[Dave in MN]

To the OP: I agree with Badpuppy's comment, it is possible to use the stock 6 volt ammeter in a 12 volt system by reversing the leads.

This is off topic somewhat: I installed an AC unit in a recent build of a touring car. Every time the AC compressor clutch would engage, the needle would peg...hard. After a few days, the needle disappeared as it was knocked right off the pivot points. I did not know what size shunt to install so I installed a second ammeter under the hood to split the load. Both meters show a good draw when it starts...just short of pegging but so far, I have not lost sight of anymore needles.
Anyone have a guess of what size shunt is needed based on my description? Thanks Guys.
Good Day!