6.2 Broken Crankshaft Revisited.

[Recovry4x4]

12 hours in the saddle alone gives you plenty to think about stuff. Whilst running the CUCV pretty tight I was considering some issues. Now let me preface this by saying that I am not metallurgist and certainly not an engineer. I do have a little common sense and horse sense. On the subject of broken cranks, most folks seem to believe that this is a result of a bad or shifted harmonic balancer. With that said, many folks caution new CUCV pickup owners about exceeding 55mph unless you hey have a spare engine. The exception to this is cucvrus and indirectly myself. My broken crankshaft experience was with an 84 half ton with a 700R4, 3.08 gears and 27" tires. I'm starting to rethink this and wondering if lugging the 6.2 might be more of a cause than the balancer. Rick has run dozens of CUCVs right up against the governed and beyond without a single crank failure. My often driven CUCV has 121,000 miles and I run it up pretty tight. When stock in the low 60s and now with 4.10s near 70 MPH alot. My crank failure truck was lugged as a result of gearing. Any substantial opinions or facts to consume on this topic? I'm really starting to consider lugging might be a primary cause.

 

[Kaiser67M715]

It could certainly be a cause, there is a lot of force being spent when lugging an engine, and it is all being transferred to the crank.


Sent from my iPhone using Tapatalk

 

[cucvrus]

I was out running my Mule hard again over the weekend.I rolled the rear curtain up on the Mule M1009 and took a long drive at about 75 - 80 MPH the entire day. I never check fuel mileage. If I was concerned about fuel mileage this would be the wrong vehicle to be driving. The weather in Pennsylvania was great this weekend. I went and looked at a few vehicles to buy and came home with 0. They were very hacked up and not running. Not in my plans. I 78 is a nice highway to cruise on. When I stoped at the fuel station a man said hey that is the shortest truck I ever seen. I explained the design and changes I made. He was in awe. But yes I agree with the lugging of diesels causing troubles. I do run mine very hard but I always check the fluids and change them on a regular basis. I just went and bought 2 gallons of Rotella T 15/40 and saw that Rotella has their own oil filters now. They may have had them for years. I always use AC Delco 1218 but they were out so I bought the Rotella compatible. Just turned 99K yesterday. That is less then 2 K on the entire power train over haul. Drives like a new one. I never changed a harmonic balancer on any of my CUCV's ever. Thank you.

 

[Two Stroke Madness]

Inline and opposed piston engines can get away with less rotational counterweight mass because there's equal weight traveling in the opposite direction of each firing cylinder. V8's don't have that advantage so to compensate they must utilize more rotational mass to keep the engine balanced. The more rotational mass you have to spin the more fatigued the crank casting can become. If you combine the V8's inherently unbalanced design with the 6.2L's subpar crank castings... Well you see where I'm going here.

 

[cucvrus]

NO where are you going? I had 2 that have over 200K on them and I drove them very hard for the past 20 years. they are still going. So I am not sure of the theory. Also I had company trucks that were V8's and some had 400K on the engines and were driven by 30 different drivers and driven hard. I still think maintenance is a key factor in the durability and long life of any engine. I was unaware of any design flaws in the 6.2 diesel crankshafts or engine in general. It is an old engine and serves well at many tasks. That has been my experience. I had a few that had issues. No more then other vehicles that are 30 years old.

 

[Two Stroke Madness]

Why do you think there are so many threads on 6.2L crankshafts. Go ahead, Google "6.2L broken crank" tell me what pops up. Inline engines are a better design. You can't throw pistons and rods at a 90° angle from one another and expect it to be an efficient design, any engineer knows this. The V8 design was utilized so more cylinders could occupy a small space. Now I won't argue that good maintenance is a big part of the longevity of any engine, and I will also say that if you have had good luck with 6.2L diesels then you should play the lottery because you are lucky. Every engine has it's Achilles heal. The 6.2L's is the crankshaft.

 

[Two Stroke Madness]

I've got a good experiment that all the V8 fans can try at home. Go to your local junkyard and pick yourself out a couple of old junker engines that still run. They don't even have to be diesel for this, but you need one V8 and one inline. Set them up on engine stands, and flip them over. Now take the oil pans off and grab a torch. Cut as much of the counterweight off the crankshafts as possible. Now put the oil pans back on, and fill them with oil. Now the fun part... Start them up. The inline engine will sit there and vibrate some while the V8 will be rocking out and head banging like it's at a Metallica concert. The more rotational mass you need on a crankshaft to balance an engine out the more forces you have trying to pull the crankshaft apart. Think of it like running 16.00R20 tires at 100 MPH. What's going to happen?

 

[Recovry4x4]

Perhaps an inline design is better. We are not dealing with that. We are dealing with a 30 year old truck subject to crank failure. Let's talk about symptoms and solutions to this. The harmonic balancer has been a prime suspect but it might not be acting alone. I know of many more 6.2 s that ran over 250,000 than have broken cranks. Rick's trucks seem to be a prime example of longer lasting engines that weren't subject to much lugging. Zinging a 6.2 is always frowned upon but I don't recall any higher rpm crank failed discussed here.

 

[Two Stroke Madness]

I was zinging all V8's not just the 6.2, and was just trying to explain the physics. I too own a few V8's, both gas and Diesel so I believe I'm allowed a little say here. By the way the solution to the broken crankshaft problem is an aftermarket forged crank. There are a few companies that make them, but I can't remember names right now.

 

[Recovry4x4]

I'm good with that. Just didn't want the discussion to turn into. " I could had an inline 6"

 

[CUCVLOVER]

I would say that the lugging caused the failure. All that fuel could have been stacking up in the cylinders and adding to the stress of lugging. Anyone ever lug a engine for a bit than really open it up? All sorts of crud gets spit out, because of incomplete burning of the fuel. The technical term is wet stacking.

Now I just ran my truck hard yesterday. I went from my house to up past Paris, then past Dover. So that's about 75 -80 miles one way, so let's call it 150miles round trip. Those miles were spent at 2600rpm or more on the engine. I rode it hard there and back, I was not babying it in anyway. Once you get that 6.2 in that 2600-3400 range she loves to run just as long as you feel like going. Super smooth and definitely a happy engine. Did the fuel mileage suck well yes but it's a brick shaped TRUCK not a Prius.

Now am I a college trained, 40 years of casting crankshafts and building engines? Nope not by any means, I'm just a redneck that has a knack for all things mechanical. I have a touch for engines it seems and I can tell a happy from a sad one. So take this information I have posted as you wish.

 

[Two Stroke Madness]

These engines run a very high compression ratio, something like 21:1 I believe. This kind of compression produces an extreme shock load on the crankshaft. If you lowered the compression and installed a turbo or supercharger it might save the crank.

 

[rustystud]

These engines run a very high compression ratio, something like 21:1 I believe. This kind of compression produces an extreme shock load on the crankshaft. If you lowered the compression and installed a turbo or supercharger it might save the crank.View attachment 618331

OK first off your theory of the inline being superior to the V8 is wrong. There is more force on the inline then the V8 it is Physics ! The inline engine always has more torque then the same size V8 engine (think levers here) so you will have more stress on this crank also. The only engine design that eliminates the downward force on the crank is a "Pancake" design, like the VW and Corvair engines. Also the M60A1 tank diesel engine and MAN diesel engines. Second your total compression would be the same if you reduce the compression and then add a blower and turbo. You really didn't save anything. In fact you will have more pressure on the crank with the blower and turbo set-up.

 

[Two Stroke Madness]

I was referring to the cancelation of forces in an inline engine. There are always pistons traveling in opposite directions from one another creating less harmonic vibrations. Fewer vibrations means less stress on the block which means less stress on the crank. The V8 design imposes stress forces in a third axis. These forces can only be canceled out by adding extra weight to the crank. The heavier the crankshaft the weaker it becomes. Those are the physics. Now the compression ratio issue. I will agree that the compression ratio would be the same in the end, but only if nothing else was added. In a diesel engine you are forcing an in-compressible liquid (diesel fuel) into the cylinders at a high pressure after the peak ratio has been achieved. In a lower compression engine (which you need if you are using a forced air induction system) there is more room for this in-compressible liquid. The injection cycle is what hammers the crankshaft in a diesel engine. I hope I was able to clear some of that up. Now my brain hurts.

 

[TGP (IL)]

FWIW
I believe there were a concern with crankshaft failures as well as the lower block in the main cap area
In the very early 6.2 production(81 1/2 to 83)
They later redesigned the block and shaft and those problems went away for the most part.

Lugging the engine and improper timing is the biggest cause for failure.
Next the balancer.

Of the 30+ yrs. of running 6.2's, I had one shaft failure and was directly caused by to much
Advance in the timing.

The engine will run all day long at it's governed RPM.
Tom

 

[cucvrus]

WOW I never had so much to think about in all the years of running V8 engines.I also have V 6 engines that I run. I never think things thru that deep. I just start them and run them day in day out. I would get a head ache if I thought about all these theories. Any way i am off and running I have an M1008 that needs a sound thrashing out on the interstate. Oil good. Fire in the hole. We're gone. Bye Bye.

 

[Warthog]

I had a crankshaft failure in a M1009. Was driving down the highway at the time. Running about 70mph. After towing it home the balancer had dry rot in the rubber but not separated.

Interesting part was that the truck would still run. Louder that a freight train but still ran.

Since I bought the truck used from GL I can't tell you the maintenance history.

High rpm or balancer?

 

[Barrman]

There is a guy in upstate NY that used to join discussions like this. JDMarris I think is his username. He claimed to have had over 60 vehicles with 6.? engines over the years. He also wrote that he worked as a Chevy mechanic back when they were new.

He wrote about his one and only crank failure. K5 Blazer, 6.2, 700R4 and I don't remember the axle ratio or year of the truck. Probably 3.42. Several hundred thousand miles on the engine and maintained by him. He had the family loaded up for a camping weekend and had driven high speed a long way. Then, once he got into the hills and lower rpm, it broke. His theory was that low rpm and high load is where it will break if it is going to break.


We have all heard stories of 6.? engines running great then get torn down for rear seal replacement or oil pan leak to find a block so cracked they throw it out. The DieselPage guys treat cracked webs as a matter of course and write for hours how to fix it or at least live with it. They don't write much about broken cranks though.


We also all know that GM was not the model for fine attention to detail and expert craftsmanship while the 6.2 was produced. Could some of the blocks and some of the cranks not been prepared for assembly as well as they should have been? Could some batches of balancers not gotten the proper mix of rubber? Could some of the tooling been in need of replacement and had that put off longer than it should have been? You get the idea. There is no perfect mass produced product out there.


I broke a crankshaft in my 1967 Austin Cooper S Mini last year. I think I broke it in an autocross in February because it was raining and I know I over revved it a few times. Oil psi was great for the next two autocross events. However, I was down on power more and more. It wasn't until Colton was coming home from a date in that car that it started making terrible noises and we realized there was a problem. When I pulled it apart, it was broken all the way through in the middle of the rod journal. Yet, it still ran? How many 6.? cranks might have been broken a long time and waited until a low rpm/high load demand was put on them before they let go all the way?

By the way, I agree with Kenny, keep it over 2K rpm and live long.

 

[Two Stroke Madness]

The 6.2L engine was designed for GM by Detroit Deisel. It was one of Detroit Diesel's first steps into the 4-stroke market. You can tell that Detroit Diesel was kind of new to that game. Though the 6.2L is not what I would consider the worst diesel engine ever. I have to give that tittle to the 8.2L V8 that Detroit Diesel produced. These two engines do share some similarities, and faults.

 

[cucvrus]

https://youtu.be/EedHegYw88c