LOL i started a thread on this stuff back in 2007! Enjoy!

http://forums.pelicanparts.com/911-engine-rebuilding-forum/328636-measurement-instruments-metrology.html

Quite right! I notice Jon that you have quite a collection of the very same tools pictured in the workshop manuals! Must be fun.

Wear Limit
 

True however, I don't think its common to find a used/re-used crankshaft that is larger than the 59.971mm spec. (material wears off making the diameter smaller...).

As such the real concern with measuring to spec is oriented toward determining if the crankshaft journal measurement exceeds the wear limit - 59.990mm - or 59.99mm, which is measurable with a 0.01mm mic.

My take - a 0.01mm mic has sufficient resolution to rebuild a motor.

Gordo

Porsche notes specs to 0.001mm for manufacturing or machine work tolerances. If you're checking a used crankshaft for wear, instruments which measure to 0.01mm are usually sufficient. Some micrometers have vernier scales, that allow accurate measurements to the next decimal. If measurements to 0.01mm are not acceptable, you can bring the crankshaft to a lab, manufacturing company or quality machine shop.

The standard 1-7 main journal wear limit is 59.960mm, standard rod journal wear limit is 52.960mm.

OK, very good. I will work with the .01 mikes and see what I get.

My guess is the larger number where you would expect a wear number might be the maximum diameter for a new unit. I thought that odd also.

I tried miking one piston this morning and I can see how the two-digit metric mikes will likely be OK. However my results did bring up another question.

The piston, as one would expect, is out of round. However to my way of thinking it is out of round 90 degrees out. It seems to me any wear should be on the thrust faces, which to me would be the diameter of the piston 90 degrees to the pin axis where the side loads on the bore are greatest. This piston was just the opposite; the "fat" was on that part and the "thin" measurement was parallel to the pin axis.

Is this right/normal, what. Sees counter-intuitive to me.

Where did you measure it? The pistons are tapered.

Someone here could suggest you to measure the piston OD at the piston skirts. The OD dimension parallel to the pin isn't spec'd.

However, I have a feeling you would follow up with a question as to where to measure the cylinder in order to determine piston-to-cylinder clearance, taper, out-of-round, as well as other cylinder measurements.

..... and so on:

Suggest you obtain a reference book of specs so you know what parts to check and to know the service limits of all the engine pieces. About $20 should do it. You could get a consensus from several responders, from a few or from one person, but why not be certain?

Sherwood

I have the info and am pretty sure I'm measuring at the right place; it just seems I'm getting some odd readings.

I haven't looked at the bores as yet. Rather moot, I'm just curious.

About messuring...
I'm in the process rebuilding my 3.6 engine.

Accurate instruments for absolute measurements are expensive. Measuring a difference like out of round with a bore gauge is easier and cheaper.
So I thought like this:
* Main bearings looked like new and no visual damage to crank => probably a 99% chance of healthy crank main journals.
* Same as above with the rod bearings.
* Check rod big end, if round = OK.
* Pistons, ring lands measured OK and no obvious damage => Pistons OK.
* Cylinders are nicasil and that don't wear, if not flaking or out of round => cylinders OK.

Kind of a shady approach maybe, but it's my engine and I can do it all over again in a year or two if needed.
The initial state of the engine has to be taken into account of course.