Rear Cover


The rear cover is where much of importance comes together, with outlets and inlets for fuel and oil, as well as a shaft geared for the distributor. The very rear of the crankshaft carries a spur gear which drives the oil pumps. Sandwiched on top of that is the impellor, which ensures delivery of the fuel/air mixture to the periphery of the rear cover, where it is guided into the intake pipes. Making it more of a challenge, since the rear cover acts as the intake manifold for the engine, the fittings must be reasonably gas tight. This is accomplished with Viton o-rings.

The rear cover is the only part which Mr. Hodgson provides with the plans as a casting. The casting of 356-T6 aluminum is of very high quality, uniform, accurate, with no voids. It also machined beautifully. He thoughtfully provided a chucking boss which I turned to 7/8 using a 3-jaw chuck. I then reversed it and used a collet to machine the portion which mates with the crankcase.

The rear cover is supported in a Kurt vise with a collet block on the chucking boss as well as a big iron angle to minimize flexing and chatter during milling. This setup was used to drill and ream the distributor shaft bushing hole, the only drilling necessary that was not axial. A 3-jaw chuck was used for other machining.

Quite a few holes were drilled or reamed. The periphery has 11 holes for securing the rear cover to the crankcase as well as an intake hole, a distributor bush locking hole, and 2 carburetor stud holes, tapped 1/4-28. All of these operations were done as pictured here, with the rear cover mounted on a lathe chuck which was clamped to the milling machine table.

All of the mounting holes were spotfaced 5/16"

Looking downward on the rear cover (temporarily mounted on the crankcase) we can see the carb mounting surface as well as the crankcase mounting holes. The brass fittings are for the oil reservoir, which is an external tank. This is typical of a dry-sump engine, which virtually all radials were at the time. The top fitting recieves fresh oil from the reservoir. After circulation, it drains to the dry sump, where the scavenge pump delivers it to the lower fitting.
Here is an interesting view of the distributor drive bushing. The original design calls for an oilite bronze bush. The only problem there is that the bush is exposed to gasoline, and none of the rear cover accessories are included in the oil-flow pathway which starts at the pressure pump; hence, I was concerned with proper lubrication. The answer was to design a bush which uses sealed ball bearings. Note the upper BB secured with an internal circlip.

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