I had Auto Instruments convert my factory tach to work with electronic ignition. Hooked it up exactly as they said to -- copper/short stuff to the negative side of the coil. Silver/long stud to any keyed fuse. Ran that wire to the radio fuse and it sparked like crazy and blew the fuse, blew the glass even. Put a test light on the wire and lit up immediately. What could be causing it to act as a ground? We tested everything on the back of the tach and everything is acting like a ground??

The two studs on the back of the tach for power and coil trigger, do they have the proper insulation spacers? Sounds like something is grounding out. I'd put the cluster on the bench and test each stud with a continuity tester and make sure there isn't one or more of the studs incorrectly being grounded out. Something is shorting out somewhere. If the proper washers are there then somewhere either the power wire or possibly the coil trigger wire is shorted to ground somewhere beyond the stud underneath the housing.

Actually, there is no spacer/washer on the silver stud. The lock washer is flat against the back of the tach housing. The coil stud does have a small insulated washer it appears. Do I need to put some kind of non-metal washer on it?

Sounds like you found your dead short.

Short of having to spend $25 for the kit online, will a rubber or nylon washer suffice? I see they used nylon insulators around the studs and then a fiber washer. Surely a rubber or nylon washer will work?

I believe you need a non conductive washer that extends over the stud and into the tach body to keep the stud form contacting the housing anywhere.

Any suggestions?

if going into the tach body, get two small pieces of heat shrink that fit the stud. shove them into the body,[one at a time] allowing them to bottom out and protrude a small amount above the back, then apply heat. after that, trot down to your local hardware store and look for a small nylon/fiber insulating washer that fits over the [double] heat shrinked studs. you may want to use one of each of the insulating washers. they can usually be found in the hardware section or the plumbing section. possibly in the electrical section as well. this "may" be a viable fix. i can't say for sure, as i can't see, or test, the cluster in person.

The insulating washers center the studs in the hole and keep the sides from shorting out. The fiber washers go on next, followed by a lockwasher and nut .

Thanks for the help guys! Turns out I was missing the fiber donut that centers the stud. I also was missing the fiber washer. Spoke to Auto Instruments because they rebuilt the tach and he had me just center the stud and use a nylon washer when tightening it down and the grounding appears to be fixed as it no longer does it. Perform graphics sells the kit that includes the fiber donuts, nylon washers, etc for $30 in case anyone needs them. Thanks again guys!