Rebuild on 300 Cummins

Coal Region Deplorable · Mar 4, 2021

Personally I'd just roll bearings in, dropped maybe 5 numbers on the button, back off the AFC adjustment a little bit, and shim the idle spring enough to give you about 2500 RPM high idle

Buttons are cheap and easy to change, if you think dropping five numbers is too much or not enough you can always change it a few numbers either way.

Go online and look for Cummins idle spring plunger, that's technically what the button is called. Last time I bought them I think they were maybe $10 each. The lower the number the more fuel it's going to inject. They're under that little cone on the back of your PT pump, the one with the plug on the end of it that you take out to adjust your idle speed.

3GN · Jul 30, 2025 at 8:31 PM

Dino soar said: ↑

We've all been watching you restore this truck and that's really great what you've done to this point. But I'm going to tell you something very serious here. There's a time to listen to what people tell you and there's a time to not listen.

The person on this site @AModelCat is a mechanic by trade and has been a mechanic for many many years. If other mechanics on the site come into this thread they're going to tell you the same thing. Everyone here is only here to help you and no one is giving you bad information except the other person that you're talking to.

Let me explain the crankshaft to you. The crankshaft only needs to be replaced if the crankshaft is so far out of round the bearings won't stay in there. They will spin and that will give you an engine knock. Engines are rebuilt all of the time with just replacing the bearings and I recommend that is where you start.

For example my truck when I worked it over I pulled the pan down and I replaced all the bearings. When you take the bearings down that's when you can tell the condition of the crankshaft. If they changed the oil and all of that most likely your crankshaft is fine and you can just put bearings in. You can do all of this with the engine in the truck. If you start replacing crankshaft especially when you don't have to you're just throwing money away. That whole engine has to get pulled out of that truck in order to replace that crankshaft. Like I said the bearings can be replaced without removing the engine and the whole engine can also be rebuilt without removing the engine.

So my first question is, why do you want to rebuild the engine? Are you being preventative or do you have a real reason to rebuild it? Generally speaking diesel engines don't get rebuilt until they start using excessive oil.

Now you said that engine has newer heads on it, how do you know it wasn't rebuilt at sometime? Those engines can run a long time even if they are using oil.

Now I've had a few trucks with those engines in them and I have a friend who is a Cummins mechanic that has been working on these engines since like the late 70s. Please listen to what I'm going to tell you because this is the best way for you to proceed. There are a couple of options.

First of all if you're going to put money into the engine to use it as it is the best course of action is to just put new bearings in and go over the rest of the engine. Meaning put a new water pump on new thermostats new belts and hoses new idler pulley put a new accessory drive on have your oil cooler serviced and have your overhead set and put new injectors in. Replace the vibration damper on the front of the engine. And put in a front crankshaft seal. If you have a clutch fan it's not a bad idea to have it rebuilt. Pressure test the radiator and the engine itself to see if anything is leaking down.At that point you have gone over that engine. Those are the things on an old engine that you want to replace. And if down the road you wanted it rebuilt then they just have to put pistons and Liners in but all the good work you did to that engine is still there.

Now if you want to rebuild the engine before you start because you feel that you want to have a new engine, there's nothing wrong with that if you have the money to spend. However, the first thing is to pull the pan down and see the condition of the crankshaft as I said earlier. Most likely the crankshaft is fine and they can rebuild the engine without removing it from the truck.

Now you have a 300 big cam. If you are going to rebuild it you should rebuild that to a 400. The engine itself the block the heads are the same. The main difference is that the Pistons are higher compression on your engine and it's pump is set differently and there's a different Turbo. They use a lower compression piston with the higher horsepower engine. The kits cost the same amount of money it's not going to cost you anything more as far as the kits go so so it would behoove you to go to the higher horsepower. But you will have to have the pump gone over and set for the right CPL for that 400 and have a new Turbo. If you want to go to 400 horsepower we can help you with the right CPL and all of that to have it rebuilt.

You're getting exactly correct information on this site. And also I'm not sure who this mechanic is that you're talking to, but the only person that can put a warranty on an engine is someone that is a certified shop. So if a shop is a certified Cummins shop they can give you a warranty on those parts. Or if a shop uses FP diesel or Interstate mcbee or whatever but they have to be an actual authorized shop that does that.

Everyone on this site wants to see you do well and to see that cool truck rolling down the road. But please take the advice because this is the best way to proceed.
Click to expand...

I need your help ASAP.
I have problem with my NTC300 CPL is 471
I need to order rebuild kit for this engine but I can't find any good source and kits wi h support CPL 471.
I read your post about upgrade 300 to 400 and need some advice from you please .
I have picture from engine with all data if u need it
Please let me know if u can help
Thanks in advance
Zack

MACK E-6 · Jul 30, 2025 at 9:02 PM

@OLDSKOOLERnWV

3GN · Jul 30, 2025 at 11:26 PM

MACK E-6 said: ↑

@OLDSKOOLERnWV
Click to expand...

He can help or ?

OLDSKOOLERnWV · Jul 31, 2025 at 2:25 AM

There’s parts available out there still, mostly aftermarket. Power Products in Kentucky has been a good source for me in the past.

Upgrading from 300 to a 400 is not always as easy as some may think, more involved than just injectors and the pump.

I would look on top the pump at the little tag to see what its calibrated for to insure it’s still at 300. Never know someone may have tried to convert it to a 315 or 350…

https://powerproductsky.com/

MACK E-6 · Jul 31, 2025 at 7:54 AM

3GN said: ↑

He can help or ?
Click to expand...

Well, he drives a Kenworth with a Big Cam that he does his own wrenching on, so for anything related to an older Cummins he’s the one I summon.

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