To install a generator or an alternator on a Continental 670 engine, you must have a generator step-up drive mounted on the accessory case. This is not an option, even though the name sounds a bit like one.
The step-up drive provides a gear ratio that drives the generator at 1.97 times the rpm of the engine.
The step-up drive mounts to the four studs near the bottom of the accessory case with a gasket in between. The generator or alternator then mounts to the four studs on the step-up drive with a gasket in between.
Although the two gaskets both have four holes in the corners, and look very much alike, they are different and not interchangeable.
The front of the step-up drive that faces the engine, has a gear that mates with a gear just inside the accessory case. The other side has a female spline that accepts the spline drive on the generator. There is no gearing inside the step-up drive, just a straight through shaft. The rpm step-up results from the front side gear meshing with the gear in the accessory case.
There are two bearings in the step-up drive. The front bearing as shown on the right, is an open ball bearing. The rear bearing closest to the generator is a shielded bearing. The shields will keep out chunks of stuff, but they do not seal, so oil will migrate right on through this bearing.
There is a round gasket on the outside of the rear bearing and then a metal disc that is screwed down and safety wired. The 1&3/16 inch hole in the disc gasket allows the outside diameter of the shaft containing the female spline to just protrude through. When this gasket is new the inside edge of the hole fits pretty tightly on the turning shaft and “discourages oil from coming through”.
The front of the step-up drive and the gear fitting into the accessory case are directly exposed to the oil in the case. This oil spreads through the front open bearing and on toward the rear shielded bearing.
There is very little to keep the oil from accumulating in the rear cavity between the step-up drive and the generator. From there the only thing preventing it from getting into the generator is the seal on the shaft of the generator or alternator. Or if the gasket seal isn’t good, it may leak out from between the step-up drive and the generator.
But all is not lost. On the bottom of the step-up drive there is a threaded plug that is screwed into a hole that leads into the cavity between the step-up drive and the generator.
There should be a drain line fitted into this hole, leading to a “T” in the vent/drain line, that goes down the left gear leg from the engine. This drain eliminates the oil coming through the step-up drive before it gets to the generator.
The rear shielded bearing on the step-up drive is an MRC 206 SFF (two shields). There are two other identical bearings available which will fit right in. One has a real seal on both sides of the bearing. The other one has a seal on one side only.
Either one of these bearings would provide a lot more resistance to oil getting through the step-up drive. One would have to decide whether to use a bearing with one seal on the back, and rely on the engine oil to lubricate the bearing from the front, or use the two seal bearing and rely on the permanent lubrication put into the bearing at manufacture.
The SKF designations are 6206-2RS for the two seal version and 6206-RS for the single seal.
At the very least, if you don’t have the drain line, put one in!!
In any event the above information is provided for reference and general interest only and should not be solely relied upon for any application.