If you are going to use this thread to do your own work on the peculiar Sachs shock read all of the thread. I had a learning experience regarding assembly order with respect to the type of Schrader valve used to re gas the shock. This effort grew out of the following thread of putting a Reiger on an older GasGas. That freed the original shock up for playing with and servicing. Here's the other thread: http://advrider.com/index.php?threads/putting-a-reiger-on-an-older-gasgas-2005.1169872/ The German-made Sachs ZF shock typical to many older GasGas and other bikes is a light, well made, simple shock that presents as un rebuildable, but is in fact, rebuildable by those skilled in such work. The Sachs is a very common design. Structurally it is similar to an Ohlins, and you can use some Ohlins parts in the shock, like shims, piston, separator piston, and even the seal head (may require some machining). The Sachs shock in this example is from a 2005 TXT GasGas. Here it is after I pulled it apart: After removing the spring, take a small punch or screwdriver and tap on the edge of the aluminum cover/dust seal to pop it out. You can then see the seal head and retaining ring. The retaining ring is impossible to remove without pushing the seal head into the shock body such that the ring isn't trapped by the step the seal head groove. Pushing the seal head in requires de pressurizing the shock. I depressurized my shock in the process of machining the body for a Schrader valve to later re charge the nitrogen. You can see in the photo above the counterbore and 1/4" reamed hole I made on an up-angle into the body chamber. Note the press-in Schrader valve I machined from a commonly available valve with 1/8" NPT male thread. Once the shock is degassed, the seal head can normally be pushed in and it will stay in or just slowly creep back out. If the shock has internally 'blown' from gas bypassing the separator piston, with the separator piston hard up against the bottom of the shock body bore, the shock will remain somewhat pressurized even after degassing the gas chamber. In that case a special fixture is required to push in the seal head to remove the retaining ring. In my case the shock was in excellent shape with no excess gas in the oil chamber. It was in such good shape with pretty clean green oil that I thought it no possible for it to be the original shock. However, 20 and 05 was milled into the cap over the seal head, so possibly it is. Regarding getting the seal head retaining ring out, Randy Lewis told me of a trick which I used. There is a tiny bleeder hole in the shock body. It is drilled on an angle, intersecting the retaining ring groove. The original hole is REALLY small at ~0.015". Per Randy I drilled the hole larger. He said 2mm (.078"). I chose 0.067 (#51 drill) to just clear a 1/16 steel welding rod to use as a push tool. The larger diameter the hole also becomes a better bleeder port when reassembling the shock: Photo error! That should be 0.067", not 0.670". When you drill out the hole, make sure to push the retaining ring around in the groove so the space between the two ends is where you will drill. You don't want to crash into the retaining ring when drilling! With the push tool and a tiny screwdriver, the retaining became easy to remove. I then worked the seal head up and out with up-pressure and a slow wobbling motion. The seal head popped right out, but the piston was then stopped by the second retaining ring. You can see here the two retaining ring grooves: The second, 'safety', ring is to keep the shock from flying COMPLETELY apart by stopping the piston. This scenario only happens after a blown shock has been repeatedly abused with hard top-outs such that the seal head retaining ring groove in the shock body fails fatiguewise. The probability of shock separation with a not-blown and not severely abused shock are just about ZERO, so that second ring just makes fully disassembly of the shock a royal pain! The second ring takes skill and patience to get out, as it is farther down in the bore and you have the shock shaft and seal head flopping around. The trick is to push a small, sharp tool behind and under one end, then push the end down deeper into the shock body to work it progressively out of the groove. It will eventually pivot in the bore, free of the groove. I belt sanded a very small straight pick to push behind the ring. A tiny screw driver with a head lamp and magnification rounded out the two-handed+ job. Three times I got the ring started out of the groove, only to have it snap back home This infernal second ring will not be going back in my shock! With the addition of a Schrader and no second retaining ring, the Sachs will become a normal, easily rebuildable shock. With the rod and piston assembly out I could get the specs on the shim stacks. This is a simple shock having fewer shims than some other shocks. I also found, as Randy Lewis said I would, the piston assembled by Sachs upside down. He said in the 2000s the assembler(s) started flipping them over. When flipped over, two through-holes port to the compression stack: ...and four holes are ported to the rebound stack. That is contrary to convention, as these piston holes are like a main jet for high-speed compression, and usually you want more maximum flow capacity in the compression direction for the rapid big hits. The Sachs has really huge high-speed damping holes, however, so halving the flow area of the compression holes may not be that noticeable? The other effect of flow area is how much pressure surface area presents to the first shim of the stack for lifting force. Sachs shocks tends to feel more bound up in rebound, and the adjuster, which affects rebound speed more than compression speed, tends to like better closer toward minimum damping or even at minimum. Having the area of four holes acting on the rebound stack may possibly have tested out on a shock dynamo as a cheap improvement in overall damping?! The piston will go back in flipped 'right side up.' To deal with the bound-up feel of rebound damping I will add one or more 'rocker shims' between the larger rebound shims closer to the piston. For additional viewing pleasure, here is a comparison of shim stacks between this Sachs shocks and two shocks I got into previously. Unfortunately I did not take data on piston holes area.