The tachometer drive unit that mounts on the rear of the accessory case on the W-670 engine, doesn’t have any seals around the shafts that hold the gears. Ultimately , the oil from the other side of the accessory case will work it’s way through the tach drive and into the cable housing that leads either to the mechanical tachometers or to the tach generator that drives the electric tachs.
There is a theory that if the drive cable inside the housing is wound the correct way, as the cable turns it tends to screw the oil back toward the accessory case. What happens when the engine isn’t running, and the tachometer end is lower than the drive end?
Anyway, the tach drive can be modified to include seals around the shafts, and that is a good thing to consider. I think any overhaul shop can do that.
Another very important consideration when putting in a new drive cable or housing, is the relative lengths of the two. When the connecting nut on the housing is screwed down tight, there must be some clearance between the flag end of the drive cable and the bottom out point on the tach drive, and between the square drive on the other end of the cable, and the bottom out point on the tachometer. If the drive cable is just a little too long, the coupling nuts will start on just fine, but in the last few turns, the drive cable will bottom out and buckle inside the housing. Then the cable will either not turn, or worse yet, wear out faster that a “you know what”.
Another potential trap. On the back of the tach drive assembly is a male 1/4 inch square drive. When the unit is bolted to the accessory case, that mates with a female square drive. That is, IF, a small coupler part about 1 3/8 inches long, and 7/16 inch in diameter with a 1/2 inch square drive on one end and the female 1/4 inch square drive on the other, has been slid into the round hole in the back of the accessory case. There is no keeper, so it is essentially loose until the tach drive is bolted in place.
More troublesome news. The coupler will slide in either end first. One way works and the other doesn’t. The square drive on the end of the coupler has to go in first. “No problem you say” because you always check such things in the Continental manual. Well, the exploded view in the W-670 manual, shows the coupler going in “BACKWARDS”. So, if the coupler is missing, or if it has been inserted backwards, when the engine is turning, the tachometers won’t.
For a drive that is already bolted on, you can put a screw driver in one of the two output ends. If it turns easily, you have one of the two above problems. If it doesn’t turn easily (don’t try too hard), then you are all right. If there is a housing and cable attached to the other output drive while you are doing this test, make sure that you don’t have a buckled cable on that side. This could keep the drive from turning easily, but you could still have the problem.