The Stovebolt Site Forums The Shop Area The Electrical Bay 12 Volt Conversion - Necessary?

Hi - I have a '51 GMC 1/2 ton truck that I'm planning to add a '55 or later engine to...Will that make a conversion to a 12 volt system necessary, or can I make the new engine & charging system compatible with the original 6 volt setup?

Thanks - Dickson

What engine?

Starter will need to remain 6v, otherwise the starter will rotate too slow to start engine and will overheat. Most starters within a manufacturer (e.g. GM) have similar bolt patterns and drive gears and can usually be swapped.

Ignition is not a problem until you get to modern electronic systems.

Other engine areas to consider are instrument sensors (coolant temperature, oil pressure,) and actuators (electric choke). Most sensors can be replaced with 6v sensors, choke can be changed to manual.

It is difficult to keep track of what was posted where.

There have been four separate posts on this topic.

I think a late 1955 or newer 235 is being referred to.

Sorry about the multiple posts, Tim. I'm trying to put my many questions in the correct forum spots.

Synopsis: I have a 51 GMC without an engine. I was told to consider a '55 or later 235, which I'm told would necessitate a swap to an open driveline, '55 or later rear, and some 12v electrical upgrades.

I'm learning that maybe this is more complex than first thought....Perhaps I should just stick with whatever the original GMC setup was?

Thanks for your advice. Dickson

Dickson,

There is no need to swap to an open drive-line just because you use a newer engine. And, there is no need to change to 12v just because the newer 235 came in a 12v truck.

I have a 1960 261 engine in my 6v 1954 1/2 ton truck with original clutch housing, clutch assembly, and enclosed driveshaft.

The 261 engine is the same physical size, with same mounting capabilities as a newer 235.

The is absolutely no "necessity" to change to an open drive line.

A 6 volt starter can safely be run on 12 volts, as it only runs for a very short period. On 12 volts, a 6 volt starter will actually turn over faster, not slower. There is a risk of overheating, but only if the starter is cranked for an exessive period.
"Sensors" will not be an issue with stock gauges, coolant and oil gauges on the original gauges are mechanical, not electric. Only the gas gauge and lighting would be an issue with a 12 volt conversion. However, as mentioned, a 12 volt conversion is not neccessary for an engine swap.
Just out of curiosity, though, is there a reason you're not running the GMC engine? The GMC 6 is considered by many to be superior to the Chevrolet engine.

Your best option would be to find a 248 or 270 GMC engine and install it. The original engine in that truck was probably a 228, but "There's no substitute for cubic inches", and the bigger engines are virtually a bolt-in swap. The biggest GMC six, the 302, has a few adapting issues, IIRC. All the GMC's have full-pressure oiling and insert-type rod bearings, even the older 270's from WW II surplus 2 1/2 ton trucks. Those engines were very popular with dirt track racers in the early 1950's when my family was running flathead Ford V-8's in a 34 Ford 3-window coupe. A good-running 270 could just about keep up with the flatheads, but the 216 Chevy engines didn't stand a chance!
Jerry

I agree with HL about the 248 and 270 being the best choices.

The 302 would be the best upgrade, but be careful about getting a military version. Some military engines might need ome adaptations done to the block in order to work with "normal" bolt-on parts (fuel pump and generator, are two examples, I think). These dan be made to work well,

The 270 might be easier to find and was used for many years in domestic vehicles. 270s from the mid 50s to 1962 might be good choices.

The 248 is about half-way between a 235 and 261 Chev engine. It would probably be the best cost as a replacement.

Note: Cubic Inches are like money, you can never have too much. lol

And, like money, if you think that it is the most important factor in life, you will never be happy, because, eventually you will find someone with more.

So true if it were a competition. Just need enough to keep mama happy. Now that's a career in itself.
The down side is when you pull up the the pump for another tank full.

To the OP's original questions, I switched over to 12v years ago and never looked back. Yes, still using the original 6v foot stomper starter which cranks over fast. This is with an early 60's 235.

I do agree with others about keeping a GMC motor in a GMC.

The original questions were: "Will that make a conversion to a 12 volt system necessary, or can I make the new engine & charging system compatible with the original 6 volt setup?"

A conversion to 12v is not necessary, and it is easily possible to make the new 12v engine compatible with the original 6v set up.

use the 6v battery
use the 6v engine's clutch housing, flywheel, and clutch assembly
use the 6v starter
use the 6v coil
use the 6v generator and all the other electrical parts from the 6v system

Changing to 12v is not difficult, and it would be just as reliable as the original 6v system. But, changing to 12v is not a necessity.