Tachometer rotation?

[TimHicks]

When I started the re-cover of my '46 BC12-D, my plan was to go back together with my gem of a motor, an A-65.
So when my tachometer was found unserviceable, I bought a new one and had them paint the redline on the face.

After years of flying with a crude slash of red paint on the glass, this looks beautiful.

Since then, I changed my plans and bought a freshly overhauled C-85-12.

Doesn't this new motor turn the tach cable a different direction than the A65 did?

So my question is, can I use this tachometer which displays anti-clockwise? Or do I need to buy (ouch) another one that displays clockwise?

There is a 90° adaptor on the back. And If the rotation works, I'm happy to send it back for a new face with the correct redline.

Tim

[woodmw]

Re: Tachometer rotation?

Tim,

The 90* adapter may let you reverse the rotation. The one I have does, and it looks similar to yours. Remove the cap opposite the drive cable and install the drive cable there. Put cap where cable is now.

[Robert Lees]

Re: Tachometer rotation?

The tach output direction at the engine is the important thing, Tim, and I'm pretty sure that it's the same on the 65 as the 85. Wait for Terry to confirm, but I certainly didn't have to replace the tach cable, so I'm 99.99% sure it's the same. (Tach cables are direction-sensitive).

I did the same as you (85 upgrade). Same cable, same 90 deg elbow behind the fuselage tank, same tach. Only difference was re-marking the red line on the tach itself, which I did by removing the instrument & replacing the red line on the face behind the glass.

Hope that helps.

[Garry Crookham]

Re: Tachometer rotation?

Tim
The tach drive on the A65-8 and C85-8 engines turn counterclockwise while the C85-12 engines turn clockwise (as viewed from the rear of the engine).

[Hank Jarrett]

Re: Tachometer rotation?

I have the same tach and 90* drive. It does work to put the cable in the other side (at least on mine) but the cable DOES need to turn so it is TIGHTENING the winds, not unwinding! I ended up with a 90* on each end and set them up so the cable was spinning the right way and the tach worked. Got a MAJOR headache trying to make sure I got every thing right, then had an A&P tell me it wouldn't damage the tach to be spun backwards, it just wouldn't work. I didn't try it, didn't want to take the risk he might be wrong. ;-)

Hank

The extra 90* angle at the back of the engine sure gives a lot more room to work with the tach cable! I just need to find a couple more of them for the 45. I probably have a few in the "junk box".

[Edwin Otha]

Re: Tachometer rotation?

Tim, The tach you have is a left hand rotation for the A65, BUT, the 90 degree adaptor will reverse the cable drive direction. So the tach with the adaptor is correct for a C85-12 installation.
EO

[n6346m]

Re: Tachometer rotation?

Tim,
If your tach does not have a built-in hour meter then ignore this posting.
Recording tach's meters are usually synchronized to the cruise RPM of the engine and are only accurate at their synchronized speed. When below or above cruise RPM they will read low or high on the hour meter. That's probably why people who rent aircraft use a "hobbs" meter instead of the recording tach so they can charge you full rate when the engine is at idle. The tach on my A65 was synchronized for 2050 RPM cruise. When I converted to the C90 I replaced it with a tach that is synchronized at 2300 RPM, my usual cruise power. Fortunately, it came with the engine and was zeroed out at overhaul time so I don't have to do the pesky math at annual time. Since I don't have a hobbs it's the best way I have to keep track of things. If you plan on buying a new tachometer, be sure to specify the proper recording RPM.

[TimHicks]

Re: Tachometer rotation?

So it sounds like my tachometer will work with the new engine, but that I need either a new tach cable that is made to wind in the other direction, or I need another 90° adaptor at the back of the engine.

If I get the new cable, I'll also have to reverse my 90° adaptor.
If I get the 90° adaptor for the engine, everything downstream will turn in the same direction as the A65 so doesn't need to be changed.

[Edwin Otha]

Re: Tachometer rotation?

Tim, I wouldn't change anything. What you currently have is correct for the C85-12. Down the road if the cable gives you trouble replace it with the wrap direction you want. I just don't know if it's that big of an issue. The cables brake when overloaded from the adaptor gear sets running dry or tight bends. Birdcaging is a problem for cables used in torque situations but the tach cable has a pretty low torque requirement.
EO