Reconditioning the Armstrong Shock Absorber or Hydraulic Damper
The shocks I reconditioned,
due to the absence of written technical material, were done by trial and error.
I had taken some others apart earlier so I had a bit of an idea what was
The whole thing, as I see it
with the shocks, is that they wear the shafts, where they rotate in the bushes,
and/or they wear the bushes in which the shaft rotates. Either way the wear
allows excessive movement of the shaft in the seals causing the seals to wear
out and allows the oil to escape, thereby starving the hydraulic operation of
its most important component, the hydraulic oil. It then no longer is a
hydraulic operation, but merely a wishbone operating in a fairly dry, elaborate
When I dismantled the front
shocks, which are in quite good quality, in that they have cast iron bodies,
the only real wear I found, was on the shafts, and that the seals were shot. I
subsequently found that the bushes were in good condition with no wear on them.
I dismantled the shocks in
the following manner:-
- remove the
bolt securing the two arms together, and remove the arm that is bolted and
clamped onto the shaft
the valve assemblies from the cylinder ends of the body, making sure to
identify which valve assembly goes in which hole. (Be careful here, there
are small springs and things that will drop out)
- remove the
alloy end plate on the body, by removing the small bolts
the plate removed it will expose the "rocker" which is a very
'heavy interference' fit on the splines on the shaft.
in the hollowed centre of the rocker, round the shaft will be a small
collar, secured by a small 3/16" bolt.
- Undo the
bolt and remove it from the collar. You will not be able to remove the
collar. This will only come out after the shaft has been pressed out. Try
and mark the rocker and the shaft, on the large end of the shaft, to aid
in reassembling them in the correct position.
- Now comes
the hard part. The shaft needs to be pressed out of the rocker, through
the body. The other arm at this stage remains on the shaft. Its a matter of trial and error on how to mount the
shock in the press, BUT when you have it mounted, make sure you have the
body very securely mounted and supported VERY close to the seal and bush
boss, NOT ON THE FLAT BODY OF THE SHOCK. I
did one of my shocks by mounting it on the flat of the body, and it
smashed the side out of the shock !!! After that
I mounted them all on the Seal and Bush boss and had no more trouble.
- You will
probably need about 10-15 tons pressure to press the shaft out !
- When the
shaft is out, it's a simple matter to remove the pistons, by pulling them
straight out of the cylinders. You will find there will not be any wear in
the bores or on the pistons. Each piston has a small valve in the top of
the piston, just check to see that the spring and valve body is in there,
and that the whole thing is still firmly mounted to the piston.
- Next check
the shaft. Mine were worn so I then pressed the arm off the shaft. Before
you do though, mark the end of the shaft and the arm so that you
reassemble them in the correct position. If they are not in the correct
position they will not align properly so that the other arm is in the
correct position to have the securing bolt mount properly in the indent in
the shaft. It might be a good idea to mark both arms, and both ends of the
shaft, that way there can be no confusion.
- To press
the arm off the shaft you will probably need about 30 tons to get them
separated. If you are having trouble separating them, heat the boss of the
arm, very quickly, while you have as much pressure as you can get on the
end of the shaft. That will allow them to separate.
- When you
get the shaft out, and its worn, send it off to
an industrial hard-chromer, one that repairs
hydraulic rams and pistons and have the shaft chromed and ground to
- When you
come to take the seals out you will find that they have been secured in
the body by having a very thin section of the boss "rolled" over
the outer edge of the seal. File this "rolled" section off, its very thin and will file
off easily. If you try to straighten it out you will only break it and
ruin it. And you will still have to file the body smooth and clean anyway.
- When you
have removed the seals, measure the brass bushes in the body. Ideally they
should only have 2-3 thou, certainly a max of 5
thou clearance on the shaft. If there is any more than that, press them
out and replace them with new bushes. At best you should be able to buy
the correct sized new bushes from any good bearing house, or at worst you
will have to have them made. Either way make sure
you buy or use 'phosphor bronze' material. When you press the new bushes
in make sure you true them up by honing them out, much the same way as you
would fit Kingpins in their bushes.
- The body
seals are a standard sized 'neoprene' seal which should be pressed into
the body and secured with "Loctite" or
"Permatex". In addition to this I made
'washers' to go over the shaft to fit between the body and the arms. When
the arms were reassembled on the shaft the washers only had about 1-2 thou
clearance between the body and the arm, and which then effectively
'locked' the seals into the body, in a similar manner to the way the
"rolled" section held the seals in.
- To get the
proper clearance between the body and arms, assemble the shaft and arms
(out of the body), measure the distance between the inner face of the
arms, and then the outer faces of the body seal boss. Subtract 4 thou from the difference, divide the difference by 2, and
that then becomes the required thickness of the washers.
- When you
have the bushes and seals in the body its a matter of the reversal of the dismantling
procedure. MAKE SURE THE ARMS ARE POSITIONED ON THE SHAFT in the correct
position, before pressing them back together.
- Cut a new
gasket for the end cover, replace the end cover and cylinder valves, and
fill with good quality mineral hydraulic oil. DO NOT USE AUTO TRANSMISSION
OIL, its got friction
modifiers etc in it that are not conducive to the good operation of
hydraulic components. I used "ENERPAC" Premium Hydraulic Oil, its quite expensive but you
should be able to buy a small quantity from a good hydraulic repairer,
without having to buy a large quantity.
- To get the
shock primed and working properly, mount it in a vice with the cylinders
down, fill with oil and work the arms through the full extent of their
travel, a number times. You will feel the 'resistance' building. Make sure
that the shock is kept full of oil at the filler whilst carrying out this
operation. When you have achieved maximum 'resistance', check the oil
again, and top up if necessary.
So there you have it. If you
get stuck with the resizing of the shaft, I can probably get them remade. I
cannot remember how much it cost to have them done, it
depends a lot too, on the amount of chrome build they have to put on the shaft.
The rear ones Shock absorbers
are a little bit different, but similar principles apply. Instead of having 2
arms there is only one arm, and the opposite side of the body is sealed with a
welsh plug, and its only the fit of the shaft in the
'rocker' that holds the shaft and arm in place. It is most important that when
taking the welsh plug out, no damage is caused to the body. Also these shocks
are an alloy body and I found that I had to go through about 5 shocks before I
found two that were not fractured/cracked in the body,
round the bottom of the cylinders.
To check for fractures/cracks
clean the shocks, but DO NOT buff them or scratch them in any way. Get a
magnifying glass and minutely inspect the end of the shock, round the ends of
the cylinders. If there's a line there or it looks like a crack, then the
probability is about 99.9% that it is racked. Once they crack they are RS, and
all you can do is check the next one! If you want to confirm that its cracked, fill the shock with any thin oil, even kerosene
would do, mount it in a vice and pump the arm through the full extent of its
travel, as fast and as hard as you can. If it's cracked then the oil will squirt
out of the crack.
Also you will find that the
shaft runs in the actual body, there being no bronze bushes in the shock (like
the front), unless it has been repaired before. If there is wear in the body
then it will have to be machined out and bushes made for it. Also if the shaft
is worn its a similar matter
to the front shocks to have it built up.