I am useing a Tower Hobbies tach. I want to know, where do you point the tach? ( If it matters ) At the tip? Center? By the hub? How far away? from the front?
The readings are sometimes all over by 1000 of rpms.
I have a OS 1.60FX, Master airscrew 18 X 6 wood, bisson pitts, NO pump.
I am getting a reading of 8100rpm. I see other post on here where guys are getting in the nines and even saw a 10,100[X(]
I can't get anymore out of it before it starts to go lean.

Ah...at the prop? With you and the tach behind the running engine (for safety's sake) point the tach about the center of the prop blade. If the inside portion of the prop is turning at 10,000, I can pretty much guarantee the outer part is turning the same.

Dr.1

When you're using an optical tachometer, remember that that the tach sees the contrast in light and dark areas in front of it. So, for instance, with your light colored (bright) wood prop, you ideally should have a darker background "behind" the propeller so that there is a greater contrast for the eye on the tach to see.

Sometimes, if you have a dark prop (like a Master Airscrew plastic) on a light background, remember that the sunlight reflecting off the blades can make the prop look "bright" to the sensor on the tach. If you're getting readings that are jumping all over the place, this is likely what's happening. Just try pointing at different areas on the prop disc or just turning the tach slightly to the left or right by 10-20 degrees or so so that the sensor isn't looking directly perpendicular to the prop disc. If the brightness of the prop is nearly the same as the brightness of background beyond the prop, that's when the sensor gets confused and you start seeing some of those wacky readings. When the sensor is seeing good contrast between the prop blades and the background, you'll have good stable reliable readings.