Front Shock Absorber Rebuild

Heinkel Shock/Damper

New shocks are not available, and your existing ones are probably leaking oil. Sound familiar? Just like everything else on your car you have another project on your hands. This one is a little messy but the payoff in ride comfort and safety makes it worth doing right.

Rebuilding the shocks consists of two distinct tasks: Replacing the bushings connecting the shock to the axle crank, and replacing the seal, o-ring, and fluid inside the shock tube itself. 

The shock rebuild is part of a comprehensive front end rehabilitation, which I will be covering in a series of posts. 

I’ll start by assuming you have removed the shocks from the axle. Once you’ve done that (which is not a trivial task) you are ready to do some shock disassembly.  Both of these jobs create noxious fumes, smoke, and possibly flames, so it’s best not to do them in the house. 

Shock Bodies after removal from axle and sandblasting

Heinkel Shock made by BOGE in "Western Germany"

Shock Disassembly:

Step 1: Remove the bushings from the bottom of the shock. The easiest way to do this is to use a torch to burn out the rubber section, then use a hacksaw to cut the outer metal band, finally punching the metal ring out of the shock.

Use a torch to burn out the rubber center

Step 2: Disassembling the actual shock: The shock is held together with a metal cap that is screwed into the top of the shock.  Use a torch (like MAP gas) to heat the shock body really hot and then use an adjustable grinder wrench (like this one from Harbor Freight tools) to remove it.  I’ll warn you, this piece is very difficult to remove. You’ll also melt the oil seal that’s inserted into the cap, resulting in a lot of smoke and odor. If this type of work doesn’t excite you, I suggest bringing the shocks to a local machine shop to have them remove the cover. 

Shock Reassembly:

With these steps done you are ready to reassemble.  Start by thoroughly cleaning all the parts and repainting the shock body. The parts needed for reassembly are shown below:

Heinkel parts cleaned and ready for reassembly

Shock Bushings: These are a stock size, I can’t tell you exactly what they are, but they are pretty much the same (although slightly narrower) than Lambretta scooter engine mounts. I used Lambretta mounts on my first set of shocks, and could hammer them easily into place. I used Heinkel Club UK mounts in my second set and I could only get them halfway in. I had to bring them to the machine shop where they pressed them into place. Another thing to consider when you are reassembling; its not a hard job if you have the proper tools.

Heinkel Damper with bushing before insertion

Oil: Use 2.5w Fork or Shock oil. You can order it online or get it at a motorcycle shop for about $15 per bottle. It may help you to do a “dry” run of this before you use the oil. Make sure you understand how the shock tube fits inside the shock body and how the gasket and dished washer are supposed to sit.There should be enough oil to do two sets (4 individual) shocks.This is a messy job, and you need to stand up parts of the shocks on their ends. I use a plastic kitty litter container since it helps me contain the mess and keep the parts upright. 

Heinkel Damper parts filled with oil awaiting reassembly

Start by filling the tubes with new shock oil. You need about 100ml per shock. Pour about 50ml into the shock body and another 50ml into the shock piston tube. Pour slowly and then move the parts around to make sure you don't have any air trapped. The oil will start to slowly leak out of the shock piston tube so once you start this process so you will want to move quickly to complete the entire job.

Assemble the following parts as shown in the diagram below and slowly insert them into the piston tube. Some oil might overflow out of the top. 

Heinkel shock piston- assemble parts in order shown above

The diagram above shows the cleaned old parts and new parts. The O-Ring (28mm x 1 mm) is new. The other new part is the oil seal in the cap, which is a 10x24x7 radial oil seal. 

Now insert the shock tube into the shock body. You will have to jiggle and turn it around to make sure it seats itself all the way to the bottom of the shock tube. Once you have this figured out you will need to slide the shock top onto the rod. The seal inside the shock top is very tight and the rod is stepped, so you must be careful sliding it over. The top of the top should line up where the wider parts of the tube is as shown in the picture below:

Heinkel Shock Reassembly

Tighten it up with the wrench and you should be good. You can test the shock action by compressing the top rod almost all the way down and slowly pulling it up. If its easy to pull up then there’s probably air in there. You should be able to work that out by slowly pumping it up and down.

That’s it! In the next piece we’ll prepare the axle knuckes for receiving the shocks.