Lets get to to the bottom of it. What are the draw backs of a 6.9, what are the advantages of a 6.9. What are the draw backs of the 7.3, what are the advantages of the 7.3. Please give me all your input. I have a 6.9 and a 7.3, I can rebuild either one I want, just don't want to rebuild both. Went to my machine shop, they told me to pick my poison. Help me pick my poison with the knowledge you all have of each. I'm sure a lot of forum users could use info on this subject as well, for the future.

 

A 7.3 is just a bored out 6.9. From my understanding I don't think the 7.3 can take much more without making the cylinder walls to thin. They are already known for cavitation issues.

-M

 

I've heared 6.9 have weak heads, 7.3 has weak cylinder walls, ect... I am trying to make an educated decision on which one to rebuild. Give me some tech data. Cylinder wall thickness, why the head gaskets fail, ect... What can stop cavitation, will a turbo work on a 6.9, will it work on a 7.3, I am getting a lot of different answers, I need some some tech info, and experienced info. Has any body had bad experiences with either, has anybody had good experiences with either. I need to pick my poison!

 

The 6.9 Heads are fine, the head bolts are the weak link, a stud kit solves that problem, also the 7.3 valve train is a direct bolt on (if I remember right), Both can be turboed, and both are great motors, The cavitation issue with the 7.3 is easy to avoid, all you need to do is keep the cemicals in the coolant at the right levels,

 

dave s is that man who i think can help ya most with this subject, im sure he will chime in

 

didnt the 7.3 head have an oil starvation issue?

 

Only with the newer-style valve seals as I understand it....

 

If you don't want to have to add DC/SCA to the coolant, try Cat Extended life coolant good to 200,000 miles I am told. Really good stuff. Additive already in it.

 

Take two IDI blocks in a row out of the foundry after 1986.

Bore one of the 4" and tap it for 7/16" head bolts.
Bore the other 4.11" and tap it for 1/2" head bolts.
All other machine processes are the same on both blocks.

The one with the 4" bore is a 6.9.
The one with the 4.11" bore is a 7.3.

The only difference in the heads is the size of the bolts.

7.3 head gaskets block off a couple coolant passages the the 6.9 gaskets leave open.

Cranks, connecting rods, cam, lifters, oil pump, oil cooler, oil pan, timing gears, CDR, intake, exhaust manifolds, bearings, lift pump, seals and all gaskets other than the head gaskets are the same.

The 7.3 had better valves, you can install them in a 6.9.
The 7.3 had better rockers, they bolt right in.
7.3 had better glow plugs, they screw right in.
Injectors and IP will bolt right on, I am running a 94 turbo IP on my 6.9.
7.3 had a better water separator, screws right on.
7.3 had a better fuel tube between the lift pump and the fuel filter, screws right on.
Since the block, exhaust manifolds and intake are the same, the turbo fits both engines.

Some of the early 6.9 engines had problems with the valve guides and valve stem seals, used a little more oil than they should have.
Some of the early 7.3 motors fixed that problem a little to well, not enough lubrication on the valves.

ARP head studs for the 6.9, 250 dollars.
DPS head studs for the 7.3, 675 dollars.
Same yield strength, 220,000 pounds.

You can bore a 6.9 30 thousandths and still have cylinder walls 80 thousandths thicker than a 7.3 has stock.
Pistons come in 20,30 and 40 over for both engines.

If you had a 6.9 that had SCA's run in it from day one.
You could rebuild it boring it 20 over.
Then if your machinist was good enough, you could rebuild it again taking it to 30 over.
Then you could rebuild it again taking it to 40 over.
And finally you could bore it 110 thousandths and use 7.3 pistons for the last rebuild.

With a 7.3, at the first rebuild you could possibly bore it 20 over and be able to run it.
But it would also have to have SCA's run it it from day one.
But for a person to get away with that, they would probably also be able to play Russian Roulette with 3 shells in the six shooter and live to tell about how they played six rounds that way.

I am not that lucky, I had four bad sleeved motors in a row, they were rebuilt by a big National Re-builder that did several hundred a month.
If you do a search for rebuilt diesel engines, about 2/3's of the places that come up resell diesel engines from the same re builder.
Under another name, they used to rebuild diesel engines for Ford.

If I dig through all my paperwork I can also tell you they had several hundred warranty claims a month as well judging by the claim numbers on the repair bills.

When I was looking into the rebuild process, I found that the cost was the same going rebuild or reman.
The difference was the downtime for the truck.

I spent 7000 dollars for a reman turbo upgrade engine, installed a new radiator, changed all the hoses and belts.
Two mechanics and myself changed the engine in 12 hours.
2700 miles later I was walking for the first time since I owned my truck, 20 years.
Two weeks later I was back on the road with a new engine under warranty.
9700 miles later I was walking again.
Two and a half weeks later I was on the road again.
17,000 miles later I was walking again, this time it took 4 weeks to get the replacement engine.
I took the heads off, the sleeves were dropping, I refused to install the engine.
Four months later I was on the road again with a salvage yard engine, unknown history.
Five months later I was walking again, this time for two months while I built the engine I have now.

I depend on my truck, I only have one vehicle.
I work it hard almost every day.
Tomorrow I have to run down the interstate 120 miles work the day and back tomorrow night.
Today I was working 70 miles east of here.
It has to go every day, I have to feel comfortable it will bring me home every night.

That is my experience, you can make your own decision.

You wonder where i learned about these engines, that was 5 engines in less than 3 years.

As far as cost, I bought almost all my parts through the machine shop.
I had about 4000 dollars in parts and machine work, I did the labor myself this time.

 

that's a nice interesting informative post. i enjoyed reading it. thanks dave. norm

 

I did too. And it explains why the previous owner said my rig has always burned oil.

-Marlin

 

That's awesome Dave, you answered every question I had on my mind. Sounds like you went to Diesel U the hard way. I'll be rebuilding a 6.9. I post pics of my progress. Thanks a million!