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Old 09-01-2006, 12:18 PM   #20
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Joined: Dec 2003
Location: Puget Sound
Oddometer: 10,718
Originally Posted by todd83-900t I've done it:)

Here's my reasoning. The total error in the valve lash measurement = error in your technique + instrumentation error.
Lets look at each contribution separately.

Instrumentation error:
My Harbor Freight test indicator has a graduation of .0005" so I'll say that it'll be accurate to within +- 0.0005 ish? for an absolute measurement. I'd be using it to take a difference so the bias in the indicator should cancel out. I'm not sure about the hysteresis effects, with this cheap-o gauge.

Feeler gauge: I'm not sure what the tolerance of the motion-pro gauge is maybe +- 0.0002" ish? (probably more accurate)

The feeler blade is more accurate and if this was the only source of error I'd be happy with either instrument, but it isn't.

Technique error

My experience (1 valve adjustment) with a feeler gauge has been that the measurement is not repeatable for the reasons listed below. I'm really big on repeatability.
  • -When I used 2 different feeler gauges, (home made & motion pro) the "feel" were significantly different, which is disconcerting. In theory they should have "felt" exactly the same. The main difference between the two gauges was the way the blade was mounted on the holder.

    -If there is any deformation/flexing in the blade as you move it back and forth you'll get a different "feel."

    -Any oil on the blade gives you a different "feel." The "feel" is really the amount of force necessary to overcome the static/dynamic friction. Yes, I wiped the blade clean before each measurement, but there was still oil residue on the shoe & rocker arm.

    -One of the two surfaces is on a gimble and positioning the gauge so you're measuring the minimum distance between shoe and the rocker arm is challenging. Challenging only because of the poor access.

With a test indicator you'd need to be careful about positioning the pointy thing on top of the adjustment nut/screw but if you're consistent on every valve I think you'll get a better measurement. It should also be very repeatable. As a sanity check you could, and probably should, go back and use a feeler gauge. Again I've never used this technique but it appears that there would be fewer variables that you have to control for. Also access would be better.

Nothing will be 100% accurate but I spent 2 nights after work trying to get the right "feel" and I'm just looking for a better way.

P.S. If you'd like to continue this discussion we should probably start a new thread or converse via email.
Interesting... so a combination of lack of repeatability on your part, based on your inconsistency of feel, and a desire to remove those inconsistent perceptions from the task warrants the use of a dial indicator.

When the touch of the hand is suspect... trust the repeatability of the machine instead... yes?

Seems reasonable... and going back and forth between two forms of feeler gauge could be frustrating in this application. What to trust... yes?

I would put it to you that extreme accuracy, in this case is not required. A "feel" of 0.0005" +/- is more than sufficient for the precision needed. I wouldn't worry about the oil... but rather use it to your advantage.

I would suggest using a micrometer (I just know you have one), set to .006", with a film of motor oil on the anvils, as a reference gauge for your feeler gauge feel.
If the valve lash feels similar to the micrometer feel... you have more than enough accuracy for the precision required.

If you feel more comfortable with using a dial indicator, and it's obvious you understand how to achieve good accuracy with one, then by all means use it in good health.
When doing our own work, we have the luxury of doing it at any pace we choose... so the method of measurement should be the one we have faith in, regardless of the time we might need to produce that measurement.

Inquisition over... have a nice day.
So... how's tricks?
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