Rod bushings: bore bauge vs. AG-300 vs. pin gauges

[PLohr]

Hello, folks. I had some connecting bushings sized for new piston pins (floating pins). There is a difference in the clearance measurements between my bore gauge and the Sunnen AG-300 that ranges from 0.0003" to 0.0005". Is it possible that my bore gauge has smaller probes than the AG-300 and perhaps my bore gauge's probes are measuring into the valleys of the surface finish?

My bore gauge is a Mitutoyo 0.7"-1.4", 0.0001" increment.

Not knowing which measurements were correct, I used pin gauges to check the clearances. I think the result of this is saying that the clearances are between 0.0001" and 0.0004". I think that's OK per the factory service manual (this is a stock rebuild). Is is valid to use pin gauges to check con rod / wrist pin clearances?

Thank you for any help.

[BillK]

How are you setting the AG300 ? Are you using the fixture or a micrometer ? Are you setting your bore gauge the same way ? If you are using the fixture on the Sunnen make sure it isn't worn where it hits the fingers. There is also some "feel" involved with the setting fixture. You can tighten it too much or not enough.

There really should not be any valleys in the finish on a pin bushing.

[Dave Koehler]

In addition to what Bill said.
1: How worn are the fingers on the AG? The original fingers were mild steel.
The later ones had carbide ends which also can wear over time. Just not as fast.
2: When was the last time the AG fingers were adjusted/calibrated?
When the planets align you should be either equal or within a tenth between the 2 measurement types.
At least that is the way it is between my AG and my Sunnen dial bore gauge.

[cgarb]

If the bore is not round or a straight cut you will get different clearance with a pin gage vs a 2 point measurement. We find this at my work with centerless ground shaft. If we mic the shaft it measures good, but it will not fit into the properly sized bore, that's because it has a barber poll finish to the shaft. it was ground with the wheel not perpendicular to the shaft and it made a spiral, when you mic in 2 places you are on a high point and a low point so you are getting a smaller measurement that across the 2 peaks.

[stock z/28]

Im not sure about the radius of the AG300 as compared to your Mititoyo, but I would think that the snap gauge would have a much larger contact radius than either of the other gauges? If thats true I would think that would all but eliminate the surface finish as a variable.

Is it your AG300, or the shop that bushed the rods. If its in a different shop there could be a lot of variables as to the discrepancies in readings, setting methods, temperature, etc..

My AG 300s have an attachment to allow you center the fingers in relation to a bore. If its yours you might check that?

As mentioned look at the condition of the contacts etc..

Personally I prefer a good dial bore gauge over the snap gauge or inside mic. any day, If its set accurately. My first choice would be the AG300 for rod work.

I have scrounged several over the years and kept one set up for big ends and one for pin ends, and they seem very accurate and durable.

[modok]

You may have to rotate the ball points in the ag300. Look and see if they have flat spots worn on them. I assume you know how to adjust it otherwise to center correct.

You can measure a hole in a lot of different ways.
The sharpness of the measuring points, and the amount of force used, will give you a few tenths difference even when everything is right.

When I see TIGHT clearance specifications for a pin, down in the .0002-.0004 range for an average size pin, what that effectively means is
"just big enough the a lubricated pin will move smoothly"

And interestingly how much gauge clearance that takes .... can vary significantly depending on how accurately ground the pin is, and the surface finish you can get in the bushing. It's really weird to have two pins that "measure" the same, and one slides right through and the other won't! But it happens!

[PLohr]

Guys, thank you for your help. This has been educational.

To answer the question about the AG-300, it belongs to the shop.

[Dan Timberlake]

One thing my boss used to have us do was "center" the Sunnen just a whisker low. Then, no matter what bore size was being measured, it the part was slid upward, the measured diameter would flash larger by 0.0001" or so. That confirmed the centering (true diameter) was decent for the finished bore size, and not just the setting ring size.

I'd expect good pin gages to possibly help detect bores that were being finished like the lower left hand image here , which no 3 point mechanical bore gage could detect -
http://www.cobanengineering.com/Geometr ... rRule1.png

[Dave Koehler]

Careful Dan,
You might reignite the banana guide thread.

[modok]

A piston pin is the same as a pin gauge

[TMeyer]

Just go for an air gauge...