Austin Healey 3000 Mk III BJ8 Overdrive Hydraulics PDF Manual

The heart of the Laycock de Normanville overdrive is the hydraulic system. Inside the overdrive case is a small pump that is driven by a cam on the main shaft. The pump produces 450-500 psi of hydraulic pressure. If the system is not well maintained properly or contaminants or dirt are introduced during a rebuild the car owner is asking for trouble. Small dirt particles can foul and block tiny hydraulic passages in the system.

An important component in the hydraulic system is the accumulator piston and its housing (Figure 1). The piston and housing in the photograph were removed from my Laycock de Normanville overdrive hydraulic accumulator housing, piston with rings, and metal ring compression tool. The housing and piston show scoring from approximately 120,000 miles of use.

I have owned the car for 39 years and in all those years the overdrive has never failed. It has always has engaged and disengaged smartly when the OD switch was thrown or when I depressed the accelerator to pass a slower Ferrari or Jag. However, close inspection of these critical overdrive components during a recent tear down and rebuild of the driveline components revealed significant damage. In Figure 1 the scoring of the inner housing bore and similar damage done to the piston are obvious. Although I have routinely changed the oil and was careful to avoid introducing contaminants the accumulator items were badly worn. It was clear that new units were required.

Note that the piston seen in Figure 1 has a series of metal rings located in a groove near the bottom of the piston. The piston and rings were purchased from Moss Motors as separate pieces and require assembly. There are a total of 6 cast iron rings. These rings are brittle and can break easily. Therefore the installation of the rings requires some care and finesse. The ring set consists of two wide rings which are installed on the piston first and line the bottom of the groove. These are followed by four narrower rings that rest on top of the two inner rings. The next step requires great care and patience, that is, inserting the ringed piston in the near zero tolerance of the accumulator housing bore. It is necessary to compress the rings so the outside diameter exactly matches the inner diameter of the housing bore. It is also necessary to align the piston recisely in line with the longitudinal axis of the housing as the piston is inserted into the bore. Failing this will ensure a binding situation that can put stress on the fragile rings and may result in breakage. Trust me on this one.

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