Thanks to ‘Bolters Kevin “klhansen” Hansen and Dan “Gdads51” Gerhards who helped a LOT with compiling this Tech Tip. Also Chris "SWEET" Sweet and Mike "Mike B" Boteler who helped with the overview.
Images in this Tech Tip that are embedded can be seen in larger views. They are after the body text, as attachments.
Need help identifying 6-volt and 12-volt starter and flywheel ring gears on a pre-1963 Chevy or GMC truck with a six-cylinder inline engine? You’re in luck because it’s pretty simple.
If you are trying to identify a starter already in a truck, let’s begin with the year of the truck. (If you aren’t sure what model year truck you have, you can consult the
Legacy Tech Tips on identifying your truck.)
From 1938 (Chevrolet) or 1939 (GMC) through first series 1955, most of our trucks used a 6-volt electrical system for smaller models. Some of the pre-1955.2 larger trucks and COE or cab forward models had 12-volt systems, but that is beyond the scope of this paper. If you have a pre-1955.2 Chevy or GMC light or medium duty truck (1/2-ton through 2-ton), and the truck hasn’t been modified, it will have a 6-volt starter. The 1955.2 series GMC trucks with 248 engines and manual transmissions remained 6-volt systems.
While a few of the earlier than 1955.2 trucks had key starters (automatic transmission equipped, COE and step van models), most used a “foot ” starter, where pressing a small pedal in the cab caused the drive (Bendix) to engage the flywheel, while completing the electrical connection between the battery and the starter motor at the same time.
From 1955.2 through 1959, *most* Chevy and GMC small trucks with 6-cylinder engines and standard transmissions switched to a 12-volt electrical system. They retained the foot starter, except for the trucks with automatic transmissions which got the electric solenoid. Some of these trucks could be ordered with optional solenoid starter.
So what about that loose starter you scored along with a bunch of other assorted parts? Or you found in a pile of other starters and generators?
Both 6- and 12-volt starters have nine tooth drives (Bendix), but the tooth design changes from 6-volt to 12-volt starters. This prevents using a 6-volt starter with a flywheel ring gear designed for a 12-volt starters and vice versa.
Measurements with a caliper allow positive identification of 6- and 12-volt starters and their respective flywheel ring gears. Measuring the starter requires removing it from the engine.
Measure the inner diameter of the bushing at the flywheel end of the starter. The measurement on a 12-volt starter (shown here) is about 0.470” while a 6-volt starter (not shown), measures about 0.502”.
You’ll just need to crawl under the truck, pull the flywheel pan from the bottom of the bellhousing and measure from the top of any tooth to the top of a tooth that is nine teeth away, as shown in the two pictures above. On a 12-volt ring gear, the distance will be about 2.363”. The same measurement on a 6-volt ring gear will be about 2.789”.
While some GMC and Chevrolet flywheels have been documented to have identical casting numbers, machining of the flywheels differ. You cannot swap flywheels between Chevy and GMC without at least major machine work.
The Chevy flywheel uses 6 evenly spaced bolts on a 3 1/4” circle and 3 oddly spaced dowels to align the flywheel to the crankshaft.
The GMC uses 4 (early) or 6 (late) bolts on a 4-inch circle with one bolt hole oddly spaced.
This paragraph is opinion, not verified fact:
Because the AEA parts catalog lists the same armatures and drives for all ’38-’54 Chevy s, I’ll bet those starters with solenoids (Powerglide and other key start cars, COEs and delivery vans) would work in a foot equipped truck if a later key switch with a “start” position or a separate “start” pushbutton were utilized.Hope this helps.
Bill Hanlon
The reference material used was:
- Remy Popular Parts Application Data Bulletin 1 A-110 dated 3-1-52
- 1954 AEA Original Equipment Distributor/Generator/Starting Motor Fast Moving Parts Catalog
- Various Chevrolet Master Parts Catalogs and GMC Master Parts Books
