The International Cessna 170 Association

I will be fitting a digital tachometer made by Electronics International , ok the wiring up is the easy bit but the hard thing is on removal of the old drive cable . So what do we do with the drive cable attach point on the gearbox . EI doesnt give any direction on this one . There is a hole in the gearbox to sort out . ( all of the digital tachometer is STC ,d of course). So that leaves only the flight instruments left on the aircraft that are steam ( vacuum driven with Venturi,s ) driven . Presently I am looking at the Garmin system also . I was told you can remove the gears from the gearbox and put a plate in but this is something I feel should not be necessary . So has anybody fitted a digital tacho to a lycoming engine . ?

In theory, you should be able to simply disconnect the tack drive cable and leave the attachment housing open. You can't remove the oil pump gears have the shaft which the tach drive cable couples. The seal #17 should seal against the shaft and stop oil leaks weather a tach drive cable is attached or not.
Missing of course from the drawing above is the entire accessory case which the oil pump gear tach shaft couple extends through and the tack drive cable attachment housing is fasted.

Thank you for that, however my tacho shalf does have some oil in it as I thought it was for lubrication reasons . I would be feeling more comfortable with a cap over the end of the housing .

A cap that will fit either The continental or lycoming tach drive housing is available from aircraft spruce, Chief aircraft or Van's aircraft. I think they are about 8-10 bucks.

My digital tach (Continental) came with a screw on cap for the tach drive.