9MM

Never had to calibrate mine as it came outta my dads first gen when he rolled it poor guy bought a second gen lol.

Bencie

Bearkiller don't know if yours is the same or not, on the back of mine is a tapered head allen screw, remove that and the back cover comes off and thats where the adjustment is to calibrate and also there is a 1/4 turn bulb socket, if I remember its a 194 style bulb, the gauge even came with colored covers to put on the bulb if ya wanted too. Mine is the same as the one Lucas has just the Z series.

BearKiller

Mine is just one solid plastic housing with nothing I can see or find that will let me inside.

The calibration screw is in a small open hole in the back-center that barely clears a very small screw-driver.

Can you show pictures, front and back, of your tach ??

I wish I was smart enough to do pictures of the one I have.

Bencie

Me too don't know how to do pix. On autometer site there is installation sections with some pix and diagrams, put in your model # and try that.