When oil manages to leak out of the tach drive on the accessory case on the W-670, it drains into the tach cable housing. Since it is going down hill, it finally ends up in the tachometer. Oil can leak out of the tach drive through any space between the two tach drive shafts and the housing. This certainly doesn’t do the tachometer any good and there is a second result that is annoying.

If the cable drive housing doesn’t have a very tight connection to the tachometer, the oil leaks out and drips on the pilots and copilots right leg. There have been various solutions offered for this problem, but one of the most creative was the Stearman owner who fashioned an apron for his right leg, with tie straps to hold it in place. The dripping oil was caught by the apron instead of his trouser leg.

Another solution that is widely presented and sometimes implemented is based on the theory that the drive cable inside the housing is wound in such a way that when the drive cable turns it pumps oil down from the tach drive to the tachometer.

stearman-book w-670 1

This theory says that if you get an internal drive cable that is would turn in the opposite direction, and it will then pump any oil that gets into the cable housing back up through the tach drive and back into the engine.

my each time I hear it, but what the heck, whatever works. Needing a technical article to replace the promised ones that didn’t show up for the May issue (what’s your deadline?), I decided to run an experiment to test this theory.

I assembled a tach drive cable housing with a internal drive cable wound in the normal direction. I immersed one end of the housing and cable in a container of engine oil (about 7 or 8 inches). The other end was positioned at about the same level as the surface of the oil bath.

I then set up to drive the internal cable with an electric drill in the direction that would move any oil toward the drill and away from the oil reservoir. At the drill end exit, I placed a shallow dish to catch the oil. If it started coming fast, I was prepared to put a deeper dish in place. I planned to stop before I emptied the whole quart of oil in the reservoir out onto my work bench.

I also oiled the internal drive cable before placing it in the housing. I then started the drill at about 800 rpm and let it run, closely watching the exit end of the housing.

After two hours of running there was no sign of oil at the exit end—not a single drop.

It became clear “That Dog Won’t Hunt” and “That Oil Won’t Pump”. This experiment verified that the internal drive cable in the tach drive housing will not pump oil from the tachometer, back uphill to the tach drive, and then through the restrictions in the housing, and into the engine. But all is not lost, and you don‘t have to go out and be measured for a yellow leg apron.

Fig.1 - Tach drives without seals.

Fig.1 – Tach drives without seals.

The photo on Fig.1, shows a stock tach drive on the right, ready to install. It has the square drive that mates to one in the accessory case.

Fig.2 - Two shaft seals, SKF 3645 or TCM 03061VM, and Tach Drive Cap.

Fig.2 – Two shaft seals, SKF 3645 or TCM 03061VM, and Tach Drive Cap.

The photo on Fig.2, shows the tach drive disassembled. The two shafts mate into the two holes in the housing driven by a center gear at half the speed. The two shafts turn in the holes drilled in the aluminum housing – no bushings.

Fig.3 - On left, seals Installed. On right, counter bored for seals.

Fig.3 – On left, seals Installed. On right, counterbored for seals.

These steel shafts tend to wear into the aluminum holes making room for oil to travel along the shafts into the drive housing. This problem can be rectified by counter boring the housing around the hole that the shaft goes in, and pressing in a lip type oil seal sized for the 3/8 inch shaft. The photo on the bottom right shows a housing on the right that has been counter bored, and the one on the left has the seals installed.

These seals keep the oil from moving into the cable housing but enough oil migrates through the seal to keep the shaft in the housing lubricated.

There are several seals from different suppliers that will do the job. This one is sized 3/8 X .687 X .156. Center bore, OD, and thickness.

I was introduced to this fix at Aero Engines, and I believe that they will provide units with the seals installed with a good core in exchange. However any good machinist could do this mod.

I have had these seals in the tach drives on both 4432Q and 77744 for many years and have never had oil leak into cable housing or the tachometer.

You should also have these seals if you have electric tachs. There still is a short drive cable and housing that leads from the tach drive to the tach generator, and you don’t want oil in the generator.

When using the short cable and electric tachs, there should be a cap on the unused drive output.