How to Convert a 1940 6 volt Ford to 12 volt

Converting to another voltage might require changing every electrical component in the car: starter, radio, lights, whatever else.

How about finding the largest 6-volt battery available, or paralleling two 6-volt batteries if room can be found? If automotive sources don't have many 6V choices, maybe check out some marine suppliers.

A very good reply. But we have come to expect nothing less from you.

Further these rods have no number stampings on the rod and rod cap to insure they go together properly as is common in many other engines.

Stamping # before disassembly is the way we do it

How fitting. An unlabeled poster is telling us how to label con rods.

Stamping is not a good idea as there is the possibility of distorting the critical cylindrical area that contacts the moving outer surface of the rod bearing and/or introducing stress risers in the rod. You likely would never connect that with a future bearing or rod failure. But in a stock engine the likelihood of such a bearing failure is low anyway. Those full floating bearings are pretty forgiving if the loads are in the level typical of light duty operation.

These Ford v-8 connecting rods are really elegant designs for the intended purpose. There is not a gram of excess material in them. Hot rodders and racers learned early on that there was little you could safely do to modify the rods. In the case of balancing, metal additions to the rods or removal is a no-no. Balancing practice involves selective matching of rods to pistons and crankshaft bob weights used in the dynamic balancing and remove/add material only to pistons and crankshaft counterweights.

Note here we are talking about Ford Flathead v-8 engines of the 1948 and earlier types. 49-53 engines had the more common insert type rod bearings and rods with slots to retain bearings from movement. Also note that it is not uncommon to find pre 49 engines with the later type parts in them and vice versa. There is a lot of interchangeability of parts between these various vintages of Ford Flathead v-8's

In a high performance engine stamping the rods or rod caps is a ticket to disaster. Best practice would be to carefully polish the surface outside the rod bolts and lightly engrave them with a vibrating engraver or better yet one of those little metal etching pens that toolmakers and machinists use. I wouldn't bother doing this unless the engine had a high quality balancing job done on it. A lot less work to learn how to assemble them correctly and keep your workspace well organized and work habits in order.

Ed Weldon

Superb answer!

Good info.

Engines of that vintage used a rope seal as a rear main seal and it is common when renewing them, hopefully with a modern type material like a graphite impregnated woven teflon rope instead of the old style asbestos material, that the installer cuts it a little too long, trying to do a good job in preventing premature leaking. It might be that that seal is a bit tight but will seat better after a little run-in.

Also, I have brought a little life back into a sluggish starter by carefully cleaning between the commutator contacts. After years of use, copper and graphite particles build up in the open spaces and short out some windings. Basically, the starter isn't hitting on all loops. Use a dental pick to loosen the particles and flush away with an electrical spray cleaner. Use a "Pink Pearl" type eraser to burnish the contacts. If the commutator is all grooved or torn up, you could take a strip of 320 emery cloth and carefully and evenly take off some of the high spots. At that point, you have little to loose and might avoid having to get a replacement.

The pink eraser is the wrong one. Use the gray erasable ink eraser for polishing terminals. There is usually a white side attached. Do not use the white side for polishing.

I use a abrasive stone made for this purpose - I am reasonably certain McMaster Carr has them.

Two things to remember.

(1) Ford in those years thought it a good idea to place the distributor where road water would keep it clean. You can always take the cap off and dry it each time (Yes, I've done that!) or you can shield it (good luck!) or you can use one of the sprays. Anyway, this made them hard to start.

(2) The generator was anemic if you had lights on, the radio going, etc. Rebuild the generator, or better yet, get a bigger one.

(3) OK, I can't count, so sue me. Check the bearings on the starter motor. They may well be worn enough that they ride low when the motor is not turning.

If your uncle "has just had the motor rebuilt" maybe it's just the added resistance to rotation of the crankshaft caused by new rings/bearings/compression/camshaft/lack of lubrication.

Has it run at all?

New battery/starter maybe?

Oh, crap! I'm so dumb. What did the rebuilder do to the cam?

converting the old cars is not all that much different than converting the old tractors and farm machinery of that era as well.

All new lights, a 12 volt ignition coil and points and condenser (if recommended by the parts place), and an alternator.

You may have a few odd relays (blinkers, horn, accessories) that may need to be changed as well but without knowing the specifics I cant say. And 6 volt wiper motors don't take 12 volts so well either.

The starters themselves actually have less problems with 12 volt than with 6. We have over a dozen regularly used farm tractors and old trucks that originally had 6 volt systems that I changed to 12 volt over the years and starter burnouts have basically become non existent! The engines start faster and easier now and that keeps the starter motors from having chances to stall and cook them selves like they used to.

I have done a number of 12 volt conversions of old vehicles and the alternator system has far greater output plus is far cheaper and more reliable than a generator system. It may not be original but if you want good starting power and a more practical car its the way to go.

You may run into voltage and polarity problems if it has a working radio though. Those typically don't take the 12 volt power directly being they where vacuum tube in that era.

As far as resale value if you do the change over but keep all of the old parts there is no real value lost. You just send the old 6 volt parts along with the next owner where then he will reinstall them and then cuss and swear about why it wont start or work as well as the 12 volt did and then change it back to 12 volt again. I have seen it twice now on old vehicles I helped convert and on 3 old tractors as well now too!

I'm pretty much in agreement with the rest.

I do like the idea of two 6 volt batteries, but overall agree that it sounds as if the rebuild was a failure.

I did have a 1954 Pontiac Star Chief, and it had a 6 volt system. It had in it a flat head eight of displacement I really never knew, and was 18 years old, when I bought it at 19.

It did seem to have some difficulty getting started when it was extremely damp, or damp and cold, and I remember doing stuff like soaking up the water around the spark plugs with paper towels prior to a successful start, but have some doubt as to whether or not that really made any difference.

I had a 6 volt Desoto which was hard to start in the winter. I just paralleled it with another 6 volt battery temporarily.

A friend of mine recently had a Hillman 1725cc motor rebuilt and he had a lot of trouble getting it turning over. Starter was fine and battery was pretty good but drained on every attempt. After he got it running (a few days of trying) and ran it for a few minutes it started way easier. It really was just tight from the rebuild. It does still happen.

Keep the 6V until you are sure that it's not just tight from the rebuild.

I agree with the keep it 6 volt if you can, for the value of original iron.

I did convert a shortened up 35 Ford from 6 to 12 volt with a Chev alternator with the regulator built into the alternator, it fit so nice my Dad said it looked like it belonged there in place of the old generator. As mentioned you will have to replace light bulbs and battery, but the starter motor worked just fine. I did that in the late 1970's.

The 6 volt system, there may be a weak battery, a problem with the starter motor, a problem with the ignition switch which allows the generator thru it's regulator to try to charge when the engine is cranking trying to start, any bad cables or connections to the battery or starter motor ? Is the transmission in neutral, clutch disengaging fully ? The points and plugs are obvious along with the carburetor, but they wouldn't cause slow cranking.

The really obvious one I have seen far too many cheap ass's do is to reuse the old carburetor, distributer and other necessary parts from the old worn out engine without touching them let alone rebuilding them or replacing them! A crappy carburetor and distributor on a worn out engine is still a crappy carburetor and distributor on a new one too!

Growing up my family taught me l about being cheap by example and what happens from it as well. Its probably why I am somewhat obsessive about proper maintenance and repair work now.

I saw too many good pieces of equipment get ruined because replacing a worn out $100 part was too much but ruining that good piece of equipment because of it wasn't.

GA. I completely forgot the ground problem with those old beasts.

Many folks have converted. You can find help through any of the Ford clubs, (Early Ford V8 Club) etc.

Don't neglect the instrument panel which is sensitive to voltages .

As stated by others, Make sure all is up to snuff, then you can live happpily with 6 volts.

The sound of a Flathead V8 cranking is like music !

"Don't neglect the instrument panel which is sensitive to voltages."

Early Ford electrical gauges can work well on 12 volts. Their internal dampening doesn't work quite as well on 12 volts; so they are a bit more "jumpy. You can fiddle around with resistors if you'd like. It really isn't necessary. But do remember that if you convert from positive to negative ground you will either have to reverse the direction the current goes through the ammeter or get over the mental shock that comes from seeing the needle drop into the discharge range when it should be reading "charge".

Why do the electric fuel, oil pressure and temperature gauges on the late 30's and up Fords work on 12 volts? The reason is that they are not just simple resistance circuits. They employ a comparative circuit like a wheatstone bridge and read out the ratio of the reference voltage (6, 12 or whatever) to the simple variable voltage that goes through the oil, temperature or gas tank sender to ground. This is done to allow the gauges to read correctly even when the system voltage varies due to the contribution of the generator, which varies with engine speed.

I had a 41 Ford pickup in everyday use, extremes of weather from Florida humidity to New Jersey winters, for 9 years and about 80,000 miles running 12 volt negative ground with the original instruments and never a problem.

Ed Weldon

If you are confident of the rebuild you may need to push to jump start or tow the car with you're good runner with a chain etc., put the '40 coupe in third gear and drag it around a little with the fuel line removed. This will get things moving and a little heat built up then reconnect the fuel line and drag it to start

Weather you have 6, or 60 volts, if the engine will not start, the battery will go dead.

Go through the basics from start. Check compression. Set your plugs. Set your points. Set the timing mark on the engine, and rotate the distributer opposite rotation till the coil fires for the #1 cylinder. That should get you ready for gasoline. Fill the fuel bowl with an eye dropper till it starts to run out the air bleeds and leaks into the throttle blade. Run another two squirts of gasoline down the throat of the carburetor. At this point your Ford should be ready to start. If it does not start in 30 seconds, fix it. good luck.

Connect 6v x2nos batteries in parallel to increase the current or change the 6v components/relays to 12v or service and or replace all electrical devices properly and use single good 6v battery.

Patrick Whowha

if the battery cables were replaced make sure they are 6 volt cables and not 12 volt cables.more than once have i seen starting problems caused by using 12 volt cables on a 6 volt system.12 volt cables are to light for a 6 volt system

An interesting and informative discussion so far though I don't see an indication if the engine does fire or not?

Do you have spark?

What color is the arc from the coil?

Is the capacitor failed?

Is the distributor worn-out?

6v to 12v conversion, replace 6v regulator with 12v regulator, replace some bulbs and use a voltage reducer on the points.

You can also call the man himself Randy Rundle at (785) 632-3450. He is the most well known guru for this situation. Call during business hours Central time zone.

When converting to 12v I would also recommend reading the "The Official 12 Volt Conversion Guide by Randy Rundle", it has everything you need to know when converting to 12v.

Might want to have the starter looked at by a good automotive electrical shop. Make sure it is not draging, good brushs Etc. I used to have a old 6 volt flat head also. True you dont get much crank time but if everything is ok they will start. The carb used to flood on mine making it hard to start. cg

Welcome to the insanity.

Thanks Looks like a pretty good group