Diddling a Tach

I've been chasing down some tach-indication problems lately, and as part
of it, I installed a borrowed tach in the Fly Baby. I'm ready to put
the original tach back in, but would like to change the hour meter to
reflect the time that was flown with the temporary tach in place. The
tach is a standard model made by AC.

Opening the tach is non-obvious. There's a set of screws in the back,
but there isn't a big enough opening in the back for the tach innards to
come out of. It looks like the faceplate of the tach needs to be
removed to let guts come out the face, but the faceplate looks like it's
formed around the mounting flange.

Anyone got a hint at how the tach comes apart...for that matter, will I
be able to just manually turn the hour-meter tumblers once the innards
are exposed?

I'm ready to rig up a power drill to turn the thing the old-fashioned
way, but since I'd flown about five hours on the borrowed tach, it'd
take about the same amount of time to run the new tach forward. Hoping
for a quicker solution.

Ron Wanttaja

Ron Wanttaja wrote:
I've been chasing down some tach-indication problems lately, and as part
of it, I installed a borrowed tach in the Fly Baby. I'm ready to put
the original tach back in, but would like to change the hour meter to
reflect the time that was flown with the temporary tach in place. The
tach is a standard model made by AC.

Opening the tach is non-obvious. There's a set of screws in the back,
but there isn't a big enough opening in the back for the tach innards to
come out of. It looks like the faceplate of the tach needs to be
removed to let guts come out the face, but the faceplate looks like it's
formed around the mounting flange.

Anyone got a hint at how the tach comes apart...for that matter, will I
be able to just manually turn the hour-meter tumblers once the innards
are exposed?

I'm ready to rig up a power drill to turn the thing the old-fashioned
way, but since I'd flown about five hours on the borrowed tach, it'd
take about the same amount of time to run the new tach forward. Hoping
for a quicker solution.

Ron Wanttaja


Simplest way is a log book entry to correct tach time to real time.

cavelamb wrote:
Ron Wanttaja wrote:
I've been chasing down some tach-indication problems lately, and as
part of it, I installed a borrowed tach in the Fly Baby. I'm ready to
put the original tach back in, but would like to change the hour meter
to reflect the time that was flown with the temporary tach in place.
The tach is a standard model made by AC.

Opening the tach is non-obvious. There's a set of screws in the back,
but there isn't a big enough opening in the back for the tach innards
to come out of. It looks like the faceplate of the tach needs to be
removed to let guts come out the face, but the faceplate looks like
it's formed around the mounting flange.

Anyone got a hint at how the tach comes apart...for that matter, will
I be able to just manually turn the hour-meter tumblers once the
innards are exposed?

I'm ready to rig up a power drill to turn the thing the old-fashioned
way, but since I'd flown about five hours on the borrowed tach, it'd
take about the same amount of time to run the new tach forward.
Hoping for a quicker solution.

Ron Wanttaja


Simplest way is a log book entry to correct tach time to real time.


Neither being a lawyer nor having seen one on TV I would wonder if
resetting an hour meter without putting an entry into the log would
equivalent a false entry.

Dan, U.S. Air Force, retired

Dan wrote:

Neither being a lawyer nor having seen one on TV I would wonder if
resetting an hour meter without putting an entry into the log would
equivalent a false entry.

I agree. When I put the temporary tach in, I noted the reading of the
previous tach and the current reading of the new one. Had an annual in
the interim, and the A&P computed the actual time based on the two tach
readings.

I know how many hours the temporary tach recorded, and plan on adding
that amount of time to the old tach before installation. Appropriate
log entries will be made, but I'd like to get back to reading the
aircraft total time directly rather than have to compute it for every
log entry.

I don't think I'm alone in that regard...if you look at the listings for
replacement mechanical tachs on the Aircraft Spruce page, you'll see
they can be set to whatever value at a slight additional charge.

Ron Wanttaja

On Jul 13, 1:51*am, Ron Wanttaja wrote:
Dan wrote:
*Neither being a lawyer nor having seen one on TV I would wonder if
resetting an hour meter without putting an entry into the log would
equivalent a false entry.

I agree. *When I put the temporary tach in, I noted the reading of the
previous tach and the current reading of the new one. *Had an annual in
the interim, and the A&P computed the actual time based on the two tach
readings.

I know how many hours the temporary tach recorded, and plan on adding
that amount of time to the old tach before installation. *Appropriate
log entries will be made, but I'd like to get back to reading the
aircraft total time directly rather than have to compute it for every
log entry.

I don't think I'm alone in that regard...if you look at the listings for
replacement mechanical tachs on the Aircraft Spruce page, you'll see
they can be set to whatever value at a slight additional charge.

Ron Wanttaja

The tach case is crimped around the faceplate. I took an old
one apart by uncrimping it and taking out those two screws on the
back. Took the tach mechanism out of the case, sawed a great chunk out
of the case, and put it back together as a teaching aid to demonstrate
the magnetic-drag type of tach. Students get a kick out of the rest of
the demo, which is to use an old rare-earth hard-drive magnet and a
strip of .032" aluminum that passes through the gap between the magnet
faces. Aluminum isn't magnetic until it passes through a magnetic
field, whereupon a current is generated in the aluminum, and that
current generates an opposing field that interacts with the magnet's
field to make the aluminum resist movement through the magnet. The
tach has a small bar magnet that spins in the aluminum cup that drives
the indicator needle. Tachs start to underread as the magnet ages and
weakens.

Dan

On Jul 12, 7:48*pm, Ron Wanttaja wrote:
(snip)
I'm ready to rig up a power drill to turn the thing the old-fashioned
way, but since I'd flown about five hours on the borrowed tach, it'd
take about the same amount of time to run the new tach forward. *Hoping
for a quicker solution.

Ron Wanttaja

Ron .........

On the shelf under my lathe lies several small sychronus 120 VAC
motors I've salvaged. It's relatively easy to hook one up to a tach,
then let it sit on the bench, quietly spinning away until the hour
reading is correct (or close enough to finish up with a drill). You're
welcome to have one.

Missed you at AWO. On Sunday, I couldn't help but notice a Flybaby
sitting out in the rain with nary so much as a garbage bag over the
cockpit to ward off the rain. Darned if I didn't see it taxiing out an
hour or so later, to brave the elements. I hope he didn't have far to
go. Do leather helmets shrink as they dry? Seems like I saw torture
like that in "Son of Paleface" . . .

Rich S.