When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
The generator in my 51 F1 239 flathead has died again. This time I’m ready to switch to an alternator. I’ve searched but only find old threads that don’t give me enough info to buy parts. I’d like to go with the “slingshot” bracket and a 1 wire alternator. GM is recommended often but I can’t find specifics. Can anyone help?
Charging systems can seem confounding, but actually are not that complicated once broken down into digestible bits.
How did you determine that the generator is at fault? I ask this because there have been many instances when the fault was misidentified and a simple low cost fix may have saved the day. A very basic one is that the charge indicator will not show charging when the battery isn't in need of a charge thus fooling some into thinking the generator is bad. Grounding issues can also rear their head in peculiar ways and blindly installing an alternator may not address that. Also, sometimes polarization/flashing of the generator may be necessary due to sitting or maintenance. No harm in making sure your wiring is correct for a type B system and flashed/polarized accordingly:
Secondly, your mention of a GM 1 wire practically involves a 12V conversion. The best advice on that front is don't convert. There are few good reasons for converting a nice stock truck to 12V. A well maintained 6V system is just as good as a well maintained 12V system. It could be possible you may have a simple wiring issue inexpensive fix - So lets start there and get that all sorted out.
But consideration, a 12V conversion can get involved. I'm sure I'm missing a few items from the list... None of this is insurmountable, but it all adds up.
*Will need a 12V Generator/Alternator and all the necessary brackets
*Will need a 12V Voltage regulator compatible with the generator/alternator
*Will need voltage reducers for the instrument cluster
*Will need to reverse the AMP gauge
*Will need to deal with coil, ignition resistor, and potential bypass
*Will need 12V equivalent lamps for the headlights, tail lights, parking lights, instrument cluster etc etc etc
*Will need to consider isolation and heavy duty voltage reducers for any Radio, Heater and similar accessories
*Will need to consider impact on starter solenoid and starter motor
*Will need to consider impact on horn & horn relay
*Will need a 12V Battery
*Will need to consider battery cables and grounding strap strategy
*Will need to consider entire new wiring harnesses - headlight, dash, engine, tail etc etc etc
*Will need to consider adding a distribution panel
*Will need to consider adding relays for lighting
Almost all 12V adoptions use a negative ground. There are few reasons to do a 12V conversion, even fewer to do a 12V positive ground conversion.
There are those who will blindly advocate for alternators are "better". But when asked, they can't objectively define better. Quantify "better charging"?
If they mean more amps, that doesn't matter for the given situation. One can get different/more amperage generators just as they can with alternators.
If one means no external regulator, well sure. It just is now located within the alternator, probably not adjustable, more susceptible to electrical damage, and realistically scraps the whole alternator when gone bad. Some avoid a 12V conversion by changing the internal regulator to a 6V one, but that option is very alternator specific.
An alternator can't charge a low/dead battery - a generator can.
While alternators may require lower minimum axial RPMs than generators, this doesn't matter in a given application because the ratio between the pulleys (should) counter/adapt for that.
Alternators generate AC power which must be cleaned, rectified, and regulated. Generators produce direct DC. Rectifiers are a failure prone part in an alternator system that does not exist in a generator system.
Alternators can come in denser more efficient packages, but we already have the space for a generator...
I'm not saying there can't be advantages to alternators or generators, just that there are considerations.
Charging systems can seem confounding, but actually are not that complicated once broken down into digestible bits.
How did you determine that the generator is at fault? I ask this because there have been many instances when the fault was misidentified and a simple low cost fix may have saved the day. A very basic one is that the charge indicator will not show charging when the battery isn't in need of a charge thus fooling some into thinking the generator is bad. Grounding issues can also rear their head in peculiar ways and blindly installing an alternator may not address that. Also, sometimes polarization/flashing of the generator may be necessary due to sitting or maintenance. No harm in making sure your wiring is correct for a type B system and flashed/polarized accordingly:
Secondly, your mention of a GM 1 wire practically involves a 12V conversion. The best advice on that front is don't convert. There are few good reasons for converting a nice stock truck to 12V. A well maintained 6V system is just as good as a well maintained 12V system. It could be possible you may have a simple wiring issue inexpensive fix - So lets start there and get that all sorted out.
But consideration, a 12V conversion can get involved. I'm sure I'm missing a few items from the list... None of this is insurmountable, but it all adds up.
*Will need a 12V Generator/Alternator and all the necessary brackets
*Will need a 12V Voltage regulator compatible with the generator/alternator
*Will need voltage reducers for the instrument cluster
*Will need to reverse the AMP gauge
*Will need to deal with coil, ignition resistor, and potential bypass
*Will need 12V equivalent lamps for the headlights, tail lights, parking lights, instrument cluster etc etc etc
*Will need to consider isolation and heavy duty voltage reducers for any Radio, Heater and similar accessories
*Will need to consider impact on starter solenoid and starter motor
*Will need to consider impact on horn & horn relay
*Will need a 12V Battery
*Will need to consider battery cables and grounding strap strategy
*Will need to consider entire new wiring harnesses - headlight, dash, engine, tail etc etc etc
*Will need to consider adding a distribution panel
*Will need to consider adding relays for lighting
Almost all 12V adoptions use a negative ground. There are few reasons to do a 12V conversion, even fewer to do a 12V positive ground conversion.
There are those who will blindly advocate for alternators are "better". But when asked, they can't objectively define better. Quantify "better charging"?
If they mean more amps, that doesn't matter for the given situation. One can get different/more amperage generators just as they can with alternators.
If one means no external regulator, well sure. It just is now located within the alternator, probably not adjustable, more susceptible to electrical damage, and realistically scraps the whole alternator when gone bad. Some avoid a 12V conversion by changing the internal regulator to a 6V one, but that option is very alternator specific.
An alternator can't charge a low/dead battery - a generator can.
While alternators may require lower minimum axial RPMs than generators, this doesn't matter in a given application because the ratio between the pulleys (should) counter/adapt for that.
Alternators generate AC power which must be cleaned, rectified, and regulated. Generators produce direct DC. Rectifiers are a failure prone part in an alternator system that does not exist in a generator system.
Alternators can come in denser more efficient packages, but we already have the space for a generator...
I'm not saying there can't be advantages to alternators or generators, just that there are considerations.
I know it looks like I wrote a book and is dense with information. Feel free to ask any questions you may have about any of this.
The truck is already 12v negative ground. The generator has been declining for a while but went dead. I did several tests with the field grounded and took the voltage from ground to armature and it flashes between voltages. I cleaned the commutator and brushes which helped the voltage but still flashes back and forth. There is no increase at the battery even when I switched the regulator. I’ve had the generator rebuilt about 15 years ago but don’t want to go that route again as there isn’t a convenient shop. I may have to go the powermaster way but I’ve seen recommendations for GM style and they are a lot less $.
The GM 10Si alternators are very reliable, parts or replacements are available everywhere. Outputs range from 65 amps to near 100. I bought a setup off eBay 21 years ago and I haven't had to touch it since. Mine is a 1-wire and there have been no issues with it kicking right off on start-up.
The slingshot style mounts are available from numerous sources. The flathead generator pulley will fit right on a 10Si. Make sure the mount you buy is for a truck, not a car, as the alignment of the belt is different for cars. Here is a thread that helps explain thiings: https://www.ford-trucks.com/forums/9...ml#post8443751
Just last year a friend of mine with a 50 F-3 was having the same thoughts. He asked me what it would cost to convert to 12. (He was running the unadvised 8 volt battery and was wondering why it wouldnt charge.) I totaled up around $1800 including the new battery, alternator ,12 volt starter, all the lights, wiring harness, etc. No labor was priced as it was just me and another friend helping him out. Your project costs may vary of course but that was what it added up to about a year back.
Look at the bracket Shoebox Central sells if you have a wide belt setup. Looking at the right side of the picture you’ll see that the bracket is stepped. It does a better job of accommodating a 10SI alternator without having to modify the alternator. I bought one that’s flat from Speedway and regret that decision, ended up buying one of these to get my belt aligned better.
@Christopher2 , speedway was very transparent with the fitment [emphasis theirs]:
The other wide belt specific options you and I listed definitely are much more suited for the wide belt setups.
I agree with you on the other solutions. I also read through Speedway’s notes about the required modifications, and that part didn’t bother me. I should have been able to remove the standoff part of the wide mount boss.
My first red flag on quality control was during mockup—the machining was so rough that the alternator wouldn’t even fit between the mounting ears. I had to hit the casting with a bench grinder just to get the alternator to bolt up.
Once I got it mounted, test fitting showed the pulley was too far forward. I ended up removing the entire wide boss protrusion I circled in green and ground it flush with the rest of the mounting surface. That was every bit of 9/16”, and I’m still off by more than 1/16”.
At this point, based on where things sit, that dimension really should have been somewhere in the 1/2”–7/8” range. The only way to correct it further would be to machine a recessed notch into the wide boss itself, which I’m not willing to do.
After the fact, I found the wider shoebox-style belt and already have one ready to install. I’ll end up adding spacers back in to move the alternator forward and get everything lined up properly.
To use this bracket on a wide belt car or truck you would need to cut approximately 1/2" to 5/8" from the wide boss on the base of the alternator. You would also need to cut a similar amount from the spacer supplied with the bracket. This must be done to get the alternator pulley to line up with the drive belt.
I got my 10SI single wire alternator from advance auto years ago for about $45. It was a remanufactured unit, it fits somewhere in the 60's thru somwhere in the 70's Chevrolet, I picked a 1968 impala just so the guy could look it up. It looks like Rock Auto has them for about that same price while advance auto is about triple that.
I used the flat top mount bracket that Hot Rod Supply was selling in order to use the AC option. Still had to cut the mounting ears apart and reweld it to get it to align.
My truck sat about 2-3 months one time, and the "residual field electrical excitement" (if I am saying that right) was lost and it would not charge. I had to use a jumper to one of the 2 smaller terminals to "re-excite" the fields and it worked again (touch the terminal only for a few seconds if I recall correctly). Just be aware ... long term non-use could cause it to lose the fields "excitement".
I still love the old generator look, but I dont regret the reliablility of the alternator.
good luck with it.
Thanks for the feedback. It seems to say that any 10Si will fit the same. I’d probably go with a 65 amp. I do see there is some variation in the brackets so I will try to find one that works without mods. Do I read right that the pulley from my generator fits onto the 10Si or do I need to order one?
Thanks for the feedback. It seems to say that any 10Si will fit the same. I’d probably go with a 65 amp. I do see there is some variation in the brackets so I will try to find one that works without mods. Do I read right that the pulley from my generator fits onto the 10Si or do I need to order one?
Belt- The Napa 25-20544 I was already using was too short, so I went with a 25-22553, which is a little over an inch longer. I still had to loosen the alternator mounting bolts and tip it in to get the belt on, but it was in stock locally so I didn’t have to wait. This belt isn’t to short as it does need to be adjusted a little tighter.
Last edited by Christopher2; Apr 12, 2026 at 05:05 PM.
I had to order the wide belt pulley for mine.
Something in my feeble mind tells me the generator pulley is larger in diameter while the alternator pulley is smaller... correct? and that the alternator needs to spin faster for good output at engine idle? am I right?
F1barry ... I assume you have the wide belt system like my '51, I went to the "cogged" belt to help around the bends. just an idea to consider if it is a wide belt. I went to NAPA and they were able to get the "non-Ford" length I needed. If you are using the Y-mount, perhaps they have accounted for the stock belt length? Mine had the AC included ...making it a non standard Ford length.
