Rebuild on 300 Cummins

Yeah I used to think they wanted all the cores back for their foundry operation.

 

There are hundreds of thousands of old Cummins NH’s and NTC’s running out there that have the original cranks and the majority of them have been overhauled many times and the original cranks have never been removed. Only the rare ones that broke were. Lots of them have millions of miles on them with 3 or more inframes. It takes a lot of abuse to wear them out.

Breaking cranks did happen in the late 60’s early ‘70’s on the original SC 290’s and 335’s that had the small tapered snout. The 350’s on up thru the big cams all used a bigger flat snout and that solved the vibration issue that caused the breakage with the higher hp’s.
But those breakages started the “must replace the crank” wives tale.

 

For the 4K labor. You have all the joirnals checked with a micrometer and plasti-gage for out-of-round and out of spec. If good just polish the journals with emory and roll in new bearings. And make sure he is Clean, clean, clean. That is the key.

 

It's hard to generate a parts list prior to disassembly. Let the mechanic take it apart & tell you what it needs

 

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.

 

A lot of good advice there!
In 1985 I was working in a big company shop. There were two guys that built all the engines for multiple terminals in the region. One quite and I got his spot. I called my job the dish washer. I had to clean everything up and pull the torque wrench on everything. That was considered a right of passage. 95% of them were BC 350's and we normally turned an inframe every 2 1/2 days. I know that sounds crazy but they had custom racks made for oil pans and such. Clean the gaskets off and put it in the rack. Roll it to the wash rack and back. All the parts were normally on hand.
Those were normally 850,000 mile engines. Almost never replaced a crank.Cam job was not uncommon and took a couple of more days.
With that said my opinion is if it does not have excessive blowby replace the rods and mains. Check the piston cooling nozzles and run the overhead. Did you have all the rocker shaft o-rings replaced? You have been getting the other stuff up to snuff so run it.
When you increase HP you often have the change camshafts and or the install an offset key and that costs a lot more money.
These engines also had some liner counter bore issues. That was corrected by cutting the block and installing lower press fit liners. Has yours been updated? No one knows but do not build it without lower press fit liners.
Would say Cummings recon heads but they did not have an ISM head in the US three weeks ago. Get the injector sleeves replaced. Skip the tested OK BS.

 

Exactly correct. Especially as part of getting your engine ready to replace the Piston cooling nozzles and The Rocker shaft O-rings. Two things that I did that I forgot to write.

And if you did need to have the offset key done for the camshaft you could have them replace the cam bearings while it's out.

Exactly one hundred percent correct!

 

You know those kind of times we're about normal for any half-decent shop back in the day. I worked for an almost all Mack fleet for a large steel Warehouse. E n d t 675 and e n d t 676 engines. If they came in, in the afternoon and I could get them washed and start draining the fluids etc the next day I could pull a long shift and be finished the in chassis when I left for the night. They'd be back to work the next morning at about 6 a.m. when the drivers came in.

Keep in mind, I already had a set of heads, injection nozzles, and rods on the shelf that I would do when we were a little slower. And if I had to home a a lot of the bores to fit oversized sleeves that could add some additional time as well.

The Mack dealer here would promise 48-hour turn around to in chassis that engine, lots of times they would be calling the customer and saying it was done even before that.

Shops are severely hampered now because mechanics are just about impossible to find. Believe me, I own a shop, 9000 square feet of shop floor space, plus parts rooms, office space, etc. 20 foot ceilings, warm heat, big doors, three phase power, the shop of my dreams when I was younger. I should be making a fortune, but I can't find much in the way of Good help. And at almost 60, I'm not doing any 48-hour turnaround in chassis anymore