This is indeed the most famous alternator of all time to the baby boomer era. Back in the day, many GM dealerships commonly installed the popular “diode trio” as routinely as a simple oil change. Many of us even rebuilt our own at home. Here’s how to rebuild one like a professional.
Observe the position of the voltage regulator plug. They are commonly found in three voltage regulator plug positions: 12 o clock, 3 o clock and 9 o clock. Some people call them top, right and left. Mark or notate where yours is, or where you want it to be when you’re done. Most popular amperage models are 37a, 42a, and 63a. These are all basically the same alternator, just the windings are different.
An extremely popular variation of the 10 si is the 10si self exciter alternator. These alternators have a special voltage regulator, that makes it unnecessary for voltages to be delivered to the plug in, to turn the alternator on and off. Hot rodders love it because they don’t have to do all that extra wiring. You just run one wire to the battery positive and you’re done.
These are the major components of the 10si dis-assembled. Front and rear plates or housings. Rotor. Stator. Roller bearing. Needle bearing. Voltage regulator. Doide trio. Rectifier. Brushes. Brush holder. Capacitor.
Remove the 4- thru bolts with a 5/16 socket. Lay the alternator on it’s side and tap it apart with a hammer. Remove the 3 stator nuts from the rectifier with an 11/32 socket. Hook your fingers under the stator and tap the four corners of the rear housing to get the stator out. Remove the battery post with 7/16 socket.
Remove the rectifier ground bolt and the capacitor lead wire. Remove the two insulated voltage regulator hold down bolts and the ground voltage regulator hold down bolt. Turn the bowl upside down and smack it on a hard surface. All the components should fall out on the table. All of the bolts in this step were ¼” when they came from the factory, but you may find them to be Phillips or torx. Often times the battery post insulator will stick in the rear plate. Use a ¼” extension and tap it out from the inside.
The most common error most mechanics in the 70’s made, when rebuilding their own 10si, was the method of removing and replacing the rear needle bearing. Get a 1” x ½’ drive socket. Place it on the flat part of a vice or a hard surface. Get a ¾ x ½” drive socket. Rest the center of the alternator bowl up on the 1” socket. Set the bearing in the middle of the socket. Place the ¾ on the top of the needle bearing. Briskly hit the ¾ socket to drive out bearing.
Place a allen wrench in a vise, pointing out to the right. Put a 15/16” closed end wrench on the allen. Put the rotor shaft on the allen. Support weight of rotor ass’y with left hand by holding pulley. Place the 15/16” on the nut. Tap CCW with hammer. OR Use a ½” impact gun, a 15/16” socket, and hold the pulley with a wrag.
Remove pulley, fan and spacer. Place plate and rotor upside down in vise. Apply penetrating oil to shaft/bearing area. Wait 5 min. Thread pulley nut on shaft so that 2 threads at the top are still showing. Get a block of wood. Hammer shaft out. Heat may be necessary first. Remove the 3 screws holding down the bearing cover. Flip plate over. Pound out the bearing with a 3/8 extension. Move around and hit in different areas so the bearing comes out straight.
HOT RODDERS AND RESTORERS: This is the time when you send your plates out to be polished or powder coated. While they’re out, or before you proceed, clean the visible part of the stator with a buffing wheel and paint it the desired color. (clear always looks nice if there’s no rust). Also paint the rotor, let it dry, then buff the slip ring with scotch brite. Wire wheel or sandblast the bolts and soak them in an oil base solution or spray the with a silicone lubricant.
Use a 3/8” extension to tap the new bearing into the drive end housing. Tap in a circle on the outer edge of the bearing. Watch from the side view, to make sure the bearing is going in straight. If it goes in too easily or falls in, make a few peen marks around the inside of the plate with a sharp chisel. Apply red loctite, then re- install the bearing.
Set the rear plate on a flat surface bowl down. Place a thin layer of oil in the hole where the bearing goes. Gently tap the needle bearing into the hole just enough to get it started straight. Turn the bowl over, and set it on the needle bearing. Get a 1” deepwell socket. Tap the plate downward until the needle bearing is all the way in the hole.
Bolt the rectifier in the plate with the ground bolt the battery stud. Bolt the capacitor in and fasten it to the rectifier. We always 8-32NC tap the hole in the rectifier to minimize chances of breaking the stud off in the rectifier.
Sand the ground tab both sides on the brush holder. Smear a thin layer of white lithium on the metal tab, both sides. Stretch the springs a little bit. Install the brushes as shown, make sure the protruding tab on the brush settles into the indentation on the brush holder. Put one spring in the brush holder. Load the brush down the tube and hold it with a pen while you shove a straightened out paper clip through the first hole. Repeat the procedure with the other brush.
Lay the voltage regulator in the bowl with the prongs pointing out, and the three holes lined up. Lay the brush holder on top of the regulator so that the 3 holes line up. Note the two line up pins on the regulator are inserted into the brush holder for stability.
Remove the old insulators from the 2 regulator hold down bolts by laying them on the side of a hard surface and pounding on the tube with a hammer. Then slide the tube down a little and pound on the circular area. Pull the plastic off. Put the new insulators on, put the paper insulators on, then apply a dab of white lithium.
Lay the diode trio in place. Install the two insulated screws loosely. Buff the underside of the ground screw, put a thin layer of white lithium grease on the underneath shoulder, and install snugly. Finish tightening all three.
Wrap the stator leads with black tape. We highly recommend 3M Scotch brands. They are much better when exposed to heat or oils. Take a hammer and lightly tap in 4 circles on the windings. Outside diameter with the flat side of the hammer, inside diameter with the ball side of the hammer. Then flip the stator over and repeat the process for the other side of the stator. This does 2 things:
First, the hammer should have a little bounce back to it. It shows that the insulation is good, and the quality of the copper is good enough to re use. Secondly, it’s an efficient procedure to check that all the wires conform to the position they need to be in. While tapping lightly, if you hit a spot that the windings are not secure in the insulation, you’ll notice a huge difference with the hammer and the sound it makes.
There are three phases to the stator winding. All three phases need to be visually inspected for color differences. Any phase that is darker, or any portion of any phase that is noticeably darker in color should be considered unusable. Find the union on the top side of the stator. This is where the phases are connected. There should be no signs of heat in this area.
Check with the digital voltmeter set on 100k ohms. There should be continuity among the three leads. There should be no continuity to the metal portion. Check the rotor also with the same voltmeter setting. There should be continuity from ring to ring, but no continuity to ground. This is where you paint the metal portions of the stator and rotor with a can of cheap spray paint, or paint the windings and everything with ignition sealer or varnish.
After the paint has dried, use scotch brite or 400 grit sand paper to clean the slip rings and the shafts. Wipe off with a clean rag, the rotor, where the needle bearing goes. Any old grease or grains of dirt are damaging elements at this point. If there are any pits or grooves on the shaft where the needle bearing rides, the rotor is unusable. See the parts list to order.
Install the stator into the housing by starting the leads onto the studs. Install the 3 8-32NF nuts with an 11/32 socket. Tap the housing while holding the stator to line up the outer slots in the housing with the slots in the stator. Make sure the first collar is on the drive end of the rotor shaft. (some are permanent).
Push the front plate onto the shaft. Push the spacer onto the shaft. Install fan. Sometimes it’s convenient to rat tail out the hole on the fan for easy installation. Install pulley. Some pulleys have a built in spacer, some pulleys have a separate spacer bushing and pulley. Install provided lock washer and nut. Use ½” impact gun and hold pulley with rag to tighten, or insert allen wrench into fitting in shaft, hold other end of allen in a vise, then hit 15/16 wrench with hammer going clockwise. Hold the plate in one hand and spin the pulley to check fan. If any blades are bent or crooked, they can be straightened with channel locks now.
Review which position you want the alternator to be turned to, and line it up in this position. Set the back ass’y on the front ass’y. With one sudden surge, push down with both hands 180 degrees apart. Install bolts. Tighten using the X pattern.
The 10 si standard version requires voltages at 3 places.
1. The battery post, where the larger cable bolts on. This requires battery voltage. Place the voltmeter on the battery positive and negative. Record the reading. Place the voltmeter on the hot post of the alternator and a good ground on the side of the case. The reading should be within .05 VDC.
Read the markings in the casting on the back of the rear plate. You will see “1.R and 2.F”
2. Unplug the alternator, the tests are performed on the plugs, not the terminals on the alternator. The plug corresponding to “2.F” is hot all the time. This wire is usually the larger of the two. Place the red probe of the voltmeter in the plug and the black probe on the case of the alternator. It should be within .1 difference from battery voltage.
3. Place the red probe of the voltmeter in the plug corresponding to “1.R” and the black probe on the case of the alternator. This can be any recognizable voltage, no need to worry about voltage drop. Turn the ignition key to “run” (NOT START) position and voltage is present, turn the key to “off” and voltage should be almost completely gone.
We no longer have this rebuild kit check out our other alternator rebuild kits