51 GMC 6 volt to 12 volt

Husker44A

Well-known Member
Alright guys, im about to get a 51 GMC pickup to play with. Always wanted one just never found the "right one". Anyways, its 6volts neg ground and I would like to go 12 volt neg ground start. What do i have to do to convert this ??? I know all light bulbs, headlights,dash lights,tail light bulbs. What about the genny or the starter ? Thanks.
 
Many 6 volt starters will still work at 12 volts okay (sure spin fast that way), although it can be a bit tough on some types of starter drives.

I forget what the truck had originally and there may have been conversions done over the years, so I must ask.......
Is it a pure mechanical push down contact type start switch (floor pedal or big switch, perhaps a saddle mount contact switch right on the starter which a floor pedal or rod activates) or an electric solenoid activated starter (perhaps a small dash push to start switch or key type turn switch) ?????? If mechanical no problem or changes needed, if its an electric solenoid it can still work provided it doesn't overheat. I've used 12 volts on a 6 volt solenoid and they worked okay.

It may be easier to just get a new or rebuilt or salvage 12 volt generator and a matching 12 volt Voltage Regulator orrrrrrrrr some folks change over to an Alternator. A genny shop can change out the fields and convert a 6 to a 12 volt genny, that's one more possibility.

John T
 
If you have any common sense and use it your starter will be fine, don't crank and crank on it, if it doesn't start in 30-45 seconds back off and let it cool a while and figure out why it won't start. I'm assuming vacuum wipers so you're good there. Change bulbs and you should be good. If you have a solenoid it may or may not need to be changed, common sense says it needs to but I ran a VW I converted for 6 months with the 6V solenoid, stopped driving it when it dropped a valve. Other advice on the generator & regulator is on track, have the generator converted, find a 12V generator and regulator or convert to an alternator. If you get a copy of Hemming's Motor News they have many advertisers that sell parts and accessories to facilitate conversion, from kits tailored to hot rods allowing you to add things to your truck it didn't come with like electric wipers air conditioning and such or generator looking things that are alternators that bolt up to the original generator mount
 
I've seen the 6v starter last for years working on 12v as for generator get a one wire Motorola alt like a hot rod. install a under dash volt meter change out bulbs,battery, gauges will peg with wrong VR.Best to run dash,radio & blower motor on 6vdc. check out "12 Volt to 6 volt" search on ebay
 

Husker44a
Nothing wrong with your 6v system when all is right. it is easier to get your 6v working correct than converting to 12v. Biggest time the problem is someone has changed the battery cables and they are too small, they got ones for 12v. Get BIG cables that is supposed to be on it and make sure you have the ground nice and shiny. One other common problem, might check this first, ground between engine and body is missing or corroded.
 
Not all that difficult a project. Did a 51 chev p-up back in 83/84. Spent 3 Saturday afternoons doing mine, with frequent breaks for a cool soft drink. Fabricating bracket for alternator took the longest.
Generator is the biggie. To keep original look, get one with matching regulator for a 55-62. If looks not important, 10SI 3 wire alternator is easy swap.
Starter will be okay, either original foot pedal or push button/key switch & solenoid if already converted. If things under the hood are in good shape, it will start in 4-5 seconds, not the 30-40 seconds as stated below.
Change all the light bulbs. No need to lie on your back to reach up under the dash. Take out that 6 by 8 or so panel in the firewall (4 or 6 screws) & reach in from under the hood. Pop out the sockets, swap bulbs.
Dash gauges. Need to calculate or experiment resisters for gas gauge & heater blower. I don't remember what resisters I used. Oil press & water temp gauges are direct mechanical type, don't need changing.
Happy motoring.
Willie
 
I can understand the need for a change from 6V to 12V. It takes a heck of a good mechanic to keep a 6V system working in tip top shape.

The big battery cables are always a major problem due to the current needed to turn over those starters. P=IV can be a real problem.
 
I have several old ford cars and trucks that are 6 volt. They start and work well. Most common mistake made is using too small size battery cables. If you must change it to 12 volt, you can likely find an alternator bracket from one of the aftermarket hot rod company's. Speedway motors in Lincoln Neb. is the first one that comes to mind. This is my 46 ford still 6 volt.
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John, i was hoping youd throw your .25 cents worth here. It 100% original. SO, it does have the foot pedal push start. Which, is C@@L i think. I would like to keep it. I havent seen the truck yet. Will tomorrow or next day. I would like to change over to a one wire 12 volt altenator. Maybe i can have the 6 volt genny changed to 12 for original look. Further down the road. I want to throw in my 327 sb and a auto. Its a 3/4 Ton also. Which the rear and front end will get swapped for a more road worthy suspension. Some day !!! Thanks John i always enjoy reading your posts.
 
MY uncle had a mid 50's something GMC truck with the foot push pedal start and in line 6 cylinder GMC engine which I would liked to have had but he got a good price more then I wanted to pay.

That starter and switch should work okay at 12 volts a long time, its an original thing Id like to keep even if its NOT perfect

While a genny is original if youre going to swap out to a V8 no sense in keeping old technology

Take care

John T
 
I did one on a 1951 Chev a few years back. I simply put on a delco 10si alternator and it was as simple to wire up as it in on a tractor. No need to do any thing with the starter it will work fine again just like on a tractor
 
The other guys have provided some good info. But, there is one clarification: You stated that it is 6v neg ground. If it is original, it will be positive ground. Chev's in that year were negative but GMC was positive. all the other info is accurate with this exception which will require you to change the polarity of the ammeter to get it to read correctly.

That and the engine are the only differences I remember between GMC and Chev in the early 50s. The GMC is a really good engine.
 
Swapping to a 327 and a auto will be at a later date. Going to get it going and drive tye heck out of it. My ten yr old boy is gunna love it when i bring it home. Wife just rolls her eyes when i talk about it. Ha ha ha. Father/son time !!!! Priceless !! John, do you whats done to the genny for the 6 to 12 volt upgrade ? Thanks.
 
Actually, I have used 6 volt gennys unchanged but just used a 12 volt Voltage Regulator AND THEY WORKED to charge a 12 volt battery. Before anyone has a calf, I'm NOT saying its electrically correct or perfect or that it set the world on fire, ONLY that it worked. A shop might change out the field windings to do it more correct.

Wish I had bought my uncles old GMC now

John T
 
The 51 Chev I converted had to push button floor start and it worked just fine on 12 volts. Had a rebuilt 216 engine in it. My son had planned to fix it up but then changed his mind and sold it
 
I was remembering the GMC was oil pump lubricated and the Chevy splash, and the GMC 10 HP more.
 
Did GMC's have stove-bolts in them in'51? Thought I saw once that GMC used a different (and much better) straight 6 than Chevy and was coveted by hot-rodders. I just converted my '46 Chevy truck to 12 volt alternator. Required a couple of relays in addition to changing bulbs and the kit I got used a resistor block for the coil. Pretty nice to touch the starter pedal and have the thing take right off. My truck doesn't have the original 215 babbited engine and has a early 235 in it - probably '51-52 with the acorn nuts on top of the valve covers.
 
I have a 49 GMC 1 ton that's been in the family since new, couldn't make the gen and reg. work anymore. They are 6 volt pos ground from the factory, put a 6 volt neg ground one wire alt on it from a local parts place, all was well but could not get the temp gauge to work,(I think it was the temp gauge) it is elect. on the GMC. finally found a 6 volt pos. ground alt 1 wire alt from one of those old truck places on line, changed it back to pos ground and all works fine. Nothing wrong with 6 volt, this truck fires right up and runs good, I have been driving this truck since I was 6 years old, runs just as nice now as then.
 
GMC had their own engine, the 49 I have has a 228 with bearing inserts, I had the crank turned years ago and it has huge journals with wide inserts, that little 228 will pull and lug like no other engine I have ever used. It is just a 1 ton but with a grain rack on it, used to load it and pull a wagon to the elevator and it would pull every hill in high gear at 20 MPH.
 
Yep, all the Jimmys (228/248/270/302) were pressure lubed rod engines. Lots of speed equipment available back in the day to. Wayne 12 port head and lots of carb capacity made those v8 killers!
 
Nothing to do with an older gmc truck but my 52 8n ford I've been temped for years to convert to 12 volt. How many still use the 6 volt system out there?
 
I've got a 52 8n that I converted to 12 volt this past summer, 6 volt batteries are a crap shoot now days it seems and combined with not using the tractor very often it was tough to keep the battery up.

I have a 48 GMC 1 ton my grandpa bought back in the 70's, it has a hoist, extensions on the sides and had a grain endgate which got lost over the years but I still have the original pickup box endgate. I'd like to switch it to 12 volt as well since it doesn't see much use but like usual there are other projects in line ahead of it.
 

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