Quality of Delco 39mt starters

There is a 65 year old family owned starter, alternator/ generator rebuilder in San Antonio that does great work. Just like new and in some cases better. They know some mods that make them better than Oe.
They have told me the newer 39’s were designed to spin faster for the sake of the sensors to get the engines to fire off more quikly. The need to spin faster causes them to get hotter faster and is a huge strain on the soliniods and that is why they burn and/or weld the contacts more frequently. They have some heavier duty solenoids that they make in house to cure this.
The 42’s are high torque but spin slower.

 

Yes I know. You had mentioned Opening them up so I mentioned them as a reference comparison on torque and speed difference. Sorry for the confusion.

 

I have had a 39mt on my 12.7l about 5 years. Do alot of short haul and try to limit idle time. No problem yet. On other equipment I can only think of changing one 39mt that failed ever.

 

What are your batteries like? Weak batteries take out starters.

From what I can see online, looks like the 39MT is pretty commonly available at a 7,300 watt rating.

7,300 watts ÷ 12.6 volts = 580 amps

Now say your batteries are on the weak side:

7,300 watts ÷ 11.6 volts = 630 amps


Amps equal heat. Heat kills starters.

 

Your spot on. Old cable too. In turn, quality batteries clean and maintained will last longer. My last set of Interstates went 7 years 8 months. Never had a no start situation. Just changed them because of age at that point.