 0.080" under the compression spring.  Select the amount
of adjustment depending on how much damping you want.  I decided to try settings
near the maximum and ease off from there if they were too harsh or if there was
some other problem.

My experience is limited, but successful.  I tightened the rebound spring nut
by 4 turns (which is almost all the way down).  I used 2 brass washers under each
compression spring.  The washers were obtained from my local hardware store for
about 15 cents each.  They measured just over 1/2" diameter and 0.038" thickness.
I used a file to smooth and slightly thin the washers.  I had to ream the center
holes large enough to easily pass the rebound spring.  The four washers were
matched in pairs and I got 0.073" combined thickness on each side.  The outer
diameter of the washers was 0.563", and the inner diameter was 0.322".

I have found that the damping is quite nice.  I am using the TriumphTune 4212
rear springs (420 lbs/inch and about stock ride height).  This
combination seems very satisfactory.  Good damping without being harsh.  Total
cost for the upgrade: $4.95 for a pint of "heavy duty" (about 30wt) fork oil
and about $1.50 for the washers.

Let me know your experiences!

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