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ecmx274 ecmx274
4/30/2020 9:52 AM

I have a brand new 05 rm125 cylinder. Manual says to measure perpendicular to crank 17mm down. I don’t have a bore gauge so I’m using a telescopic gauge and measuring it with micrometer. I have measured it about 10 times and get 53.96 almost every time. Should I get an a piston? All the manual says is if cylinder measures more than 54mm it needs serviced. I am using a 04 manual so I could be doing it all wrong. Let me know if I should measure another way. Thanks in advance

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Paul_Pitzonka Paul_Pitzonka
4/30/2020 11:10 AM

Measure in several different locations... one location of the bore won’t tell you the true shape of your bore... I take 3 initial measurements top, middle (This measurement can be hard on two strokes because of the transfer ports) and bottom; Then 3 more measurement perpendicular to those... this will not only tell you if your bore has any taper but will also show you it’s concentricity... if it’s a used bore I doubt you need an A piston... probably more along the lines of a B or C... but I’m not the one measuring the bore... the good news is in most cases you can run a D piston in a used A bore and be ok...

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SBD SBD
4/30/2020 11:18 AM

I would not trust a telescoping gauge, although you are getting a consistent measurement its just not accurate enough.
A bore gauge is much more accurate, maybe take it to your local shop or borrow a bore gauge just to be sure.
Your current measurement is .0015 inch under MAX, that's too close to call with a telescoping gauge.
.

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Paul_Pitzonka Paul_Pitzonka
4/30/2020 11:30 AM
SBD wrote:

I would not trust a ...more

I agree with this as well snap gauges are good for getting a rough measurement... not very accurate when getting down into the ten thousandths...

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ecmx274 ecmx274
4/30/2020 11:36 AM
SBD wrote:

I would not trust a ...more

it’s brand new never been bolted on a bike. I’m assuming it will end up being an a piston but figured I should check anyway. I guess I will be purchasing a bore gauge can’t hurt to have for future measuring as well. Thanks for the help

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Paul_Pitzonka Paul_Pitzonka
4/30/2020 11:38 AM

If it’s brand new most cylinders are stamped with the bore size on them

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