Author Topic: Piston interchangability  (Read 133 times)

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Offline KilliansRedLeo

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Piston interchangability
« on: January 16, 2014, 11:17:56 am »
I have a saw that uses a 47.95mm diameter piston (factory spec) just measured an after market replacement and it measured 47.96mm. Should I use it? Difference in diameter is .01mm or 0.0003937. That would make a reduction in piston to cylinder clearance of 0.00019685 each side. What to you all think?
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

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Offline adam32

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Re: Piston interchangability
« Reply #1 on: January 16, 2014, 11:32:03 am »
Use it.

Offline 1manband

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Re: Piston interchangability
« Reply #2 on: January 16, 2014, 07:26:46 pm »
what are you using for the measuring.......mic or caliper?

-joe

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Offline KilliansRedLeo

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Re: Piston interchangability
« Reply #3 on: January 17, 2014, 03:09:05 pm »
Mitutoyo 500-196-20 Digimatic caliper.
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

Thomas Jefferson
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Offline 1manband

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Re: Piston interchangability
« Reply #4 on: January 17, 2014, 07:32:17 pm »
you may be trying measuring to tighter accuracy than the caliper can actually do.  most calipers, mics, whatever....... you end up using, show numbers outside their actual capability.

example:
in order to measure to the 0.001 inch, the instrument rounds to the last digit to the right of the decimal point, ("1").  so, if it actually reads 0.0005 it may show as either 0.001 or 0.0005, depending if it's a dial or digital.

as a rule of thumb, gauges are 100% to accurate only one number preceding the last digit they display.  digital gauges which are capable of mathematically calculating the measured result from metric to english, show digits beyond thier actual accuracy.

example:
0.01mm = 0.0003937 inch   ...........  a trusted reading of the caliper, given that displayed reading, would be 0.000 inch.  the "3937" digits are outside the capabilities of the instrument, even if they were shown on the gauge display. its common that a  micrometer is used to measure if you need measurements to the 0.001, because they are accurate out to the 0.0001.

not a fan of digital gauges, but they do have an advantage of being able to display numbers that are easy to read without a magnifying glass, can convert between metric and english.  suggest to measure with the units you need to use, and take multiple measurements.  when things are this small, even the temperature of the measuring device can alter the result.

hope it helps.
-joe

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Offline KilliansRedLeo

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Re: Piston interchangability
« Reply #5 on: January 17, 2014, 09:09:16 pm »
According to Mitutoyo the caliper is accurate to 0.0005. I do understand your comments Joe. So will check again with a mic.
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

Thomas Jefferson
1743-1826

Offline aclarke

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Re: Piston interchangability
« Reply #6 on: January 17, 2014, 11:45:43 pm »
He's saying a micrometer is more accurate than your digital caliper. There is a. 0005 variation on the Mitutoyo so it could read .0005 and the true size could be  .001.   Don't think you are gonna have fitment issues with your piston, splitting hairs at this point!! Lol

Offline KilliansRedLeo

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Re: Piston interchangability
« Reply #7 on: January 18, 2014, 05:23:54 am »
Thank you all, pulled several pistons off the shelf, mic'd all: from the same and different sources some OEM some after market. It seems that they vary in diameter any where up to 0.01mm. So it appears that a manufacturing tolerance of 0.01mm between pistons from the same or different manufacturers is about the norm.

Let's face it, a 50-100$ cast chainsaw piston is not a Formula 1 engine component from the likes of Ferrari or McLaren.
"When the people fear the government...you have tyranny....When the government fears the people....you have liberty"

Thomas Jefferson
1743-1826

Offline 1manband

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Re: Piston interchangability
« Reply #8 on: January 18, 2014, 07:31:01 am »
He's saying a micrometer is more accurate than your digital caliper. There is a. 0005 variation on the Mitutoyo so it could read .0005 and the true size could be  .001.   Don't think you are gonna have fitment issues with your piston, splitting hairs at this point!! Lol

+1

measuring tools wear or loosen up over time.  when they are still new, i like to measure a guitar pick, and write the measurement on it with a permanent marker.  keep it in a drawer.  this way, you can always go back and measure it again to see if the tool is still reading the same.  a better idea is to use common "feeler guages" like those used to set points for example.

this is good peace of mind if the caliper was dropped on the floor, or starts to show wear marks on the slide or tips.  some low buck calipers are surprisingly accurate.

you can buy "checking standards", but a set of those costs more than an expensive caliper.  some manufactures may offer a service to recalibrate the tool.

0.01 mm is too small of a difference to be concerned about.  at about 0.05 mm,  get questionable with a caliper.

regards
-joe
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