Rebuilding a Montreal Spica TA

Brenno de Zwart (Holland)

About two years ago I rebuilt my TA using the following procedure:

The plunger of the TA, a 4 mm diameter chrome-plated steel rod about 4 cm long, was corroded so I replaced it with an old Festo pneumatic cylinder rod. Hardly any machining was necessary. To open the TA I separated the two parts of the actuator housing by using a file to carefully remove the material from the bottom part that overlaps the top part. Inside you will find a long spring, a brass ring that guides the plunger rod and a small ring against which the spring rests. The rod has a retaining ring in the middle. You can easily cut a groove in the new rod for the retaining ring with a small file while rotating it in an electric drill. (Protect the surface in the chuck with some tape).

Now comes the trick: To allow the system to be filled, I carefully drilled a 4 mm hole in the top of the actuator and tapped it with an M5 thread. I inserted a small screw (length 6 mm or so) with a so-called Dowty seal or bonded seal under the head of the screw. The O-ring is a 3.78 x 1.78 mm 70 deg Shore one. I used a Buna-N O-ring but don't use this material if you refill the TA with brake fluid, for which Viton would be more suitable. I refilled the TA with SAE 15 hydraulic oil, which is a very thin oil rather like sewing machine oil. I think it is better to use this than glycol because of possible swelling, lubricating and corrosion issues.

Re-assembling the actuator requires the machining of a brass or steel ring that fits around the bottom part of it and carries 3 radial socket-head screws. If I remember correctly the inside diameter of the ring (the outside diameter of the actuator) is about 20 mm and it can be about 12 mm high. I made the wall thickness of the ring about 3 mm. The ring has to fit precisely and you can secure it with glue or locktite. Once the ring has been fitted, drill 3 holes at 120 deg and tap them with an M4 thread. When assembling the parts, some additional polishing may be required to achieve an accurate fit. Grind sharp points on the 3 screws and insert them lightly in the ring so that they make small indentations in the top part of the actuator. Drill shallow (1 mm deep) 3 mm diameter holes at these spots and assemble the actuator, taking care not to overtighten the 3 screws.

Now for the filling procedure: Open the top screw with the bonded seal, push out the plunger and clamp it to keep it extended against the spring force. Now fill the TA as far as possible with the fluid. I used an injection needle for this operation. I then heated the bulb with a flame, taking great care not to overheat it. You will probably see that some dirty fluid comes out of the actuator and you may also see some air bubbles come out. Now cool the bulb in a glass of cold water and you will see the level drop. Keep injecting fluid during this so that no air enters. I repeated this process 4 times.

The last step is to insert the screw with the bonded seal under it when the bulb is cold. At this point the plunger is extended too far. I brought the TA to room temperature (left it overnight) and then, referring to the graph on the Montreal website, adjusted the extension at room temperature by loosening the screw. This is a bit tricky as if you loosen the screw too much the seal starts leaking rapidly. As a result I had to do this twice.

As I said I used hydraulic oil but I checked the extension at 100 deg C and it was nearly perfect - a little bit too much, possibly due to air in the system. In fact I think the extension should be perfect at 80 deg C because that is the operating temperature of the engine.

Altogether - Work time: 3 hours, Thinking: 5 hours, Cost: USD 20, Satisfaction: 100%, Getting this written down: 1 hour.

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