Thanks Jim. It seems like I remember a trick where you could run the engine at night looking through the prop at a florescent light and adjusting the RPM until the prop stops. This will give you a multiple of 60 RPM, for instance, you should be able to stop the prop at 3660 RPM. Any difference in the tach reading from that value, would be the error. And then there's the trick of flying behind another airplane with a know accurate tach and having him set it upon an agreed upon value and adjusting your rpm to stop the prop. I've done both. Thanks for the reminder, I'll check it out.