First, here is a look at the very unique shaft and piston design from the original Rocky:
From the above picture the shock looks almost normal. It has a black piston and an e-clip to hold the piston at the top of the shaft. But there is a spring under the piston between it and the e-clip. What does that do? Let's look at the parts individually to find out.
The shock shaft has a small piston head built in.
The piston is hollow with very thin sides that contain notches. The holes around the center shaft hole allow the shock oil to flow through but they are partially blocked by the head of the shock shaft when the piston is pushed against it by the spring at rest.
The spring.
How it works: The spring holds the black piston against the head of the shaft when the shock is at rest. When the shaft is pushed into the cylinder, like when the car hits a bump, the force pushes the shaft into the shock cylinder. The black piston, which is the same diameter as the inner cylinder wall of the shock, wants to stay in place due to the oil in front of it, so it pushes against the spring, compressing it. This action allows oil to flow past the shaft piston head (which is smaller in diameter than the black piston) and flow through the black piston holes to the other side. This allows the black piston to move forward further into the cylinder providing the damping action needed by the car to absorb bumps.
It is fairly complicated by today's standards but very cool.
Unfortunately these unique shock shafts are impossible to find, so when the original shafts wear out from use or are damaged there is no way to fix the shock while keeping the original piston design. The simple solution is to find a modern replacement shock with the same specifications (length, diameter), but I wanted to find a way to use the original shock cylinders, springs and external hardware while swapping the piston and shaft for something modern in order to keep maintenance easy while keeping the original look.
Here is what I came up with. The design is based around the re-released Kyosho Optima shocks.
I used the following parts:
Alternatively, Kyosho W5181-04 Shock Seal O-Ring (S) can be used, but this part is getting harder to find, is more expensive, and only comes 4 to a pack.
The original Rocky shock parts that will be used are:
How to build the shock assembly:
Step 1: Install a small white washer from W5181-03 into the end cap.
Step 2: Install a 3mm o-ring from ORG03BK.
Step 3: Install an end cap seal o-ring from TD310671 (or W5181-04).
Step 4: Assemble the shock shaft and piston assembly using OT239 and OT209 and the supplied e-clips. There are three piston styles (A, B, C) to choose from in the OT209 pack. I used style C, but you can use whatever piston you want.
The small spacer in OT209 is used to limit the maximum extension travel of the shaft to match the original stock Rocky shocks. In the following picture the spacer is slid up to the piston.
Step 5: Install piston shaft assembly into the Rocky cylinder and fill the cylinder with silicone shock oil up to where the end cap threads start. Remove all air bubbles by tapping on the cylinder and moving the shaft up and down.
Step 6: Install the aluminum retaining washer into the cylinder, making sure it seats uniformly on the inside lip of the cylinder wall. This washer prevents the o-ring from popping out of the end cap.
Step 7: Put a drop of silicone oil on the inner o-ring in the end cap to help prevent damage to the o-ring when pushing the threaded shock shaft through the end cap. Screw the end cap in to the shock cylinder. Make sure the retaining washer remains seated properly while this is done. Also, test and make sure the shock shaft can be fully pushed in to the cylinder and fully extended. If the shock shaft is hydro-locked when pushing it in, loosen the end cap to allow some oil to come out and then tighten and re-try. Since there is no bladder and the shock volume is small it is difficult to have a perfect smooth feel to the shock movement and some air will likely be present, but with some trial and error it should be possible to have a good feeling shock with minimal air inside.
Step 8: Install the external hardware (spring retainer w/screw, ball end, etc.) and you now have a finished shock.
Here is one of my Rockys with the shocks installed:
There may be cheaper solutions for replacing Rocky shocks, but in order to keep the vintage look I think the extra cost is worth it.
If you have found another way to restore or rebuild your Rocky shocks, I would love to hear about it in the comments below.
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