Crank grinding questions

[83Blzr62]

I recently got my engine block machined and was talked into having my crankshaft ground. Now I am fearing that it may have been a mistake. I have heard many things about getting 6.2/6.5 crankshafts ground. From Benny Avant I heard that you can grind them up to .020" and they will be fine. From many diesel page members I have heard that they should never be ground under any circumstances, but I have seen that Ronnie Joe builds engines with Index Grinds that usually end up to be about .010" undersize. I have also heard that they should not be ground because of the nitride coating or the fillets at the end of the bearing journals. What is the real reason that they should not be ground? WHat is the purpose of the nitride coating? I am buidling a performance 6.2 or atleast trying to get the most performance I can out of it and I do not want to worry about crankshaft failure. My current setup for the new engine is studgirdle kit for the bottom end, 6.5 F code heads with the new turbo precups, the crank and pistons were balanced, timing gears, gm-4turbo, new balancer and serpentine pulley, and db2 4911 injection pump.
Would I be fine using the ground crank, should I get it renitrided, or should I get a new crank? Also I have asked about getting the crankshaft nitrided and was told to make sure there was no compound layer left over. Are there any specifications like depth of coatind or anything else I should know?

Thanks,
Kyle

[doncannon]

hello all,

blzr: the nitride coating is a harder surface for the bearings to ride if i understand it correctly. Once you grind the crank that is gone and has to be redone. I am not sure about how far to grind the crank, but i am sure someone else will chime in on this.

hope this helps,
Don

[Robyn]

The nitriding is a hardening process that has been used on crank journals for many many years and provides for a harder wear surface than normal.
The nitriding is only a few thousands thick so if the crank is ground it is gone.
The Nitriding is a chemical process I believe.
Now if you want to get real trick have the crank gound .010" under size and then take it down town and have it "Hard Chromed back to about .005" oversize then grind it back to standard. Now you have something that is harder than the hubs of hell and will wear far better than any nitriding ever thought of. We used to do this on our Blown Rat race motors years ago.

The fact is that loads of engines have ran millions of miles and never had a nitrided crank.
Having it is nice but not essential to make a long life engine.
Unless your crank is out of spec more than .0005" or gooved I would simply pollish it and put it back in.
Hot tank and magnaflux the crank before doing anything. No sense in spending any $$$$$$$ on a cracked shaft

[ronniejoe]

The cranks are not nitrided from the factory, to the best of my knowledge.

Hard chrome plating is much harder than a nitrided hardened surface, but there is risk in doing the chrome plating. If not done properly with the right controls, stress corrosion cracking and hydrogen embrittlement can occur. There are also issues with bonding of the plating to the substrate. Chrome is also not really good in parts that have tension stress because the chrome is not strong in tension and will crack.

If you nitride your crank, make sure they remove the compound layer (or white layer). Most outfits don't do this.

[Robyn]

These are some great points Ronnie.
We had one outfit years ago that could chrome ice cream and make it stay.
Just kidding but these guys knew their business and we used them exclusively on all our cranks. Dad even had one cylinder block chromed to see if we could reduce wear and stuff.
It was spendy. Actually we chromed up sleaves and then installed them.
I really think that just grinding the crank and using the proper bearings is gonna work just sweet

[ronniejoe]

I really think that just grinding the crank and using the proper bearings is gonna work just sweet

That's my recommendation, and what I do.

[83Blzr62]

OK sounds great, I just needed some extra reassurance that I would be alright. I will be running synthetic oil and a bypass filter so I don't think the bearing journals will be worn much anyways so the nitriding or chrome is not very necessary. Thanks for all the input.

Kyle

[gmctd]

Also, you must remember, Robyn, that very few hi-po - or lopo - rat or mouse motors rolled off the assy line with cast-iron cranks.

Beginning with the 327-325, then the 302-325, 350-350, 396-375 and 425, 427-425, the 454-450, and probably the 502-XXX, the hi-po cranks were all forged steel and nitrided from the factory - that manly gray sheen was the tip-off.

The intermediate engines got forged steel, but no nitride - we always checked for the wide, ground forging flash along the plane of the crank B4 plunkin' down the cash.

You may also have noted that none of the production cast-iron cranks - in the 283, 307, later 265, 305, and ld 350, identifiable by the flakey sharp-edged casting flash, with never a hint of the gray sheen - were ever nitrided.

Dunno - maybe ronniejoe could answer this - but iirc, cast-iron does not take well to hard-chrome plating, particularly in any frictional moving\rotating service, subject to wide temperature variation

Not even the cast nodular-iron cranks

[Peter J. Bierman]

Funny thing is that with a new moderator we get a whole new perspective on this subject.
The good old doctor always told us to never grind a crank becouse of the nitrate layer.
Next to the wear part, he was/ is convinced that the layer did contribute substantial to the strength of the fillets.
I'm not an expert on this and the doctor is, so there must be somthing in it.
On the other hand, why all the fuzz on those little diesel engines, normal common engine rebuilders practice must work on the 6.2 and 6.5 too, they are not that special.....

We probably never get complete consensus on this subject

Peter

[ronniejoe]

The fillets on these crankshafts are undercut, then cold rolled. When grinding the crank journal, the fillet is not touched. As long as the diameter of the journal remains larger than the minimum diamter of the section through the fillets, no real reduction in strength will occur.

Nitriding to harden the surface of the crank will increase it's fatigue strength, or it's resistance to fatigue cracking. As I said earlier, I don't believe these cranks are nitrided from the factory. Correct me if I'm wrong, but have some documentation in hand.

[Robyn]

I agree iron cranks are not treated.
The standard rat cranks may or may not have been treated. The high perf stuff was. Tufftriding I beleive they call it. Its all nitride treatment.
I replaced a crank in a 6.2 a few years back and it did not have the grey look that most rat performance cranks have.

[Robyn]

One thing I am currious about is the number of 6.2/6.5 cranks that break. I really would like to know what is going on with this.
Something is hinky somewhere????
Any input RJ?????

[ronniejoe]

Weak, cracking blocks and/or bad torsional dampers.

Almost all broken cranks that I've heard of were accompanied by broken blocks. Most people put the cause and effect order as crank breaking then block. I believe it is the other way around. Block breaks causing a loss of support for the crank. The extra flexing causes a rapid propagation of fatigue cracks and breaks the crank.

Failed torsional dampers accompany some. If the damper fails, the dynamic stresses in these cranks become enormous.

[gmctd]

And the occasional break at the 4rth main where the dual-mass flywheel has deteriorated.

[Bobby Etheredge]

I may be mistaken, but the most important part of these crankshafts are the fillets of the journals. "Car" engines have no fillet. Most automotive machine shops do not have and do not buy the more expensive stones to grind these fillets. To gey them ground properly you need to find a machinist that is qualified to grind crankshafts for Aircraft Engines; all Aircraft engines have the same kind of fillets as the diesel cranks.

The theory is that you do not want any sharp corners, because that is where the stress will concentrate and cracks usually start at these points. You need a "Radius" of the proper dimension to remove these stress concentration points. The nitriding only gives you a "Hard" bearing surface that will withstand the constant pounding it recieves from "High" output engines. The crankshaft is made to "Flex" to some extent; even though it may be negligeable. If the crank was as hard as the nitrided surface it would shatter like glass the first time it tried to start. Thanks.