OEM Rear Shock Rebuild ( ST1100 ) *

[KoTAOW]

Original article written by Jim Randall, STOC #1230 from Kingsport, Tennessee.

It can be found here:  http://webpages.charter.net/jrandall/st1100/shock/st1100shock.htm

Jim has graciously given permission for this article to be copied and reposted on the AOW thread.

Rebuild (actually an oil change) for ST1100 Rear Monoshock

The Honda ST1100 is a fine sport-tourer but being a large motorcycle that is often ridden long distances heavily loaded, it is hard on the rear monoshock. Many people believe the OEM shock is good for only about 30K miles. Since 98% of my miles are one-up and as I'm probably less critical of shock performance than many, I ran my original shock about 75K miles before replacing it with another OEM shock. At 40K miles on the second OEM shock, I could tell it was beginning to get a bit flaccid so I could see another replacement coming.

However, being somewhat of an experimenter myself and also being of a frugal nature (I'm being nice here), I decided to see if I could "rebuild" my old ST1100 shock. This was the original shock from my '94 bike and as noted above, I removed it at around 75K miles - it was very tired but didn't have any oil leaks (i.e. the seal was good) so I thought it would be a good candidate for the experiment. Shocks dampen movement by forcing oil through small orifices or around tightly fitting pistons and as they wear the clearances around these parts become greater. Hence the oil can pass more easily around or through these clearances and the dampening capability declines. It also possible that the constant shear forces may actually break down the oil itself. My intent was to try an old dirt bike (we're talking 1970's DT-1) trick of replacing the original oil with fresh oil of higher viscosity to compensate for the wear.

It took a bit of experimentation but I have a working shock now. The process was this...

1) Remove the old oil
Start by cleaning and sanding the work area. In order to get a good weld all dirt, grease and
the zinc (or whatever it is) anti-corrosion plating must be removed.

Work area at the base of the shock prepped for drilling and welding

Drill a 3/16" hole near the base of the shock to remove the old oil. The ST shock is not gas-charged so there is no danger of a "pressure event". I don't think this technique would work on a pressurized shock unless one welded on a threaded bung for a Schrader valve to allow re-charging with inert gas - even then it would be a guess as to how much gas to put in. It turns out, however, that one hole is not sufficient since the fluid hydro-locks in the shock and will not drain. I ended up drilling two holes (one on each side) and got about 100 mls of fluid out. Also, with two holes I was able to use compressed air to force out all the liquid and a few metal filings from the drilling. And, as it turns out, the two holes would have been necessary to re-fill it anyhow.


The drain/refill hole - 3/16"diameter and low enough that the rotating preload collar won't hit it. There will be another on the other side.

2) Determine the viscosity of the removed fluid.
The drained oil was obviously VERY thin. I needed to know the relative viscosity of this fluid so I could determine what to put back in so I used a homemade comparative viscometer to qualitatively measure the viscosity. This was a plate of glass upon which I placed equal sized drops of the removed fluid along with drops of shock oil of known viscosity. After the drops were placed, the plate was inclined to approximately 60 degrees for 30 seconds and then laid flat again. I then measured the distance the drops "traveled" down the incline and mathematically interpolated the viscosity of the unknown oils from the known oils. I ran the test three times at approximately 65 degrees F and got consistent results so I feel the data is valid. The known fluids were 10W Honda suspension fluid and 20W Belray shock oil. Since I have successfully used ATF (Automatic Transmission Fluid) in the past in MC suspensions, I also threw in Advance Auto Mercron/Dextron fluid of unknown viscosity. To make the long story short, the fluid removed from the shock had a calculated viscosity of 7 wt and the ATF calculated at 12.5 wt (assuming a linear relationship). At this point I elected to use ATF as the replacement fluid.

3) Weld on closing fixtures
I had my son (he's a better welder) MIG weld #10 machine scew nuts on both sides of the shock. This was the first one we tried and it wasn't the prettiest weld but it appears to be oil tight. The second one got better and if we do any more they may even start to look professional at some point.


One of the nut assemblies (actually two nuts stacked) MIG welded on the shock.

4) Refill the shock
I used a standard pumper type oil can to refill the shock with ATF. The first 75ml went right in then I started to get overflow from the secondary hole. Because there is no obvious way to take apart the shock assembly (see update in the post script below) and the spring is a sturdy one, there was no easy way to pump the shock to get it to take the rest of the fluid. So, I put the screws in, re-installed the shock on the bike, set the spring preload to minimum and, like Tigger, I bounced. For the first 6-10 bounces the shock felt like a pogo stick. Then, all of a sudden, the shock was working again. Apparently it picked up the new oil and just simply started dampening. I bounced it a few more times for good measure then removed the shock and was easily able to get the rest of the fluid in it with both screws removed. At this point I lost track of exactly how much fluid went back in but basically I filled it to the point that fluid was running out the other hole. It was approximately 100ml but it's not possible to say exactly since some fluid is retained in the oil can's pump and I had to put extra oil in to compensate.

5) Seal it up
I used #10 machine screws cut off to an appropriate length so as not to strike the inner cylinder and aluminum washers (backing washers for 3/16" pop rivets). In retrospect, I think copper washers like the ones used on fork drain screws may be a better choice than aluminum.


Installed on the bike

6) Re-install and ride
Well this is the fun part, huh? With the shock dampening screw set in the minimum position (i.e. fully counter-clockwise) the shock is quite firm. In fact, it's more firm than the one that came off (~ 40K miles) was at the most firm setting. After about 500 miles, the dampening properties seem unchanged from when I installed it and all-in-all, I'd say the experiment was a success. The "rebuilt" shock is firmer than the original but still compliant enough to provide a comfortable ride. Plus there is LOTS of adjustment left in the dampening screw.

7) Final
To get a good seal the screws had to be snugged down pretty tightly but, in the end, the aluminum washers worked fine. Also since the welds and exposed steel of the shock where I sanded off the anti-corrosion coating will rust, I painted the area with an aluminum paint.  Obviously, I have no idea how long this will last but all I have invested is a few hours of time and an old shock.  I guess time will tell. Check back later for updates on long term durability.

Jim Randall
2/1/2006

Post Script
The following additional tips were provided by another ST1100 Lister. I haven't tried these methods but they seem logical.

1. The spring can be removed with a press. Push the inverted shock down with the hydraulic ram while supporting the the shock's upper (now at the bottom with the shock inverted) spring perch on blocks and the lock-ring will be apparent. Remove the locking ring and release the press to remove the top spring perch and spring.

Jim's comment - Yep this should work. The retainer ring is obvious in the parts diagram below as part number 6 - I shoulda checked this first.



2. The shock is filled at the factory without doing any welding, so it should be possible to empty and refill without drilling any holes too. The rear of the damping adjuster screw is staked in position; I assume to prevent its removal. The top shock rod is hollow so the oil should be able to be changed through the damper adjustment block once the damping adjustment is removed.

3. Drilling and welding only one hole should work with the spring removed so you can pump the shock to remove trapped fluid and suck new stuff in. In fact, you can apply a vacuum to the shock using a Mity-Vac, pinch off the hose, and then stick the hose in a pre-measured container of oil and it'll suck it in for you. (we do this on old V45 and V65 Sabres)

4. If you do the drill & fill method, use copper crush-washers to seal up the shock - that's what's used on the fork legs. Aluminum might work too.

~~~

Jim's comments: May 5, 2008:  The shock was still working well when I sold the bike. I put around 15K miles on the "rebuilt" unit before selling the ST.

~~~

Comments from: "Bob Peloquin"

1. The spring can be removed with a press.  Push the inverted shock
down with the hydraulic ram while supporting the the shock's upper
(now at the bottom with the shock inverted) spring perch on blocks and
the lock-ring will be apparent.  Remove the locking ring and release
the press to remove the top spring perch and spring.

2.  The shock is filled at the factory without doing any welding, so
it should be possible to empty and refill without drilling any holes
too.  The rear of the damping adjuster screw is staked in position; I
assume to prevent its removal.  The top shock rod is hollow so the oil
should be able to be changed through the damper adjustment block once
the damping adjustment is removed.

3.  Drilling and welding only one hole should work with the spring
removed so you can pump the shock to remove trapped fluid and suck new
stuff in.  In fact, you can apply a vacuum to the shock using a
Mity-Vac, pinch off the hose, and then stick the hose in a
pre-measured container of oil and it'll suck it in for you. (we do
this on old V45 and V65 Sabres)

4. If you do the drill & fill method, use copper crush-washers to
seal up the shock - that's what's used on the fork legs.  Aluminum
might work too.

~~~

Here are the part numbers and approximate costs for the OEM Rear Shock that have been researched as of May 2008:

1992-1995:  52400-MY3-781  $365.00

1996-2002:  52400-MAJ-G01  $253.46
                                   -G11
                                   -G41




From archives of John O:

~~~~~~~~~~~ Shock Absorber PNs.txt ~~~~~~~~~~~~

ST1100 Shock Absorbers' PNs and Prices (Sep 05)[Sep07]

 -- PNs/applications from Service Honda's 'remote desktop':
http://69.213.66.54/TSWEB/Default.htm

Part Number      Yr/model    SH  HOP RA   BB  PSP DHP    [ZAN]
----------------------  ------------    -----  -----  -----  -----   -----  -----
52400-MT3-611 91-94S      237  257  267  351  355  428
                                        [245 269  372  341  290  253]
52400-MY3-781 92A-94A   341  371  362  505  511  617
52400-MAJ-G01 95-02S      237  257  267  351  355  428
52400-MAJ-G11 95A          237  257  267  351  355  428
52400-MAJ-G41 96A-02A   237  257  267  351  355  428

Notes:
 model: S=std/A=ABS
 nearest whole dollar

 SH - Service Honda:
http://servicehonda.com/parts.php

 HOP - Honda Online Parts:
http://www.hondaonlineparts.com/

 RA - Ron Ayers:
http://www.ronayers.com/index.cfm

 BB - Bike Bandit (1st-STOC start; 2nd-direct):
http://www.bikebandit.com/PartsBandit/popup.asp?id=55
http://www.bikebandit.com/PartsBandit/popup.asp?beginorder=yes

 PSP - PowerSportsPro (PartsFish):
http://www.powersportspro.com/oempartsearch.asp

 DHP - Discount Honda Parts:
http://www.discounthondaparts.com/fiche_select.asp?sid=01879236X10K20K2004J2I37I41JPMQ1354R0

 ZAN - Zanotti Motors:
https://www.zanottimotor.com/shopping/partLookUp.html