Hello All,
I have a tacho that I’m sure is under reading by 200rpm or so. At 140kts the tacho only reads around 2250rpm (O-200A with a 62X64 prop). On previous flight test reports, apparently the engine and prop combination would achieve 2450 at the same airspeed. The O-200 red lines at 2750, but the prop is very coarse. Flying wise the Ts & Ps are spot on and the engine is running beautifully. I’m concerned as I’m trying to sell the aeroplane but cannot in good faith sell it when there’s that much discrepancy in the RPM. Any advice would be much appreciated!
I should mention its a mechanically driven tacho!
Some ideas to start your troubleshooting 
Old RPM tacos work on the same principle as car speedos. One remedy is to remove the indicator, and either replace it with an equivalent new or used, upgrade to electronic sensor/display, or send it to an overhaul shop… problem being those have been disappearing in the latter years, but I’m confident some must have survived in the UK.
An item worth checking is the mechanical drive cable… running freely?
The engine and flight data you give are of no real help, but since you sense a misreading, and not at ease for a future sale, not many other options. Last one would be to either hook up another tacho, or use an optical digital tachometer to confirm your misreadings. Or maybe setting a bench device with a know RPM input as in the vid below?
A digital tachometer with a laser beam you can aim at the prop is your best friend. You can get one at Aliexpress – it’s sufficiently precise for your purposes and, being digital, won’t lose calibration.
In 2018 our small local FBO lent a device to check our tach. Easy to check.
There is also a phone app called something like sound to rpm. Or it can be done optically with an app (need to tell it how many blades the prop has).
After noting some apparent inaccuracy with my tach I used a digital photo tach through the windshield, on the ground and in flight, taking notes. My situation was interesting because when using that tool I had three sources of prop rpm data: the mechanical tach, the photo tach through the window and the setpoint on my closed loop electric prop speed/pitch controller. As expected the mechanical tach was the issue, while the photo tach aligned with the control setpoint fairly closely.
Having done that, I tried a new Mitchell tach thinking that would be easiest way to solve the problem. Don’t do that, nowadays unless you spend an exorbitant sum new certified mechanical aircraft tachs are Chinese junk. The new Mitchell tach was worse that the one I was replacing, even after being returned and recalibrated by the manufacturer. The best solution ended up being to have the old AC Delco tach rebuilt, and now it is close enough. FWIW I had it done by Airparts of Lockhaven They were slow but did good, reasonably priced work for me. It now reads within about 50 rpm, with some variation depending on… who knows what.
The tachometers original to most Cessnas and Pipers have crimped shut cases, and not intended to be opened They did not want people resetting the hours. They can be opened, and I used to do it often when I worked at the instrument shop, but special tools are required to do a neat job of uncrimping the case, or it’s a mess. Once opened, bending the rotating drag cup arms in or out, and adjusting the pointer spring will create the adjustment. But, you have to have a known and adjustable RPM drive to calibrate the tach with. By the time you do all that, for a 50 year old tachometer, you’re probably just better off to buy a new electronic one.
Thank you all for your replies! I’ll get hold of an optical tacho and go from there. It probably would be counter intuitive to replace it with anoyhrr mechanical.
I gave up on my mechanical tach after two overhauls and replacement by a new unit. I replaced it with a digital tach.
