setting pto backlash help

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by Collie, Oct 12, 2019.

  1. Collie

    Collie Guest

    I have a muncie 489 unit. Mounts uderneath a rtlo trans. Im reading onlne it says to set backlash at (.006-.012). What does that exaclty mean? I swear the place whre the unit came from said to set at .015 but not 100 percent sure.
    What is a safe number? what does .006-.012 mean in idiot terms? .006 is the minimum and .012 is the max?
    Thanks
     
  2. Goodysnap

    Goodysnap Road Train Member

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    Yes, anywhere between 6 and 12 thou. Dial indicator measuring gear lash with the side cover off. You are measuring slop in the gear contact for oil clearance and no binding of bearings. Too tight add thicker gasket and too loose take some out at the PTO mounting.

    I usually mic the old gaskets for a starting point and start there. If it was set up right to begin with you should be spot on.
     
    Last edited: Oct 13, 2019
  3. Collie

    Collie Guest

    thank you.
    Reinstalling a pto on my new trans. and want to set drive gear lash of trans gear
    yes i will mic the old gaskets for a starting point to see where it was at. but this is old truck and i found that after working on it, it spent its life dealing with half assed mechanics so i just want to be spot on. I picked up two .010 gaskets and two .020 gaskets to make the correct adjustments
    Thank you again my friend
     
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  4. Goodysnap

    Goodysnap Road Train Member

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    Yes for sure to measure when changing components as the gaskets required will surely be different.

    Same parts is almost always the same thickness.
     
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  5. spsauerland

    spsauerland Road Train Member

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    489 would be a Chelsea PTO. Dial indicator is a must. With used gearing, shoot for the higher end of spec. 0.010" of gasket changes backlash approximately 0.006". When setting up, I usually only put four fasteners to get in ballpark and start with a thick and thin gasket. Final torque with all fasteners usually crushes gaskets an additional 0.002-0.003".
     
  6. Collie

    Collie Guest

    yeah i couldnt figure out why i couldnt find the part number. I guess m unit is old. Says muncie on it. I bought a new air shift for it ut all the gears and bearings are solit as can be. And thank you for the additional info for setup brotha
     
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  7. BoxCarKidd

    BoxCarKidd Road Train Member

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    I normally take the shift cover off the pto. Put a pry bar through the the cover and lock down the countershaft that drives the pto. Then move the mating pto gear to check clearance - backlash.
    That is where you would use your dial indicator. Must have a little freeplay.
    If to tight it will be screaming. If to loose it will sound like a rattle trap - knocking.
    Follow the other folks directions and you should be fine.
     
  8. pushbroom

    pushbroom Road Train Member

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    A 10 inch extension on a dial indicator makes it alot eaiser
     
  9. Working2party

    Working2party Medium Load Member

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    I’ve mounted lots of PTO’s on ‘01 and newer trucks. I’ve got a feel for it, but it’s pretty simple. A thin gasket adds .006 backlash, a think adds .012. You can do it without a dial indicator by Adding a stack of gaskets and removing thins till it’s out of lash the add a thick (or two thins) to land in the range .006-.012

    I usually start 2 thick & 2 thins, tighten 4 bolts, if the idler gear gets tight while tightening. add a thick and thin then re check.

    If you think it’s right, (has about the same lash on the transmission gear to idle gear as it does on the slide gear to idle gear). remove a thin and check for “zero lash”.

    If it goes zero lash, add a thick and bolt it up. Your done.

    If you still have lash, remove a thin and recheck for zero lash. Then add your thick (or 2 thins) your done.

    But normally since 01 transmission machining has gotten good enough 2 thick, 2 thin gets most of them.

    If you are using a dial indicator and find your self close but still alittle loose, give it about 15 min and retighten. After the gaskets at given time to compact, sometimes, your can get just alittle more That will also help keep them from seeping oil later.
     
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  10. starmac

    starmac Road Train Member

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    It is always better to be a little loose than a little tight too.
     
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