I am building my own battery cables for my 70 Charger RT and I was wanting to know if 10 gauge wire would work for the small red wire that goes from the battery to the starter relay on the firewall.

And if 10 gauge wire would also work for the grey wire that goes from the starter relay to the starter???

my 65 Imperial wiring diagram shows a 12ga from the relay into the bulkhead protected by a 16 ga FL so 10ga would ge a step up. Which other (grey) wire are you asking about?

10 gauge would work. Either I'm guessing it's goin' to your ignition switch or your fuse box. If the grey one is going to your starter it's gotta be the starter wire to the solenoid, which should be no different than a field wire to your alt. Aslong as it feeds 12 volts and excites it (engages the solenoid/washer).Make sure there is no diode in the system your replacing(sorry worked on boats, motorcycles and airplanes the last 2 years and have forgotten the systems). But should be pretty straight forward.

The Mopar Junky

If going into the bulkhead connector, most likely is for the amp guage. Use 10 guage..

Just my $0.01...

Do you still have a fusible link?

is more important a good alt, then the 10 gauge wire into alt wire, more than the batt wire itself where 12 could be enough... some other procedures are also advised depending on alternator used and equipment options

If you are rewiring the car. Why don't you just run the heavy wire from the alternator to the starter relay (which is actually the connection to the battery.

What is this grey wire from bulkhead to the starter?

The only wires to the starter should be a constant power which is the big positive battery wire and a small brown wire from the starter relay.

I made a mistake. I went and made corrections in my first post so you know what I am talking about.

The wires I am referring to are the small red wire that goes from the positive battery post to the starter relay on the firewall and the grey wire from the starter relay to the starter itself.

I notice that the stock wires that route to the starter relay have hard black rubber blocks near the connector ends...what purpose do these rubber blocks serve? I will try to post a pic of them.

What are the black rubber blocks for near the end of the wire connectors which go to the starter relay?

simply terminal isolation...

oh ok thats the grey wire... I think normally is black... maybe faded ?

here too

yes those are... rubber blocks are simply terminal/wire splice insulation