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Discussion Starter #1
Hi All,

Please try to contain your laughter at this question. ;)

I'm checking my crank to see if its in-spec but I'm concerned the journal diameter measurements are not very precise because its tough to know if the mic is on the exact diameter or slightly away from it. The other thing is the anvil face...I try to get it on the tangent on the fixed anvil and then carefully approach it with the movable side, but I'm not always sure its all square.

Looking at all the resources I can find, this measurement seems to be a no-brainer. I'm thinking maybe a ball attachment might help with the anvil face issue, but you still need to be on the diameter.

What am I missing?
 

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Take it to the machine shop with specs and have them measure. Otherwise, take a machining class at your local college and learn how to use a micrometer.
 

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You're thinking too hard! (that always makes my head hurt...)

Your micrometer needs to be at right angles to the length of the crankshaft and measuring the maximum diameter of each journal - typically in a few different areas & about 90 degrees apart. A ball end would be used for measuring an inside diameter. The flat faces of the micrometer can allow you to make sure it is at a right angle to the bearing surface.

However, it is possible to have a journal measure the same in differnent areas and yet not actually be 'round'. See: Is It Round?
 

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Discussion Starter #5 (Edited)
Thanks all for your help...sorry for the long winded response below.

I hesitated asking this question because I knew it would make me look like a moron. However, I'm at the age where learning stuff is way more important to me than looking like a moron. :laugh:

I do intend to take this to a machine shop to do the real checks, but I'm going to milk this engine rebuild for all the experience and knowledge its worth. I would love to go to a CC and take some classes in this, but that's not fitting my present schedule.

I know how to read a micrometer and a use a dial bore gauge. I've used a dial bore gauge to measure the cylinder sleeves and various con rods and main bearing diameters but can't see how you'd use it to measure this, maybe someone can show me.

I also know the concept of what I'm trying to do and keep the anvil faces at at the tangent of the diameter endpoints. Actually doing it physically within the 0.0004" tolerance is the problem because it doesn't take much to tilt one anvil face or the other and lose the precision. A long-reach caliper would make this easy, but calipers don't have the accuracy and precision that a micrometer has so you can't use that. Another physical problem is that the counterweights crowd in on the area so that measuring the different angles and reading the mic isn't easy. None of this inspires confidence in my measurements (witnessed by the fact they weren't very repeatable to 0.0001").

Eric, thanks for that link...good stuff there. I did exactly what that picture shows with the mic. I know ball ends are used to measure thickness of curved surfaces like the bearings, but I was shooting in the dark. Actually the opposite would work better...like some long flat attachment. I think the answer is LASERs, but maybe I am thinking too hard.

I'll probably pay the machinist a few bucks more to let me see how he does it (or bring a box of donuts).
 

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Another option are dial or digital snap gages. When I am processing a job that has very tight tolerance and inexperienced personnel I always use them.



And they can be sourced via eBay for well under $100.
 

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Discussion Starter #8
Thanks Iljayr and classic Alfas. Snap gage makes sense, not sure about the 3-point mic since that seems to be for measuring inside diameters, not outside....anyway I can't afford it.

I forgot to mention that I ended up using my mic as a snap gauge basically by setting it to the largest permitted size within the journal spec and then pass it over the journals. Unfortunately on some it passed pretty easily making me think the crank may not be useable. It probably would've been better if I attached some disk anvils that had the sides shaved, but it clearly was smaller than the allowed size.

Still I wanted to know if there was a sure fire way to measure it and compare to the main bearing inside diameter as a double check on the bearing size to get.
 

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One of my micrometers has a vernier scale on the barrel of it that allows for reading to .0001". I don't take any notice of it, but just estimate by 1/4 'thou increments on the main scale.

As far as technique to ensure repeatability in measureing an outside diameter is concerned:

If you are measuring against the specs, zero the mic against the master calibration anvil first. Note that the loading you use on the adjustment dial should be the same for all measurements. Outside micrometers have a litle ratchet knob that is supposted to be used to get the same loading each time.

For measuring journals: I am right handed. I hold the body of the mic near the fixed anvil with my left. The anvil stays stationary while you rock the mic in line with the journal curve and adjust the dial down onto the journal. Getting the adjustment correct is a combination between the weight on the dail and the feel of the drag accross the journal (should be of the lightest possible drag). The rocking while adjusting centrs the mic and ensures it is square. You should be moving the mic less that the diameter of the movable anvil. Lock the mic, remove and read.

Measure in line with the counterweights and 90 degrees accross, 60 if you are pedantic. Orient the adjustment of the mic away from the counterweights. Compare journal to journal. That will give you an initial idea that the crank may need grinding. Look for visual clues. If it is a 2000 engine, then usually only very high mileage engines or those that have suffered a bearing failure will require grinding.

When dissassembling an engine, you should always look at the size of the bearing in it when you take it apart. If they do not say 010, 020, 030 etc, then it is standard.

For main bearing ID, I use tetescopic gauges to transfer the size and ensure the correct clearance (in three directions).
 
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