How are out of round and taper of journals measured?

[NoGood]

Can someone explain?

Thanks?

[PatBuckley]

With a micrometer.

[NoGood]

Yeah, but I need a more thorough explaination, how do you meaure it with a micrometer?

Sorry, I'm very new to all of this.

[Big-Foot]

Okay - here's the skinny on measuring crank pins.

First off the journals you are concerned about are the Rod Journals.

Draw an imaginary line through the centerline of the crankshaft and the centerline of the crank pin. Orient that line so it points straight up (12 O'Clock).
With your micrometer, you will be measuring the crank pin from the 6/12 position and the 3/9 position (as though you were looking at a clock). You will take 2 measurements (minimum) starting from the front of the crank pin's bearing surface to the back of the bearing surface for each rod bearing. Mind you there are two bearings per journal (pin) in V6 / V8 engines.
You are looking for out of round, which comes from Detonation mostly - and taper, which comes from a bent connecting rod or bad piston.

Tolerances will vary from one manufacturer to another. I typically will call it quits on a journal if it is more than .00025" out of round or taper. If it's tapered more than that, make darned sure you have the rod's checked out or replaced. You cannot straighten a bent rod.

Contrary to the beliefs of many, you can turn a crankshaft as much as .060" without causing weakness in the crank that would ever be of concern in a street engine. You are, however, running a lot of material in the bearing and that could cause some degree of excess wear in a higher compression engine. I typically stop turning a crank when it hits .040" just because of the thickness of the bearings.

Hope this helps!

[stengun]

Howdy,

Big foot hit the nail on the head. If I was you, I would take it to a good auto machine shop, and see what they think.

Randy, we miss you over at FFCobra!

Paul

[Excaliber]

Measuring with a micrometer is as much "art" as it is "science". It's easy to say, "Just mic it for size", but the fact is it IS difficult without some experience to measure the SAME PLACE twice and get the SAME READING!

A lot of it has to do with how much pressure one might be applying to the screw end of the mic. Or perhaps your holding the mic slightly off center or a bad angle. Being able to dupilcate your results is the key, measure it over and over until your comfortable with getting the reading "right" everytime.

It is actually pretty easy for a "new guy" to mis-measure a thousands of an inch off, that would be .001 Bigfoot mentioned a measurment as small as a .00025, now THATS REALLY TOUGH for a new guy to measure! The way you hold your left foot while you measure it could throw you off!

Practice for the "feel", THATS the art of it all.

I recently spent an "evening" with my crank shaft, literally in my living room watching TV and "measuring" and writing down the various readings. When I was done with the mic, I then moved on to Plasti Gauge (the poors man's micrometer). I measured the big end of the rod, the thickness of the bearing itself, the crank pin, I did the "math" and calculated the "clearance". So the plasti Gauge should pretty much match the "numbers". If it doesn't your math is off or SOMETHING is wrong.

Consider this: Even the BEST race teams and FACTORY race teams use Plasti Gauge to double check the math just before final assembly. If THEY do it, you can bet your ass I will do it!

[Big-Foot]

Hey there Paul - Good to hear from you! Say howdy to the folks back at ffcobra for me!

Excaliber is right - measuring correctly is almost an art form and requires exceptional attention to detail and feel... I've been doing it for so many years now, it is almost second nature, but I still have to focus my complete attention so as to not overlook something. That's how you gain repeat business and racing championships!
In regard to Plastiguage:
I did a bunch of testing with different crankshaft main bearing journals and rod journals a number of years ago during a slump - just to see how accurate Plastiguage really was. I found that the AGE of the plastiguage was more important than anything. Fresh plastiguage would give you a measurement that was accurate to the .00025" when fresh. When it was old and brittle, it would always read high on rods (by as much as .0005") and would read about the same on mains. As strange as it seems, rod caps would seem to flex a little with the old brittle stuff.

The alternative to plastiguage is using micrometers and snap-guages. Understand that when you first seat a bearing, you are "crushing" the bearing into the saddle - the first reading you take with a bearing that is freshly crushed will give you the best reading. Once you un-torque and re-torque the bearing cap, you will get a different reading (off only by 10ths of a thou, but different).. Positioning snap guages and then pulling them out of the bearing without moving the pin can be difficult for some folks - then you need to read the pin length with an outside (Caliper) Micrometer without compressing or moving the pin. Honestly - it is a PITB even for those of us that have good equipment and know how to use it. I use both methods of measuring, depending upon the build and the type of rods and fasteners being used. Many other shops in my area do the same thing.

In my opinion, there is nothing wrong with Pastiguage as long as it is fresh. Make sure you wash off the residue of the smushed plastiguage with lacquer thinner and use a liberal dose of assembly lube before you re-assemble the bearing for good.