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I saw a lot of YouTube videos referring to crankshaft grinding, while watching videos about working on crankshafts. I don't understand what crankshaft grinding does.

If we grind a crankshaft, then how does it stay compatible with the vehicle where the crankshaft was taken from? If it is to make it lighter, then doesn't it invalidate the crankshaft maker's purpose of making it thick and heavy?

  • This doesn't seem like a maintenance or repair question. Do you have a maintenance or repair question? – cory Jan 11 '17 at 15:50
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    You can grind a crankshaft as part of maintenance or repair. The question seems valid to me. – Chenmunka Jan 11 '17 at 15:56
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    The question is completely on topic for Mechanics.SE ... Thank you for asking it! – Pᴀᴜʟsᴛᴇʀ2 Jan 11 '17 at 16:21
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Grinding a crankshaft is a process of removing material from the journals in an effort to refurbish and reuse an expensive, yet vital component of an engine. It is usually done during the process of rebuilding an engine when needed, but also has some performance aspects which come along with the process.

Let's first off describe the anatomy of a crankshaft. The different parts of the crankshaft are:

enter image description here

  • Main journals
  • Rod journals
  • Snout (or nose)
  • Flywheel mounting flange
  • Throws
  • Counterweights
  • Web

The main parts we are working with when grinding are the journals, both main and rod.

During the rebuild process it will be determined if the journal surfaces are within sufficient tolerances to be used as they are or if they will need to be ground. There are several reasons why they may need to do the machining:

  • If the surface of the journal has wear which makes it no longer smooth
  • If the journal is out of round
  • If the journal is not square (same diameter at both ends of the pin)
  • Can also be ground to create more stroke (this would make for another answer, so will not include the details here)

If the decision is made to reuse the crankshaft, but machining will be necessary in order for it to work, the machinist will grind part of the top layer of the journal away to make it smooth again. Usually (in the SAE world), the amount taken off is measured in 0.010" (usually 0.010", 0.020", or 0.030" - depending on the severity of the wear on the journal). Once the journal is ground down close to the final dimensions needed for the process, there is then a finishing process of polishing the journals. This involves using using a long circular piece of emery paper. The process looks like this:

enter image description here

The crankshaft is spun while in the opposite direction and the emery paper is put in contact with the journal. This process creates an ultra smooth surface on the crankshaft, which reduces friction, which improves overall power and torque.

To get the crankshaft to work once again in the engine, you have to take up the excess space which is taken away during the process. This is done by using under-sized bearings (undersized because you are making the journal smaller, not larger - the bearings are matched undersized by negative numbers to match). The bearings are matched to maintain the proper clearances for oil flow at the new diameter of the journal.

The main reason for grinding a crankshaft is as stated, but there are some side benefits to doing the grinding.

First, yes it does lighten the overall weight of the crankshaft, but overall it doesn't do that much to create a huge difference. If you wanted to reduce weight, you'd do things like drill the pins and other things which are better left for another question.

Second, grinding the journals and reducing their overall size. By reducing the size, you have less surface area. Reducing the surface area reduces friction losses imposed by the journal faces. The difference is measurable on an engine dyno. If you are an engine builder doing a max effort, this is definitely one area to look at for creating greater efficiencies and thus making more power/torque.

Third, the grinding process also makes the corner of the journals so they have a greater radius. This actually makes the crankshaft stronger by reducing the stress riser at the corners.

There are other reasons which I may not have covered, but I believe I should have answered your question.

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    You had us at "round" and "smooth". +1 for the undersizing for performance reasons. The friction is not so much of a problem with age (maintenance angle) as balance/play when vibration/harmonic resonance becomes an issue, This is one of the single points of failure for oil pressure too. Just putting in new shells is a short term solution as they will wear quickly if they do not match evenly. – mckenzm Jan 12 '17 at 0:36
  • In some cases extra material is first welded onto the crankshaft and then it is ground down back to the original size. Usually done when the bearings are not replaceable or the crankshaft is so worn that grinding it further down would jeopardize structural integrity. – Tonny Jan 12 '17 at 11:33
  • Very nice answer! could you confirm that the 'Rod journal' you are talking about in your text is identical to the 'Crankpin Journal' mentioned in your picture? – loonquawl Mar 12 at 7:02
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A crankshaft wears with use. After a lot of miles, the play between shaft and bearing becomes too much. The more play, the lower the oil pressure, the worse the lubrication. And then it wears out even faster.

You can grind a crankshaft, camshaft, or any shaft to make it round again when it's worn out. You replace the bearings with thicker/oversize ones, so the crankshaft fits nicely in its bearings again. The shaft becomes a little weaker, but only marginal. It doesn't really compromise safety or strength. It is done with crankshafts, camshafts, cylinders(although naming is a bit different here) and much more.

If there's been to much wear on the shaft to be ground while keeping enough strength, you just have to replace it. With crank- and camshafts, it's still profitable to grind them, because they're generally really expensive. No one would grind for example a waterpump shaft, or it has to be a very special car.

  • I never heard anything about Crankshaft grinding here in India. – InQusitive Jan 11 '17 at 16:56
  • @InQusitive are there machine shops generally available there? If not, that might raise the cost of grinding higher than the cost of replacing the crank shaft. Just a guess. – cdunn Jan 11 '17 at 17:10
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    I think very less, it looks like the machinery costs very much not affordable to small shops. I think I should start one. – InQusitive Jan 11 '17 at 18:09
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    It's profitable only for cars with high (sentimental) value. Not sure if you're serious about starting a shop, but I woouldn't recommend it before you have a lot of experience in both company management and machining. If there are not much expensive cars in your area, buying machinery for grinding shafts may be a waste of effort and money. – Bart Jan 12 '17 at 7:35
  • I am seriously thinking about it. Can u point me to some link where I can see how to check the feasibiltiy of such mechanics realated business. btw I have no experience in machining. – InQusitive Jan 12 '17 at 13:10

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