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Old 03-11-2013, 11:44 PM   #1
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What is the stocks size of a BP's crank bearing journal diameter? how much can be cut on a crank without fear of catastrophic failure on a boosted motor? I'm looking at a used crank that has been lightened and knife edged. The previous owner threw a rod due to one of the connecting rod caps was loose, he states there is no damage to the crank journals but he did state when the motor was assembled they did cut the crank. Im wondering if this crank is worth it? how much would one expect to pay to have a crank knife edged and balanced? What should i look for when checking out this crank? i know there should be know scaring on the journals and nothing more then what i can feel with my nail?

thanks for any advice!
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Old 03-11-2013, 11:45 PM   #2
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Leave the crankshaft alone.
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Old 03-12-2013, 01:33 AM   #3
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Dont cut the crank. If your crank is fucked up, find another one. There are lots around.
When you cut it youre losing the hardened surface and will just have more problems. You cant beat an OEM crank.

Also, knife edging is bullshit. If youre concerned about windage, run the 01+ girdle and windage tray.
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Old 03-12-2013, 02:38 AM   #4
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Leave Brittany alone.
Fixed.
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Old 03-12-2013, 10:11 AM   #5
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This is the crank in question


Thanks for the advice ill guess il pass on it
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Old 03-12-2013, 03:10 PM   #6
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Using my internet micrometer, I can tell from that picture alone that
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Old 03-12-2013, 03:51 PM   #7
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I like how that crank has been knife edged. How much has been taken off the journals?

There is an advantage of removing material off the journals. If you take it down, you reduce the amount of surface area that the bearing contacts. This creates less friction, and also makes it a tad lighter. It was a known tactic to take big block cranks and turn them down to small bock size to make more power. The only downside is if you hurt the crank, you have little or no room for repairs. You will have to get the crank rewelded, or source another crank. I guess this is ok for the higher budget racers, but not so good for the Joe Shmoe.

If you do decide to take the crankshaft down further, find out what size bearings are available from the supplier of your choice. You should be safe to make as much power as you desire from a crank that has been turned down to what a bearing supplier like ACL has to offer for your crank.

As far as the nitriding, you can typically take it down to 10/10 size and still have the nitride hardness. However if you want to take it further, there are nitriding services available. It depends on what you can afford.

I found this operation that does various hardness services of many types.
IBC Coatings: ION Plasma Nitriding | Boriding | TD | Vacuum Heat Treat | ION Nitriding
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Old 03-12-2013, 04:01 PM   #8
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The seller say the crank is fine. Befote they did the work the crank was cut. He ran it fot a short period and threw a rod due to one of the rod caps was loose. He fixed that and melted a piston. He is now swapping the bp for a fe3. If the crank visually looks good should i buy it? If i measure the journals what spec should i be looking for?
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Old 03-13-2013, 10:10 AM   #9
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So should i pick it up or leave it,? Does anyone know the size so i can measure it?
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Old 03-13-2013, 11:17 AM   #10
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I do not know right off the top of my head, I am not looking it up for you. It is a very simple google search, coupled with very simple math.

*Look up the standard size for both rod and main journals.
*Look up what ground down sizes are available.
*Inspect the crankshaft for any damage.
*Measure the journals on crankshaft.
*Factor in what you think the crank will need to be ground to if any.
*Make a decision based on the above knowledge.
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Old 03-14-2013, 09:24 PM   #11
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Crankshaft Main bearing measurments
Journal Diameter:
STD 1.9661-1.9667
.01 Undersize: 1.9563-1.9569
.02 Undersize: 1.9464-1.9470
.03 Undersize: 1.9366-1.9372

Oil Clearance w/bearings: STD .0007-.0014 MAX .004
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Old 03-14-2013, 09:51 PM   #12
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There is an advantage of removing material off the journals. If you take it down, you reduce the amount of surface area that the bearing contacts. This creates less friction, and also makes it a tad lighter.
Basic Freshman engineering class will say otherwise. Surface area does not affect friction force.
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Old 03-14-2013, 10:00 PM   #13
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Im no expert but the above is true for 2 dry surfaces.

Fluid bearing surfaces... I doubt you are right at all.

Dann
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Old 03-15-2013, 01:08 AM   #14
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Quote:
Originally Posted by 240_to_miata View Post
Basic Freshman engineering class will say otherwise. Surface area does not affect friction force.
So sure? Back up your claim, and I will back up mine.
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Old 03-15-2013, 01:35 AM   #15
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Old 03-15-2013, 08:48 AM   #16
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I just was saying that the fundimentals claim that. I agree that in reality it does come into play for reasons other than F=uN

Basically I was being a smartass.
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Old 03-15-2013, 09:39 AM   #17
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Quote:
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I just was saying that the fundimentals claim that. I agree that in reality it does come into play for reasons other than F=uN

Basically I was being a smartass.
OK. You almost got my jimmies rustled.

Anyways, I did some searching, and apparantly this is typically done on the rod journals. I am guessing leaving the mains larger helps reduce flex. I failed to mention that earlier.

I guess what happens is; the smaller the rod journal, the slower the bearing speed in relation to the journal. It has been an engine building practice for many years.

I remember the redneck engine builders, in my younger days, would cut the cranks so far that they would stack 2 bearings on top of one another. They said it was to make power. I thought they were too cheap to buy another crank. Either way, it was way too ghetto for me.
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Old 03-15-2013, 08:26 PM   #18
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Continuing the discussion on surface area affecting friction... Would this mean that two tires made of the same compound but one a 195 and the other a 225 would have the same rolling resistance and grip?

-Psych major having trouble grasping engineering concept
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Old 03-15-2013, 08:49 PM   #19
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^^ There is some cool articles out there explaining why tire width actually makes a difference on grip, I just cant remember where I read it. Hopefully someone else will chime in.
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Old 03-15-2013, 08:55 PM   #20
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Quote:
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^^ There is some cool articles out there explaining why tire width actually makes a difference on grip, I just cant remember where I read it. Hopefully someone else will chime in.
In the real world, the coefficient of friction isn't a constant, and it varies ever so slightly (with pressure IIRC). This (among other reasons) is why wider tires generate more grip than narrow tires.
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