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Join Date: May 2009
Location: Bergen, Norway
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Messed up my crankshaft.... (With pictures)

Yesterday I was tearing apart the shortblock of my 951 as I am rebuilding it. Everything inside looked very good. I cleand the block, checked all bearings (All was fine), and cleaned the crankshaft.

Then I thought I'd check the main journal and rod journal diameter with a micrometer to verify they were within the tolerance limits. The crankshaft looked flawless before I started.

Well, it all went wrong from there. I haven't used my micrometer for many years (And I have only ever used them once before), so I had forgotten how to use them. Probably used 15 minutes just to figure out how to read the measurements from it.
I found it hard to find the center of the journals, so I made the mic gradually smaller and smaller in 0.0010" steps and tried sliding it across the journal, to see what was the smallest opening that would pass. I know, stupid stupid stupid. It was late at night, I was tired, was working on the garage floor under the car that was raised all the way up on the lift, and had bad lighting. I actually tried wiggling the mic across in the narrowest.. I know, flame away..

Well, when I put the flashlight to the crankshaft once more in the end for one last close inspection after measuring, I found all the scrathces my dumbass procedure had produced..

I stayed up till 3 AM that night, searching the web and trying to find out how to mend this. I then checked how I should have used the mic, and also found I should have used a ballpoint mic to get proper readings and avoid scratching, and that I of course should not have dragged it across like I did.

Anyhow, here are some pictures of the mess I made:










I can not feel the scratches with my fingertips, but I can feel them with my nail. There was one nick that was raised and caught my finger. After reading a bit about how people polish crankshafts themselves and different online recommendations, I used a 1200 grit wet paper and WD40 as lubrication, and carefulle removed the nick. You can't tell that the surface has been touched as the surface doesn't look changed. But the small raised edge is gone.

What do you think, can this crankshaft still be used? As far as I could tell from what I read online, as long as any raised microareas around the scratches are removed with polishing it doesn't matter that the scratches are there. They will simply hold a bit extra lubrication. But any material sticking out/raised area will destroy the bearings.
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Paul E. Johannessen from Bergen in Norway

- 1972 Porsche 911, rebuilt to '76 Carrera 3.0 spec
- 1986 Porsche 944 Turbo
Old 02-03-2013, 01:08 PM
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If it were me I would have the crank turned 10 thousands under and get undersize bearings to match. Don't feel too bad about this either. **** happens. There is a very good crank place in my neck of the woods that is reasonable so there should be one in yours as well. You could even take the crank to a place near you and get their expert opinion.
Old 02-03-2013, 01:17 PM
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Would emery paper help?
Probably not...
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Old 02-03-2013, 01:20 PM
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Hmm. I already have bought new std size bearings.... So I'd really like to be able to use those as long as it is possible.

Also, I live in Norway. Fairly large country in size, but small by number of inhabitants. Machinist shops are hard to come by and expensive.. I would have to send the crankshaft by mail, so probably around $60 in shipping costs each direction. And a grind is $408 the places I've checked so far.

It's from reading stuff like this I thought maybe it could be salvaged as it is:
Polish Crank - Sandpaper, Emery Cloth, or ? [Archive] - Chevelle Tech
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Paul E. Johannessen from Bergen in Norway

- 1972 Porsche 911, rebuilt to '76 Carrera 3.0 spec
- 1986 Porsche 944 Turbo
Old 02-03-2013, 01:57 PM
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You will have to use emery cloth then, it comes in a roll with different grit ,and get some thin oil and you will have to remove all the bad material till you can't see any of the scratches by going around the entire circle one side at a time but that will be taking down the diamiter some so you will have to plastigauge it when you are done to see if your standard bearings will be to loose. Do some research on the net.
Old 02-03-2013, 02:09 PM
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Wrap whatever you use to polish completely around the journal. Wrap some string around that and rotate it. Do not let the string (or whatever you use) to sit in one spot as you are rotating, should be able to move side to side a bit.
1200 may work. You want to go as fine as possible as to not remove too much.
IMHO, small scratches will not be a big deal as long as there are no raised areas.
I have rebuilt engines..Chevrolet and Buick....not a Porsche......yet.
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Old 02-03-2013, 03:59 PM
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Old 02-04-2013, 05:44 AM
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Thank you for all the suggestions! :-)

I talked to a machine shop in another city, they thought they could polish it up to spec for $130. So I think I'll bite the bullet and ship it to them and let them balance it at the same time. Far cheaper than the other shops I have gotten quotes from.
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Paul E. Johannessen from Bergen in Norway

- 1972 Porsche 911, rebuilt to '76 Carrera 3.0 spec
- 1986 Porsche 944 Turbo
Old 02-04-2013, 08:59 AM
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Glad to hear
Old 02-04-2013, 09:52 AM
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Update:

Well, they managed to polish away all the scratches and still keep it within spec.

Next up is balancing the whole rotating assembly.

Before shots:



After:




So this just shows that quite pronounced scratches can be polished away without removing noticeable amounts of metal.
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Paul E. Johannessen from Bergen in Norway

- 1972 Porsche 911, rebuilt to '76 Carrera 3.0 spec
- 1986 Porsche 944 Turbo
Old 02-11-2013, 02:01 PM
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Awesome!
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Old 02-11-2013, 02:36 PM
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UPDATE: JOURNALS SLIGHTLY UNDER SPEC... Is this a problem?

I got an email with the measurements today. They are:
Rod bearing journals: 51.965-51.970mm (Std tolerance limits are 51.971 - 51.990mm)
Main bearing journals: 69.970-69.975mm (Std tolerance limits are 69.971 - 69.990mm)

So, the main bearing journals are within spec except one which is only 0.001mm / 0.000039" too small. Thus within spec measured in inches. :-)

But, the rod bearing journals are from 0.001mm to 0.006mm too small. Thus 0.000039"-0.000236". Which means the largest rod journals are within spec, but the smallest is 0.0002" too small (Spec only goes to thousands in inches).

I would think this is ok?
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Paul E. Johannessen from Bergen in Norway

- 1972 Porsche 911, rebuilt to '76 Carrera 3.0 spec
- 1986 Porsche 944 Turbo
Old 02-13-2013, 12:52 AM
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I believe you are good to go with Standard Bearings. Just going up the next Bearing Size is Extreamly Expensive. The Crank can usually be polished out.

Cheers,
Larry
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Old 02-13-2013, 12:58 PM
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The numbers you mention could be due to how the micrometer is used. You may mic the thickensses of the bearing shells and use the thickest shells with the smallest dia journals (you must use a ball point micrometer for this). This should even it out a bit.

What matters the most is the actual clearance you get between the shells and the journals. That measurement should actually be taken by measuring the crank and the bore at the same time using one single micrometer and a dial bore gauge. Using two different micrometers could be enough to introduce a margin of close to what you are worrying about.

Check out these videos on how to blueprint (measure up) an engine.
Blueprint 104 - The Crankshaft - YouTube
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Last edited by bebbetufs; 02-14-2013 at 12:12 PM..
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