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How to check play?

This is a discussion on How to check play? within the Drivetrain forums, part of the LSx Technical Help Section category; I now have 500 or so miles on the new 12 bolt so it's time to drop the fluid and ...

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    How to check play?

    I now have 500 or so miles on the new 12 bolt so it's time to drop the fluid and look things over. To check the lash, do I simply set up my dial indicator on a ring gear tooth and rock it back and forth against the pinion gear? I should know how to do this but have never had to check one before. Other than lots of metal in the oil, anything else I should be on the lookout for when I pop the cover?

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    Moderator Cutlass's Avatar
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    yep that's how I check backlash. Put the dial indicator perpendicular to the ring gear tooth and rock it back and forth trying not to move the pinion gear. You can also check it in a couple different spots around the gear to make sure the even all the way around. If its little tighter in a different spot then maybe the ring gear isn't fully seated correctly to the differential.

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    What is my reference range that I should be within if it is properly set up?

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    Moderator Cutlass's Avatar
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    exact spec, I'm not positive, but most gears I've setup is somewhere in the .005" to .010" range. maybe a little more but probablly not much smaller.

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    Thank you! I'll probably wait until our heatwave departs before doing this as my garage gets rather warm in the evenings.

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    Dropped the fluid last night and it looked pretty good. There was a little bit of metal sludge in the bottom, but certainly not enough to worry about. I set up my dial gauge and ended up with just under 0.011". I didn't have any help, so it's possible I moved the pinion a slight amount as I rocked the ring gear up and down.











    I did have an utter fail during the process of draining the rear. I went to unscrew the drain plug in the aluminum cover and it seemed a bit stubborn. I just figured that Moser probably had a thread sealant of some sort on it. Wrong. It ended up pulling a thread and then buggering the rest on the way out. I had never pulled the plug when I did the installation, simply checked to make sure it was snug. Looks like I will be tapping the hole and maybe drilling it out to the next size up tomorrow evening.

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    Moderator Firebirdjones's Avatar
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    Ya, anti seeze or some form of pipe thread sealer is your best friend when you install steel or brass plugs into aluminum.

    Is this supposed to be a brand new rearend?? How many miles??

    If so,,,and you are sure the pinion didn't move on you while checking,, I'd say .011" is a little on the excessive side,,,especially if you have a stick car.

    On new gears (depending on manufacture) they recommend somewhere around .006" to about .010".
    I generally like to set them up as close to .006" as I can because after some miles things settle in and wear,,,and this dimension will increase .002" to .003". I've actually had them wear in a little more and had to go back and reshim for proper backlash.

    If you are at .011" now, you might find that rearend whining a bit,,,and would get progressively worse as the miles wear on.

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    Moderator Firebirdjones's Avatar
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    Never mind, I see where you posted 500 miles.

    I personally would like to see it a little tighter than that on my builds. I'll take a wild stab at it and guess this is a moser build??

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    Quote Originally Posted by Firebirdjones View Post
    Never mind, I see where you posted 500 miles.

    I personally would like to see it a little tighter than that on my builds. I'll take a wild stab at it and guess this is a moser build??

    Nail... on the head. I still have the dial gauge set up and will try it again when I can get someone to hold the pinion from moving. While I have it open, should I check the torque on the cap bolts, or leave them alone?

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    Moderator Firebirdjones's Avatar
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    Wouldn't hurt to check the torque on the cap bolts.

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    Quote Originally Posted by Firebirdjones View Post
    Wouldn't hurt to check the torque on the cap bolts.

    Any idea what they should be?

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    I am going to guess 60 ft-lbs...if I am thinking of the right thing. WAIT for someone else to concur of course

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    i agree with cutlass and fbj. everytime i have built a gm rearend which i cant even count, new i always set them around .007'' and they always are quiet unless they are bad gears. but also, not saying you are setup wrong but i have always set the base/magnet on the bottom of the housing and aimed the dial up to measure backlash. seems to give good results that way. make sure to go back with some good amsoil in there and also i am totally jealous that you have a nice new 3rd member

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    Quote Originally Posted by 35th-ANV-SS View Post
    I am going to guess 60 ft-lbs...if I am thinking of the right thing. WAIT for someone else to concur of course

    Has to be somewhere around there -- I wouldn't think anything less than 60 ft. lbs. by the size of the bolts.


    Quote Originally Posted by blue02Z View Post
    i agree with cutlass and fbj. everytime i have built a gm rearend which i cant even count, new i always set them around .007'' and they always are quiet unless they are bad gears. but also, not saying you are setup wrong but i have always set the base/magnet on the bottom of the housing and aimed the dial up to measure backlash. seems to give good results that way. make sure to go back with some good amsoil in there and also i am totally jealous that you have a nice new 3rd member

    I'll try it from the bottom. As much as the arms on the magnetic base articulate, it was still a PITA trying to get lined up on a tooth. Interestingly, the Moser instructions advise against a synthetic gear lube. Not sure why.

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    might be due to the differential you have. some specify mineral oil

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    Took 5 tries, but I finally located a tap this afternoon. I ended up running a 7/16" drill down the hole and then tapping it on the car as I was not able to hold onto the cover. I had to get the tap deeper than the stock threads for everything to clean up. This worked well as my replacement plug was longer than the Moser one, so it flushed out when tight.

    I also hit the cap bolts at 50 and then 60 ft. lbs. and all of them immediately "clicked". I'll fill it tomorrow and be ready to roll. As suggested, I did try the dial indicator from directly beneath the ring gear. Still came in at 0.010" so that has to be my lash. Maybe I'll shoot Moser an e-mail and see what they spec it at.

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    Quote Originally Posted by pajeff02 View Post
    Took 5 tries, but I finally located a tap this afternoon. I ended up running a 7/16" drill down the hole and then tapping it on the car as I was not able to hold onto the cover. I had to get the tap deeper than the stock threads for everything to clean up. This worked well as my replacement plug was longer than the Moser one, so it flushed out when tight.

    I also hit the cap bolts at 50 and then 60 ft. lbs. and all of them immediately "clicked". I'll fill it tomorrow and be ready to roll. As suggested, I did try the dial indicator from directly beneath the ring gear. Still came in at 0.010" so that has to be my lash. Maybe I'll shoot Moser an e-mail and see what they spec it at.
    If ya ever have to do it again.. Little trick, get yourself a small block of wood that spans the cover from one side to the other. Grab a couple washers and secure the cover to the block of wood with screws and the washers. Lock the block in your vise and tap away... Sorry i'm a little late spotting your thread...
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    In my ASE gm book it says backlash is generally .005-.009. A little loose for my taste but the book doesn't have your rear in it. Side bearing caps are 55lb' for aam 7.5 and 8.5". Why did you mess with the side bearing caps?
    If you want to check the lateral runout of the gear it shouldn't be more than .003.
    Did you paint or spray the gear to check the pattern? I see I posted a bit late.

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    Thanks for the info guys. Smittro -- yeah, I've done that trick quite a few times. Last time was when I polished the rocker covers. I screwed them to a board so they wouldn't go flying across the garage. c5z28 -- I didn't build the rear so I just wanted to ensure that they hadn't backed off after the 500 mile break in. The gears came with the pattern dye intact so I could see the mesh. Everything looked good.

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    Just pulled my differential because of the excessive whining during acceleration. The bearing directly behind the seal wasn't even touching the race. Is this normal? Other than that, the bearings look to be great. However, there was an 1/8 inch play pinion/ring. Think I may have crushed the collar by driving too hard. Is this possible?

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