RPM cable leak

Have oil leak at RPM cable. What to use for a gasket

JGT said:

Have oil leak at RPM cable. What to use for a gasket

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It doesn't have a gasket there. There is a small oil seal in there that has failed and is now leaking oil out past the tach drive shaft. If you don't replace it soon the tachometer will slowly fill up with oil! It is a very difficult seal to replace without the right tool and with the engine in the airframe. Watch the short video at the bottom of the linked page.
https://www.aircraftspruce.com/catalog/topages/atstachdrive12-04820.php

I had the same issue. Bought a cap to seal the mechanical tach drive, and now use the RPM in the new engine monitor.
It's a made-for-purpose cap with proper O-ring seal. Bought it from Aircraft Spruce, IIRC.

Edit: Found it... https://www.aircraftspruce.com/catalog/inpages/tachcap.php