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I think you have the Stewart Warner tach like the other 300 series. It is the drag cup type that has a permanent magnet spun by the tach cable. The magnet is inside the drag cup and as it spins, creates a magnetic field that forces the drag cup to move in the same direction. The rpm of the magnetic determines the strength of the magnetic field and so the number of degrees the drag cup turns. Attached to the drag cup is a spindle with the pointer on the end. A spiral spring is attached to the spindle on one end and to the frame on the other. The spring is wound to oppose the magnetic field and serves to force the pointer back toward zero as the rpm drops or stops. In addition the spring has a nut to increase or decrease tension and that is the tach's range calibration. The pointer has a socket that is pressed onto the chamfered end of the spindle. The pointer can be pried off and pressed back on to the spindle in any position and is the zero adjustment and also used to help correct range errors. So you can see there are two adjustments to make the tach read more accurately. This type of movement is non-linear. To compensate somewhat the spaces between the increments on the panel face are wider on the ends than in the mid range I.E. the space between 2 & 4 is about twice that of between 10 & 12, this is particularly noticeable on the speedometer since there are less increments. There is no way to make this movement completely accurate throughout the full range but range adjustment you can make it repeatably accurate at one rpm and it will follow very close with a known error rate throughout the rest of the range. The tach was probably factory calibrated at pto rpm. If you want to calibrate to say pto rpm, run the engine up to pto speed on a known tach, adjust the spindle spring tension to line up with pto rpm. Stop the engine, the pointer drops toward zero. If it is close enough to zero, start the engine and incr rpm by 100's checking against the known tach and determine which way you need to move the pointer on the spindle to lessen the error between zero and pto speed. Be careful prying the pointer off, it's been on there 45 years, if it is close to zero and mid range error is liveable leave it alone. If you pop it off, press it back about 1/2 the error you determined above. And now you will have to readjust the spindle spring tension for pto rpm. Both the pointer and spring tension will affect the full range, pointer being major and spring minor. If you spend the time repeating the adjustments, you can get it as good as it was new. While you are in there put some oil in the drive bushing. It will either have an oil hole or a wick container. Joe
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