Hi everyone, I'm new to this forum and suspect this question has been asked long before me. I have a 1971 Mach1 with a boss 351c with a MSD ignition which includes a 6al ignition box, a MSD Blaster SS coil and a MSD distributor. Which all runs sweet as. The previous owner did all this work but the tacho has never worked he told me. As a result he put in a after market tach which works with a MSD 8920 tach adapter. I've disconnected this to try the original tach but it doesn't work as he previously said. Other forums suggest that it needs the 8910 model tach adapter to make it work. Any suggestions please.