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Old 09-01-2006, 11:27 AM   #19
todd83-900t
Gnarly Adventurer
 
Joined: May 2006
Location: SLC, UT
Oddometer: 159
Quote:
I want to understand why you would perceive a dial to be a better choice of measuring device in this application
.....now 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.

Todd
P.S. If you'd like to continue this discussion we should probably start a new thread or converse via email.
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