Question about starters

[Prowbar]

The original GMC V6 starter works OK for the 478 but cranks over a bit slow. As far as I know the 305/351 V6 always used a standard or low torque starter.



I managed to score a Chevy high torque starter for cheap and was wanting to swap the high torque housing and rotor over and use the GMC V6 nose, resulting in a high torque starter for the V6.
But I ran into a problem. These is a difference in shaft length. This means I'd have to shorten the shaft and cut a new groove for the metal clip, which is doable.



Another thing I'm wondering about is the length or the starter gear. As the GMC nose is much shorter, these is a lot less travel for the starter gear to engage, provided you're using the longer gear.
As you can see the gear that came off of the GMC starter is a lot shorter. Is this original? In the rebuild kits you always get a longer gear, and the Chevy starter is longer so you have plenty of linear travel. The longer gear in the GMC starter makes the gear almost contact the flywheel in its resting position, which is not ideal.

I can shorten the gear too, I'm just curious why the lengths of the shaft and the gear are not the same and if anybody else ran into these problems when they rebuilt their V6 starter.

[AZKen]

I realize you live overseas and shipping is way high along with whatever tariffs are doing. So maybe you sourced that Chevy starter there. Not sure what country you are in now.

I believe this endeavor has too many issues to work. I know you are handy and quite knowledgeable in mechanical design. Me too. Whatever the correct GMC starter was used was good then and if refreshed will be good now.

Your issues, if I understand correctly, are quite likely:
1. Battery power, I.E. breaking down under load, need more amps, More CCA's, etc. Get a big CCA battery and make it fit the holder.
2. Corroded, dirty or loose cables.
3. Ground cable breakdown. Tripple check. Things get old or messed up in certain climates and don't get noticed.
4. Battery cable gauge too small. Get larger ga.

Don't know history of the issue, just started happening?, was good but gradual slower crank? truck won't start sometimes? crank speed faster at times and slower at times?......something new done to something?

A "bit" slow is good news. It means my list is the answer. All these "little" things will fix slow crank if starter motor is refreshed.

Stop before you screw up your only in-country GMC starter.

FYI: The power of a starter is in the number of field coils. 2, 3 or 4.

Anyway, this is my RSVP,
Willing to help.

[Prowbar]

Thanks for the help, Ken.
Yes I'm overseas now.

Battery, cables and connections all check out. I actually made new battery cables and they are beefy. None of them run hot. Battery is good, an old Optima but still potent. Tests and checks out OK.
Battery ground cable actually hooks up to the engine and its contact is good.
I've rebuilt the original starter already and it is in good shape.

The 478 has a lot of piston to move up and down and it bogs the starter down. It was fine with the 305.
The 478 uses a bigger starter from the factory in combination with the SAE bellhousing. The Chevy high torque starter should help to crank it over.

Now the nice thing is that I won't modify the original GMC starter. I wouldn't want to if that was the case. The only thing that I use the GMC starter for is the original nosepiece. And it will not be modified. So I can always convert it back to original.

The Chevy starter is readily available and no problem if it's modified. The high torque starter has a larger diameter rotor so it should have more torque.

What I'm wondering about is whether the original GMC gear is shorter than the Chevy gear (both are the same 9 tooth gear) to compensate for the shorter nose piece and shaft length on the GMC?

[AZKen]

What pinion gear are you going to use, the chevy? Have you solved the fit, the shaft, bendix, and etc. already? Were are you on this hybrid starter build?

[Prowbar]

I haven't started yet, I wanted to have the information before I started.
The GMC gear is pretty worn out but usable, not ideal.
The Chevy gear would have to be shortened to match the GMC gear length. Now this would be do-able with a CBN cutting insert. Not sure if these are through hardened but my guess is that they are.

I'm not modifying the Bendix or solenoid or arm either. Everything should swap over when I have the right length of gear.

I should add that I rebuilt the GMC starter with the longer gear as is used on the Chevy starter (this gear was provided in the kit) and I had a lot of problems with the starter grinding when disengaging from the flywheel.

Do you have a original GMC starter so you could measure the length of the pinion gear to verify the shorter length?

[AZKen]

The fact that both gears have 9 teeth means nothing. They need to have the same pitch diameter, tooth form and other factors which you can not measure. The pinion gear needs to match the flywheel perfectly or all manner of noise, wear, mis-mesh and damage will occur. You need to use the GMC pinion.

[Prowbar]

Originally Posted by AZKen

The fact that both gears have 9 teeth means nothing. They need to have the same pitch diameter, tooth form and other factors which you can not measure. The pinion gear needs to match the flywheel perfectly or all manner of noise, wear, mis-mesh and damage will occur. You need to use the GMC pinion.

The gears are identical and mesh properly with the ring gear (and themselves)
The 478 initially had a coarser ring gear with the bigger 11 tooth Delco Remy starter, but I changed out the ring gear to match the 305. (191 tooth ring gear)



I measured the gear lengths, the GMC gear is 1/4" shorter than the Chevy gear.
Today I shortened the Chevy gear on the lathe. Quite a chore to because the one way clutch freewheels in the usual rotation of the lathe, meaning I had to cut with the tool upside down or from the other side (reverse direction). But the gear needed to be supported by the live center as well. Ended up slowly cutting off the bulk material with an angle grinder with the lathe spinning and the bed covered, making sure I did not introduce too much heat. Then used a CBN cutting insert to clean up the face. I made the chamfers/lead-ins with a Dremel tool. The tape is to protect the one way clutch from shavings/grinding dust.
The gear is through hardened so it should hold up.


Finished gear.


Converted GMC gear vs original worn out GMC gear.

Tomorrow I'm planning to modify the Chevy high torque shaft to match the GMC. It needs to be shortened, a new ring groove needs to be cut. The old ring groove will be sleeved as it sits in the bronze bushing now.

I must admit that I like to do these re-engineering jobs

By the way, I've seen rebuilt starters offered for the GMC V6 and I think the 366/427 big blocks that use the longer Chevy gear. Looking at how close the starter is to the dipstick tube makes me wonder if these starters fit properly, as they are longer than the original V6 starter. They use a different (longer) nose piece to compensate for the length of the gear. I've read one post where the starter indeed interfered with the dipstick tube.


Converted GMC gear vs stock length Chevy gear.

[Prowbar]

Did more digging on the forum, apparently there are 2 lengths of GMC V6 starters.
The longer starter allows for direct swap with a Chevy starter. All you need to do is swap the nose piece/nose cone.

The shorter starter (like mine) is the trouble child. The nose cone is shorter, as is the shaft and the gear.

https://6066gmcclub.com/showthread.php?t=17751

Low vs high torque starter explained:
https://6066gmcclub.com/showthread.php?t=47899

Rebuilt starter (maybe?) interferes with dipstick:
https://6066gmcclub.com/showthread.p...hlight=starter

[AZKen]

Sounds like you have a good solution. Best of luck.

[Prowbar]

Finished the starter. Here's the modified high torque starter shaft (top) vs the stock GMC shaft (bottom). I shortened the shaft, cut a 2nd groove (didn't end up bushing the original groove, don't think it poses a problem). I also shortened the Bendix stop to retain the geometry (I cut the shoulder further back on the lathe) Note how the windings stick out further.


This posed a little challenge. The windings on the high torque starter stick forward more. Since the shoulder where the Bendix stops against has to be spaced the same as the standard starter, it results in the fork that actuates the Bendix rubbing against the windings with the modification.

Ended up making an 1/8" spacer ring, see picture. This spaces the fork away from the windings but throws off the geometry. I had to bend the top part of the fork forward somewhat to get the required travel.



Here's the Bendix with the spacer installed.



Here's the fork clearance with the spacer installed.



All in all more work than I figured but, the end result should be worth it, a high torque starter with the stock GMC body so to say. Bench tested the starter and followed the manual checks, then painted it gloss black.



Happy with the end result, hope it works as good as it looks!