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Straightened crankshaft revisited

General engine tech -- Drag Racing to Circle Track

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AC sports
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Straightened crankshaft revisited

Post by AC sports » Tue Nov 05, 2019 1:16 pm

Some time ago I posted about a couple of cranks I'd straightened. One under discussion was from an engine that suffered oil starvation and chewed up the centre main bearing shells, in the process overheating the crank and bending it 0.010.
I put the crank in v blocks and pressed against the bend and got it to 0.0004 which I'm happy with.
That was a year ago. I was reluctant to use it since I had another 2 on the shelf.
Now I need it! Supply has dried up. My question is ,will it stay straight in use?
I didn't hear any creacking sounds. It rings like a bell.
Crank is from an inline 4. 5 main journals, forged construction.
All I did was press it. No stress relief of any kind. I've not much experience in these matters. Opinions welcome.

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Re: Straightened crankshaft revisited

Post by Vintagewrench » Tue Nov 05, 2019 2:50 pm

Here is another way to straighten shafts: I rebuild early racing engines and have had good luck for years straightening crank and camshafts by peening the radius' of a journal with a chisel carefully ground and polished to match it. By doing it this way the shaft will remain straight in use.

Set the shaft up on a solid surface on a set of V-blocks and check it with an indicator (usually on the center main) to find the lowest spot. Then carefully and slowly peen the radius at the low spot on both sides (lengthwise) of the radius' on the journal. Keep checking with the indicator and proceed slowly. This will straighten the shaft by bringing the low side of the shaft back to true by upsetting and stretching the metal and it will remain this way in use.

If the shaft is going to be ground take care not to remove any of the metal from the peened areas. If the shaft relaxes during grinding this area repeat the process after grinding.

You should be able to get the shaft straight within +.0005 or -.0005 or better using this process unless it is cracked (mag first).

The photo below (click on it to enlarge) contains a 1914 Simplex 4-cyl 600 CI crankshaft with a 6.5" stroke that was bent over .020" because it was run for a long time after the babbitt bearing lining had cracked and lost its bond from the originals bronze shells and disintegrated.

Included in the photo are newly machined and babbitted main inserts, and semi-finished rod and cam bearing shells all machined here in the shop.

Image

See and hear it run in this video:


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Re: Straightened crankshaft revisited

Post by BillK » Tue Nov 05, 2019 7:28 pm

Did you magnaflux it before and after straightening to see if there are any cracks ? That would be my main concern. The guy that grinds crankshafts for me has straightened several over the years and I have never had a problem BUT he always magnafluxes them and always regrinds them undersize.
Bill Koustenis
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www.enginerepairshop.com

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Re: Straightened crankshaft revisited

Post by mag2555 » Wed Nov 06, 2019 6:17 am

The undisputable fact is this, if the Crank ever gets subjected to the same stress loads that caused it to bend the first time then it will bend again .
If it where me and I did not want to go thru that again and if the Crank was not a 150 lb truck Crank then I would send it out to be cryogenicly treated .

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Re: Straightened crankshaft revisited

Post by modok » Wed Nov 06, 2019 11:04 pm

Strightened it with a press, cold....hmmm
I'm glad it worked for you, usually it does not.
That's why we don't do it that way. Usually doesn't work.


You did it, you tell me if it worked. :lol:
You should have asked how to do it BEFORE you did it.

if it's a fairly skinny MILD steel crank, you'll probably get away with it.

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Re: Straightened crankshaft revisited

Post by AC sports » Wed Nov 06, 2019 11:46 pm

Well its straight. But my question was will it stay that way in use?
If you don't think it'll stay straight..can it be fixed?

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Re: Straightened crankshaft revisited

Post by modok » Thu Nov 07, 2019 12:27 am

What? No, no , the question is............. will it CRACK?
and........ probably not, but as I said depends on the material and design.

why do blacksmiths HAMMER the steel?
Because if the area you are working is always in compression.... you CAN'T crack it.
That's why we forge cranks, and swords, keep it in compression, rather than just STRETCH them to length and shape. Putting it in tension can form cracks or micro-cracks.

the bearing rapidly heated, and heat shrunk a spot, then when it cooled it became bent. What you did was PULL on that shrunk spot it and stretch it back out, so, that was risky, but, if it isn't cracked yet then your PROBABLY ok, although not all cracks are visible.
Last edited by modok on Thu Nov 07, 2019 12:32 am, edited 1 time in total.

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Re: Straightened crankshaft revisited

Post by exhaustgases » Thu Nov 07, 2019 12:29 am

Vintagewrench wrote:
Tue Nov 05, 2019 2:50 pm
This will straighten the shaft by bringing the low side of the shaft back to true by upsetting and stretching the metal and it will remain this way in use.

If the shaft relaxes during grinding this area repeat the process after grinding.
Relax and stretch??? Maybe compressive stress is relaxed when grinding? Doesn't peening cause compressive stress?

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Re: Straightened crankshaft revisited

Post by modok » Thu Nov 07, 2019 12:33 am

You grind out a little of the area you peened, so, it that compressive stress is gone, because that material is gone.
Also the heat of grinding may relax some of the compressive stress also, just slightly, usually only a small amount. A few thou. Half the reason you straighten a crank BEFORE grinding is SO you can grind it.
There are service limits for how bent it can be and still work, and they usually do seem quite extreme, but it's true.
Say, .004" for a four cylinder, (as long as that's a gentle curve), is ok. Really? yep.
Cranks do flex a LOT in use, so a very slight bend just makes for a very slight bit of bearing drag, which you would not even detect. the same is true of main bearing alignment.

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Re: Straightened crankshaft revisited

Post by Circlotron » Thu Nov 07, 2019 1:22 am

modok wrote:
Wed Nov 06, 2019 11:04 pm
Strightened it with a press, cold....hmmm
I'm glad it worked for you, usually it does not.
Would you be more likely to have success using a press on a forged crank rather than a cast one?

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Re: Straightened crankshaft revisited

Post by modok » Thu Nov 07, 2019 2:35 am

There is such a thing as a special crank straightening rig, that may have hydraulics....so, "it's a press", but, made for the job, made to not get in your way,
Physically bending the crankshaft can help, but IMO should only be done in conjunction with peening AND with it heated it up to.... at least 300 degrees F, and maybe hotter, maybe a lot hotter. I mean....think about it. heat got you INTO this mess right? An area got hot rapidly and cooled, with the crank held STRIGHT. So, maybe the way to REVERSE that is heat up the area and let it cool with the crank held bent, and release the force on it and it will spring back to straight. Probably the safest bet. If it doesen't go.... I'd rather use more heat than more force.....if you know what I mean. Peening works very well but that only reaches a limited area and sometimes you need more. You don't forge cranks nor swords at room temp.


really depends on the crank tho. Can you straighten a grade 2 bolt? ok sure bend it back. not much to worry about.
Can you straighten a grade 8 bolt.....well, that's more difficult. Same thing with cranks.
The harder the material, the closer the break and yield points become.
I think the softest crank I ever found was a corvair. Super soft. soft like cheap bolts that don't even have a grade :lol:
Sure makes them EASY to straighten

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Re: Straightened crankshaft revisited

Post by modok » Thu Nov 07, 2019 3:09 am

how about this.
Lets bend a snap-on wrench to a special shape....for that hard to reach spot.

if you just try to bend it, it will break in two.
Maybe I'll try to peen it on the outside fo the bend AS I'm bending it, and that could work, tho it would take forever.

The way any sane man would do it is heat it up to glowing and bend it.

But if you have a cheap wrench made of SOFT steel, you can probably JUST bend it, and it'll work.

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Re: Straightened crankshaft revisited

Post by David Redszus » Thu Nov 07, 2019 11:17 am

Many years ago, I had my engine machining performed by a FAA certified engine repair shop.

One day the owner asked me if i would like to see how a bent crank from a Merlin aircraft engine
was straightened. Sure, I said.

He placed the crank in vee blocks on a surface plate. He then used a large brass hammer and
smacked the crank very hard. Using an indicator to determine the high spot, he repeated the
process until the crank was straight.

Cranks are much softer than we might think but with hard surfaces.

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Re: Straightened crankshaft revisited

Post by dwilliams » Thu Nov 07, 2019 11:37 am

modok wrote:
Thu Nov 07, 2019 2:35 am
There is such a thing as a special crank straightening rig, that may have hydraulics....so, "it's a press", but, made for the job, made to not get in your way,
I wound up with a forged steel 231 Buick crank that had about a .005" runout on the crank snout. I determined this was due to the crank being slightly bent at the #2 rod journal. Not having a proper press, I used the one in a friend's shop. Then I wound up having to repair the press after breaking it...

The crank was smooth all over and had no markings. I have no idea who made it. But they had heat treated it until a file just skipped across any surface you tried to check. Not only was it harder than the clappers of doom, no matter how much I bent it, it just sprang back to its original shape when I let off the pressure. Yes, that was including peening...

My customer decided he wanted a different crank anyway, so I never did manage to straighten the stupid thing.

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Re: Straightened crankshaft revisited

Post by ClassAct » Thu Nov 07, 2019 1:39 pm

I have straightened many cranks shafts. None ever bent again, unless the same scenario occurred which caused the bend in the first place.

Several cranks I have straightened more than once. One was one of my own, personal cranks. Three times IIRC...may have been 4 but I can't remember and I still have it. Had it not cracked on the last explosion.

Grind it after you get it straight and don't think about it.

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