6 Bolt Crank

[force_feed]

Hey guys,

I know there has been tons of debates on whether or not you can turn a crank and it hold up and last.

So here is my question. I have a stock crank that the rod journals are in perfect condition. But for some reason 3 main journals look like they got extremely hot! I will post some pictures up a little later.

Is there a limit that the crank can be turned and not suffer the integrity of the crank?
Does that limit effect the main journals?

Thanks!

 

[Jeff99GS]

You can have it micro polished. That is about as much service I would do to the crank.

 

[gordonbradford]

You can get a crank machined. But i would only do it as a last resort.

 

[Colt4G63 also]

If it got extremely hot, wouldn't that comprimise the integrity of the steel? In which case it would be garbage no matter how much you turn or polish it, right? You should be asking if a crank can be re-tempered.

 

[Jeff99GS]

How do the journals "look like they got extremely hot"? How did it get SOOO hot?? The crank can handle a great deal of heat before losing strength.


If you ran the car out of oil or something like that, then the crank is probably scratched and scored. In which case polishing WOULD NOT work....However the factory service manual states that these cranks should NOT be serviced in any way. Other than polishing of course.

Lets see some pics.

 

[BoredDSM]

Cranks can be cut as long as you get it re-coated. FFWD I believe does nitride coating for cranks, so if you do need to get it cut you can just send it to them and have them cut it and then coat it. Its a process in which the crank is heated in an oven, then all the oxygen is pumped out, and then lastly they fill the entire chamber with pressurized nitrogen. All stock 4g63 cranks came DEEP nitride coated from factory and that is what made them so durable. When you cut them you are removing the coating which makes it weak.

 

[Jeff99GS]

Anything "CAN" be done....we ALL know the crank "CAN" be cut and RE-coated....but I STILL DO NOT reccomend it if you plan on building any decent power with this engine.
Just buy a cheap 6 bolt NON turbo block and use that crank....the NT's are usually in great shape cause they are NOT as beat on as the TURBO cars.

 

[force_feed]

Thanks for all the info.

I know the crank got hot because the journals are discolored.

I do plan on building a high power car, and I've heard many things about getting the stock cranks serviced.

I will post some pictures today after work.

 

[Jeff99GS]

Run your fingernail across the crank journal. Does it feel grooved? It should be perfectly smooth (no grooves or scratches) like a new Socket set.

You can take it to a machine shop and have it checked/Magnafluxed for any hairline cracks. They will be able to tell you if it just needs a polish or if it needs turned(ground) re-treated. If they say it needs ground...I would throw it in the scrap pile and get a new or used crank in great shape. Then have that one polished.

 

[mindset]

Well what was the cause of the problem? Maybe a lacking amount of oil was getting to the specific bearings. Be sure to change your oil!

 

[dsm-onster]

Nitriding is a surface treatment done to the journals. It does not make the crank stronger WRT horsepower. It helps the crank survive a dry bearing. Do you run your bearings dry? It can happen. But my motor ran for only a few seconds with an very low oil situation and an uncut crank (all the nitrid still on the journals) and the crank still had scoring that had to be cut out.

The laser rebuild manual actually says not to cut the crank, then it says literally on the next page that if the crank is scored cut to undersize for reuse. . .

I'd like to see where all these cut cranks break.

You need a crank to be cut PROPERLY. This is much more important than whether or not the crank has the surface treatment for tolerating incredibly brief moments of incredibly light contact. We may beat the he!! out of our motors, but we don't drive for 50K with the same oil in it. T/E/Ls and other sirius powered cars were marketed to many different kinds of people.

 

[mikelv]

Nitriding is a surface treatment done to the journals. It does not make the crank stronger WRT horsepower. It helps the crank survive a dry bearing. Do you run your bearings dry? It can happen. But my motor ran for only a few seconds with an very low oil situation and an uncut crank (all the nitrid still on the journals) and the crank still had scoring that had to be cut out.

The laser rebuild manual actually says not to cut the crank, then it says literally on the next page that if the crank is scored cut to undersize for reuse. . .

I'd like to see where all these cut cranks break.

You need a crank to be cut PROPERLY. This is much more important than whether or not the crank has the surface treatment for tolerating incredibly brief moments of incredibly light contact. We may beat the he!! out of our motors, but we don't drive for 50K with the same oil in it. T/E/Ls and other sirius powered cars were marketed to many different kinds of people.

Yes, I agree, and I've heard from someone very reputable that a crank can be cut, but like you said it has to be done by a damn good machinist, preferably someone who is a Mitsu guy.

I can show you a picture of a cut crank from a car we dynoed.
#4 journal snapped right before the flywheel. Took it apart and the flywheel was just flopping around. Lucky, rest of the motor was fine, don't ask me how.

 

[dsm-onster]

But did the crank break because it didn't have the .030" amount of nitriding treatment on that journal? How much was cut? 6-bolt or 7-bolt?

 

[mikelv]

Well, that's a good question.
It was a 6 bolt crank from a 4G64
Bearings were .05 over so I would say yes.

 

[force_feed]

I bought this car not running. It was suppose to have a rod knock.
When I toar the engine down, I found that the balance shaft was causing the knocking noise.

But the crank is questionable.

I will post pics when I get home!

 

[dsm-onster]

Well I have my crank cut .01" under. We'll see this season if it lasts. I think this still leaves nitriding left. 0.05" under is too much IMHO. That makes the crank 1/10 of an inch smaller .

I still don't think it has to do with nitriding and more has to do with a bad grind and/or reducing the size of the journal too much. I would of course get the crank checked. Don't just get it cut no matter what. A typical machine shop will say, just get it cut and you'll know your crank is fine. But if they can't tell whether or not the crank would be fine with a polish job, then they shouldn't be cutting on it.

 

[mikelv]

Right, as I said as long as you have someone who knows thier way around a machine shop, and not the guy behind the counter at Advanced, then you are ok. I mean hell, doing anything the right way I'm pretty sure always work. ha ha

The crank snapped because of bad machine work, bottom line. Car would run fine, but slap 30 pounds of boost on it and jack the revs to over 7-8K, and you got problems.

 

[force_feed]

I just got off the phone with a local well known machine shop.

I had mentioned that the cranks are NOT SERVICEABLE. He immediately knew what I was talking about, and told me that if the crank cannot be turned no more than .01 under. If the crank requires more than .01 than it completely removes the Nitrate treating.

Has anybody heard this before?

 

[mikelv]

I'm pretty sure it's a little more than that, but I'm sure that's correct in saying you don't want to go over that.

I think someone mentioned looking for another crank. Find an old car in the junk yard that no one modded or boosted the shit out of. Take that to the guy and see what he can do. Either that or spend the money on a nice Eagle like I did, and never worry about your crank again.

The reason behind this coating stuff was Mitsubishi as a company was trying to cut corners and save some money. The manufacturing process was getting costly, so to save they used a cheaper material and used a coating to help keep the integrity of the crank. IMO, it's all worthless.

 

[dsm-onster]

Yes. That is what my machine shop told me. If I wanted enough of the nitride treatment to remain to be useful, this is all that can be removed. I asked if the nitriding is important. He said "Do you make it a habit of running your motor with no oil or compromised oil?"

My crank grinding/polishing cost $130 and was done by a reputable shop. I could have hunted around and probably got a good crank for that cost plus the cost for polishing. I think it will all come down to costs. Price out what you can get uncut and polished vs. what the machine shop will charge you for this crank.

 

[force_feed]

Thanks for everyones input.

 

[dsm-onster]

Just to add a little more. Jean-Paul Riccio's (coltboostin) 708whp colt has a .01" under crank. and Kevin Jewer has used a cut crank. They both trust their machinist .

 

[BoredDSM]

I just got off the phone with a local well known machine shop.

I had mentioned that the cranks are NOT SERVICEABLE. He immediately knew what I was talking about, and told me that if the crank cannot be turned no more than .01 under. If the crank requires more than .01 than it completely removes the Nitrate treating.

Has anybody heard this before?

There are two different processes of nitride coating. One is deep nitride coating and the other one is just a surface version. I forgot the specs but I have a book on machining that I will search through for the specs. I know for a fact some American engines did not come with nitride coating or only a surface version. From what I was told from another reputable Mitsu machinist the 4g63 crank comes deep nitride coated.

 

[bpestilence]

Nitriding makes the steel harder and more brittle. However since it is only a thin layer at the surface, the majority of the crank is still tough enough to take a lot of abuse. The point of the nitride treatment is to make the journal hard enough to avoid journal damage if incidental bearing contact occurs. Mitsubishi uses a tin/copper babbit material vs. lead/tin/antimony like most domestic cars. The tin/copper is much harder than the PbSnSb babbit and can actually do damage to untreated steel.

Most of the crank failures (in many kinds of engines) I have seen have been due to microfractures developing due to overheating of the substate material. If there is any discoloration of the steel, a simple magnaflux can determine if cracks have developed. Surface polishing can actually hide the cracks. So if you are going to reuse a questionable crank, magnaflux it before polishing.

 

[dsm-onster]

This is VERY good to know. So it is very *possible* that the cut crank failures were from the cracks that developed from the heat due the dead bearing cutting into the crank in the first place. Not from having it cut down.

I assume a good machine shop always magnafluxes the crank. Mine did.