6.5 rebuild - grind the crank?

[brherrmann]

I bought 2 599 6.5's from a wrecker to build one good engine. Ofcourse the least worn has web bolt cracks and the other will need oversize pistons. I think I read on "The Diesel Page" somewhere that you should not grind the crank and that they generally don't need it anyway. Plastigage shows clearance on all bearings average about .003" with the old bearings. Should I be OK with standard bearings and no machine work on the crank? Magnefluxing is good practice but do 6.5 cranks generally develop cracks (aside from when they snap in two)? If the crank has cracks where would they develop and would they be visible with the naked eye if I know where to look? Personally I think the cranks are good and I would like to save a few dollars but this would be the time to check.

[john8662]

Take the crank to a qualified machinist/engine builder and get their opinion on your specific crankshaft. They can establish size and surface condition of the journals.

If they want to turn the crankshaft that's not an issue. I'm surely not afraid of a .010"r .010"m crankshaft. Just ensure they stay out of the rolled fillets.

Mag the crank? It can be done... I will have to be de-magnetized afterwards.

J

[ronniejoe]

Theoretically, you can grind these cranks down tangent to the rolled fillets. However, I have never taken one below 0.010 under. I suspect that 0.020 under would be safe as well, but have never needed to go that far.

Some folks (I think even GM if I remember correctly) say that you can't grind these cranks. That's an extremely conservative position to take. I have well over 270,000 miles on my current crankshaft. Over 100,000 miles of that have been since grinding it 0.010 under size to blueprint the index and balance it. My engine is making a fair amount of power, too.

[Kennedy]

I had mine index ground and also nitrided. A .010" grind took care of the index. The block broke in short order, and the crank survived. I worked up another block with splayed mains and as far as I know it's still running today.

[moondoggie]

RJ: "My engine is making a fair amount of power, too." What a master of understatement...

[Robyn]

Been through a few of these little monsters and I would not use another GM Cast Iron crank that had a lot of miles on it.

Look really hard at the SCAT steel cranks. I bought mine for just under $300.

Zero hours, cast steel and ready to go.

When you find cracks in one of the 6.5 cranks, its usally after it has grenaded.

The web cracks in the center mains can be fix easily with Full Torque inserts on the 6 outer center holes.

"Lock and Stitch" inserts

The thing is this, if the cracks have not progressed beyond the bottom of the hole, the inserts will save the thing.
If the cracks are at the register fit where the caps fit in then the block is a door stop.

I have a fresh Scat crank sitting here for my fresh Zero hour AMG Block.

The only thing better is a forged steel crank from AMG $$$$$$$$$$$ :eek:
About $900

My rule on the cranks, is unless its worn badly, I will put them back with a std set of bearings on a quicky overhaul.

The big problem with the Cast 6.5 cranks is that they fatigue over time and use and then they just up and snap.

Gm never used a cast iron crank in any of the HP big blocks, but they used the cast crank in the diesel which just beats the snot out of a crank.

Go figure. I forged crank would have been a great thing from the gitgo.

Missy

[ronniejoe]

Well, 280,000 miles and no kablooey. These cranks aren't as fragile as their reputation would have us believe. Show me a broken crank that wasn't preceded by a cracked block in the main webs, then you might have something.

Would a forged crank be better? Yes! But I am confident that a properly prepared production crank will handle anything that a 6.5 can give it.

[Robyn]

Ron
All things being equal I would agree.

I have torn down a couple 6.5's that had broken cranks and the block was still intact.

The small cracks that we see on a regular basis in the main webs, really do not cause any issue. The problem comes when the cracks progress through the main web and either up into a cylinder wall or into the cam bore area and the main web actually gets "Loose"

Nobody has really proven beyond the shadow as to what really happens in these engines.

I have seen 599 Blocks with 250K ++ miles and no cracks at all, then again I have seen engines with less than 100K miles that were total junk.

The broken cranks IMHO are due to a poor casting. Now its damned hard to predict that issue.

I dont think GM exrayed every crank so bad ones did get through.

I have found one crank that showed indications when magnafluxed.
We cut that one up and it had some serious issues with some nasty porosity and other problems.

That crank would have broken eventually.

My feeling is that with the tremendous amounts of structural vibration and internal flexing that these engines are subjected to that the cast crank is just a bad choice all around.

Another theory I have is that poor cylinder to cylinder timing due to various things such as injector POP pressures not being equal can cause a rise in the stress levels and flexing of the crank.

Cast iron just does not like to flex, as compared to steel that can handle far more flexing before it fails.

With the advent of the P400 engine from AMG and the massive girdle they designed into the lower end, plus the addition of the forged crank, it becomes very clear that there was a design flaw from the gitgo.

Now we have spoken for years about the 6.2 NA engines being of such a low power level that they were "MORE" imune to the cracking and breakage that we see in the 6.5 TD.

I have torn down a couple old Red blocks that looked like a road map in the crankcase.

My overall opinion is that GM cut corners and took a gamble that the numbers of units that failed would be small enough that they would still come out on top of the heap.

Whether GM actually came out on the 6.2/6.5 engine design, I have no idea, but one thing is for sure, just mention he name 6.5 diesel and people usually give you a disgusted look.

The reputation and bad image due to many things made this engine family an outcast.

Over the years I have had great luck with several 6.2/6.5 rigs, but, being able to fix them made it worth while.

My Dually is one case in point. The truck was purchased with what was likely its second engine ?? the rebuilt engine I installed had issues when it got off the ground and I took a Gamble with the MLS head gasket, and LOST.

OH well, I knew the risks going into that deal.

I will be the first to admit that a factory crank can run a long time, it can also break.
If I were looking at a crank with a lot of miles on it and then considering spending $$$$$$$$$$ on grinding that crank, I would toss the thing on the trash pile and buy a brand new Scat crank. Zero run time and better materials.

The cost of buying a new Scat crank vs paying to have a used crank ground ????????????

Peace of mind is worth a lot to me.

My last go round cost me a lot of $$$$$$$$$ and its pretty much a total loss.

I sold the .020 piston set, the head were sold as was the crank. Still the overall cost of the rebuild will never be recovered.

That said, once the fresh AMG block I have is built with the scat crank, AMG heads and other parts, the total cost of Two engines is still less than what an optimiser 6500 in a crate would have cost me.

We can debate this stuff till the cows come home and really never have any more answers than we do now.

Missy

[brherrmann]

I am going to grind this crank .010" and install .020" pistons. As john8662 said .010" shouldn't hurt (unless I'm taking off a hardening). Also, with a high milage crank, factory defects should be ruled out. I can't do anything about fatigue if this is an issue but I'll take my chances. Thanks for all your input and I will keep you updated.

[Robyn]

Unless the journals are scuffed and show some serious wear, I would just have that crank pollished and use STD bearings.

Clearances in the .002-.0025 range are perfect on these engines.

The rods can go as high as .0037 but thats taking it a tad too far.

I have used a lot of 6.5 cranks that had 200K miles + with just a quick pollish and then back into the battle.

Be sure you have your machine shop fit the pistons at .004" Clearance on Cyls 1-6 and .0045" on Cyls 7-8

The back two always need to be a tad looser as they run hotter.

Missy