A. The cable from the starter switch to the starter is 13" long and has a 1/4" eyelet at the starter end and a 5/16" eyelet at the switch end.

B. The heavy current cable from the battery to the starter switch is 95" overall and has a 5/16" eyelet at the switch end. Do not go shorter than 95".

C. The cable running between the original twin 6 volt batteries is (15" long overall ???) and is short enough to be self supporting and not drag on the propshaft.

D. The battery to chassis ground cable is (12" long ???) with a 5/16" eyelet at the chassis end. The original chassis grounding point is a bolt in a frame flange near the center exhaust hanger.

E. The cable that goes around the left motor mount for engine ground return is braided, uninsulated, 6.5" long and has 5/16" eyelets at both ends.

The four original battery cable connectors were the vintage helmet type connectors with a single round head lag screw (full thread wood screw) in top to secure the terminal to the battery post. Replacement cables may have bolt type pinch connectors. The battery terminal posts are tapered and different sizes, roughly 5/8" and 3/4" diameter with the positive post being larger. If you convert the electrical system from positive earth to negative earth you can turn the batteries around 180 degrees in the carriers. The center battery cable can switch ends, but the main and ground cables must change connectors for the different post size. It may be convenient to buy a new ground cable to change connector size there. For the longer main power cable it is more common to cut the cable to remove the original terminal and install a new cable connector for the different post size.

The original main cable from battery to starter switch is 1 AWG (about 0.31" diameter stranded copper wire). The much shorter cable from starter switch to starter is smaller 4 AWG (about 0.20" diameter stranded copper wire). The other shorter cables may similarly be 4 AWG. All cables originally had black insulation jacket, except for the engine ground strap which was bare and braided. Replacement battery cable insulation jacket is commonly either red or black regardless of terminal size. Do not let the cable color fool you when it comes to determining vehicle electrical system polarity for battery hookup or for using jumper cables.

Currently available 12 volt batteries are physically larger than the original 6 volt batteries (which were nearly square). If you install a single 12 volt battery you will most likely need to cut and weld to extend one of the battery trays. It is common practice to put the single battery on the right side to avoid having to lengthen the main battery cable, and to improve left to right weight distribution when driving with no passenger (left hand drive car). Then you need to install a longer battery ground cable, or relocate the chassis cable grounding point closer to the right side battery carrier. If you use a long battery ground cable to connect to the original chassis grounding point, be sure to secure the cable for support so it cannot drag on the propshaft. Tie-wraps work well to attach this cable to the large round frame tube just under the battery cover panel. If you have a right hand drive car and are concerned about weight distribution, you might put a single 12 volt battery on the left side. Then you need to either replace or extend the main battery cable to reach to the left side. Also secure this crossover cable so it cannot drag on the propshaft.

You might consider installing a battery cut-off switch, which I like to use for maintenance purposes (or could be a casual theft deterrent). I installed mine in the rear plywood bulkhead panel, easily accessible behind the seat, near the original cable grounding point. I then connected the original battery grounding cable to the switch and used a duplicate of the original engine grounding cable to connect the cut-off switch to the original chassis grounding point. If you use a battery cut-off switch that mounts on one of the battery posts you will not need any additional cable, but you may need to change the original cable connector to attach to the switch (depending on switch design).

Installing the cut-off switch at the battery to chassis grounding point makes it impossible to accidentally short circuit a hot battery terminal or cable to the frame with a metal tool (when it is switched off). This follows the traditional workshop manual instruction to disconnect the battery ground cable when working on the vehicle electrical system.

Accidentally shorting a metal tool between the battery grounding post and the chassis (with cut-off switch open) will restore the normal battery connection to the vehicle electrical system. With two 6 volt batteries, shorting the intermediate cable to the chassis (with cut-off switch open) will apply 6 volts to the normal 12 volt electrical system. This might cause a small spark depending on what might be switched on at the time.

WARNING: When all cables are connected (and cut-off switch closed), accidentally shorting a metal tool between one of the hot cables and the chassis will cause a catastrophic arc similar to an electric arc welder. This involves enough electrical power to melt a steel tool and possibly burn your fingers. It could possibly cause a hydrogen gas explosion to destroy a battery, spattering sulphuric acid about in the process.

Rules for some vintage racing organizations may dictate installing a battery cut-off switch in a different manner. Consult the local organization rule book for this information.

If your battery(ies) is(are) in good condition and all connectors are clean and tight, the standard size cables do okay, and the current will spin the starter over well even in cold weather. Modern batteries (at lease 12 volt batteries) have considerably higher cold cranking current capacity than the original twin 6 volt batteries. The higher CCA current means less internal resistance for less voltage loss inside the battery and better current for faster cranking. If the battery is a little weak or you drive in extremely cold weather, you may benefit by using larger cables for slightly stronger current. You could use larger welding cable (1 AWG or 0 AWG) which is more flexible and carries more amperage with less voltage drop (making the starter spin faster). But don't get too carried away with the idea of switching to monster battery cables just yet.

The main battery cable is about 8 feet long. 1 AWG copper wire has resistance of 0.124 ohms/1000ft, or .000124 ohms/ft. For the 8 ft long cable this works out to a total of 0.001 ohm. All of the shorter cables combined are about 4 feet long. 4 AWG copper wire has resistance of 0.2485 ohms/1000ft, or 0.000245 ohms per foot. For the 4 ft length of smaller cables this works out to a total of 0.001 ohm. I will venture a reasonable guess that the large steel chassis frame between engine ground and battery ground points may be equal to about 2 feet of 4 AWG copper wire, or 0.0005 ohm resistance. Adding cables and chassis resistance together gives a total of 0.0025 ohm for all of the main conductors.

Rule of thumb indicates that cranking the engine with the starter motor may require about 1 amp (at 12 volts) for each cubic inch of engine displacement. The (current) engine in my MGA is 1625cc, or 99 cubic inches, so it should take about 100 amps to crank it over. Let's suppose a strong battery may give it 200 amps during hard cranking in cold weather. Using Ohm's Law (E=IR), 200 amps going through a 0.0025 ohm resistor will have a voltage drop of 0.5 volt. This would be 0.2 volt on the main cable, 0.2 volt on the smaller cables, and 0.1 volt through the steel frame.

To put things into proper perspective, changing all of the smaller battery cables from 4 AWG to 1 AWG would cut the resistance there in half and reduce the voltage drop by half, recovering all of 0.1 volt to be added to the starter cranking input. If you increase all of the cables to 0 AWG you could cut the cable resistance by another 20%, which would recover another 0.06 volt. All together this could give you an additional 1/16 of a volt for the starter motor, on top of 9 or 10 volts it may see during cold cranking.

So now I can ask, is this really worth the effort to change out all of the battery cables for such little gain? Apparently there is a point of diminishing returns where big enough really is big enough. If you have a slow cranking problem in cold weather, perhaps you should be looking elsewhere for the cause of low voltage at the starter motor. Check the condition of the battery first. Then check, clean and tighten all of the cable connections to avoid any high resistance contacts. Also check for voltage drop across the starter switch with full cranking current. Happy hunting.

Addendum: Follow-up to main battery cable size.

At 09:56 AM 10/19/2006 -0600, Randy Brown wrote:
"I have Questions about the accuracy of the battery cable page on your website. Where did the size information come from? My measurement of original cables show them to be 2 gauge not 1 gauge."

Considering the frugalness of the MG factory system, I would have been inclined to believe that the main battery cable would have been 2 AWG, figuring that it would be marginally good enough to perform the function. I would be tickled pink if there was factory documentation somewhere showing the original construction of these cables.

My note about 1 AWG main cable comes form measuring the one in my car. I have measured the conductor a few different directions at both ends near the end terminals. No matter how hard I try to convince myself it should be smaller, it keeps coming out .300-.310 with my dial caliper. I might concede some problem measuring over the outside of the radius of the outermost strands, so maybe it's only .295, but no way could it possibly be anywhere near as small as .250. It is definitely 1 AWG.

I bought this cable from the parts counter at an MG dealer in late 1977 as part of the first parts order when I began restoring my car (the first time). It was OEM replacement part with a factory part number tag. I would be hard pressed to believe that the factory would have paid the bill to fabricate these parts in mass production with larger wire size than was intended (on the assumption that the original size should have been 2 AWG). While I concede this is not the cable that came in the car from original production, it is a standard OEM factory replacement part, which I will assume is true to original form until someone may produce proof to the contrary.

I have spent hours searching the internet for any well founded information on original cable size for these cars, but so far all for naught. I suspect in the end the closest we can get to the truth is measurement of enough verified original production parts for reasonable statistical analysis. Certainly the parts from any single car cannot prove that 100,000 others should be the same.