changing bearings, final drive and shimming of the MTX-75 a how-to article
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Thread: changing bearings, final drive and shimming of the MTX-75 a how-to article

  1. #1
    NECO Member rbls4ever's Avatar
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    changing bearings, final drive and shimming of the MTX-75 a how-to article

    So since the passing of our beloved transmission builder TH, I have been left to my own devices for building up my transmission.

    Part numbers:
    Old style output shaft with 3.82 final drive $121 F8RZ-7061-SA
    Ring gear to match 3.82 $171 YS4Z-7F343-FA
    Input shaft front bearing CV6Z-7025-A
    Input shaft rear bearing CV6Z-7025-B
    Output shaft rear bearing F8RZ-7F431-AA (discontinued)
    Output shaft front bearing (pain in my ass from FAG in france) 578184B
    Differential bearings F7RZ-4221-AC x2
    Input shaft shim kit $107 F5RZ-7L172-AA
    Input shaft measuring shim F5RZ-7L172-DA
    Didn't buy this because of cost output shim kit F5RZ-7N135-AA $350
    Differential measuring shim 91ZZ-4067-XA
    The differential shims are listed separately by size and have the same part number as above but a different ending depending on the size. check www.fordpartsgiant.com under focus 2004 or above for which shim you will need.

    To get the bearings or the final drive apart you need a shop press of at-least 15 ton capacity and a bearing splitter like this one

    I bought a "rebuilt" transmission from a member here who had the trans rebuilt by a reputable shop in the area. All I needed to do was open the transmission and replace the bearings on the Torsen diff I picked up and drop it in... or so I thought. All seemed well inside, all new synchros, later style shift tower with roll pins, roller forks, 3 piece synchros. That was until I inspected the bearing surfaces..


    As you can see the bearings are scored to hell. For the cause of the issue I identified two main problems. First of all the trans magnet was left out upon reassembly which allowed metallic bits to destroy the bearing surfaces. Now this probably would have caused no more issues than some noise and possible premature bearing failure except for what I found next...
    I then assembled the cases with input, output and differential installed separately and turning torque was examined. The input shaft had axial play when installed into the transmission and made noise. The output shaft and differential could barely be turned with quite some force.... Obviously this wasn't right and couldn't be put back together like that.

    So I figured if I'm going through the hassle of doing all the bearings I might as well go a head and change the final drive while I am at it. I have the "old style" output shaft and as such the information on the forums told me I could not change final drives. As I learned this is simply not true.
    The old style shaft is still available at Ford for a small sum of 121. The part number is F8RZ-7061-SA. You will also need the ring gear to accompany it which is a cool 171... part number YS4Z-7F343-FA. So about 300 to change the gear ratio in ford parts plus bearings.... Now here is where things got real screwed.
    The input shaft bearings and differential bearings were straight forward enough to locate albeit a bit expensive. The part numbers are as follows input front (updated part numbers) CV6Z-7025-A rear CV6Z-7025-B. Differential bearings F7RZ-4221-AC x2. The rear input shaft bearing is discontinued but I was able to find one F8RZ-7F431-AA.
    The front output shaft bearing gave me all the grief.

    I got confused by part numbers and thought that the front and rear were the same but part numbers slightly off. Not the case. The rear of the shaft and front have different sizes where the bearing press fits. So I ordered another bearing which has the same size but is not grooved for a snap ring. Why does this matter you ask? let me show you.


    See that small snap ring laying beside the output shaft? That "holds" the pinion gear on the output shaft and must have a home on the inside of the bearing or "grooved for a snap ring". Really the pinion is pressed on and the snap ring is probably not needed. I toyed with leaving the snap ring out and using the bearing I had that would fit on the shaft but the female cup side of the bearing was too fat and isn't the same as the original. Not sure if the case was later redesigned to take this bearing or it was simply incorrect.

    The original re builders had used some weird metal caged equivalent they had found but not only was that bearing destroyed when the final drive was taken off it would have likely made noise as all the other bearings are plastic caged.
    The solution believe it or not came from France. You need this FAG bearing which was the original Ford supplier. FAG bearing number 578184B. A quick google search will bring up 123 bearing.com which is in France where I bought the bearing. The turn around time? 1 week. Yeah. I was surprised too.

    Okay now onto the how-to.

    I am not going through the process of exactly how to use the press or how to press the bearings but basically you need to get this top bearing off by use of the bearing splitter and the press.


    Also the snap ring under the bearing will need to be removed and then the same process on the final drive pinion which is actually press fit tighter than the bearings, it took a few attempts and took some gouges out of my tool to finally get the two to separate.



    Here you can see a few things. The snap ring is sitting down there next to the input shaft, the Torsen with both bearings and both final drive pinions. Also the end of the input shaft can be seen.
    Now to put it all back together


    Make sure everything is pressed very tight back together and only press on the inner cone of the bearings by whatever means you must. I used a combination of sockets and the old bearing cups to achieve this.
    To remove the old female or outer bearing race from the trans case you must heat up the case around the bearing surface making sure not to heat the bearing directly or you will discolor it and destroy it if it is a new bearing. The best method I have found is with a map gas torch, patience and a rubber hammer. when the case is nice and hot you can turn the case over and smack it hard on a flat piece of wood and the bearing should come out along with the shim if it's on the bell housing side.


    Tapping the back side with a rubber hammer works as well as tapping the front side with a screw driver and hammer against the race. The latter isn't ideal as you can mess the shims and the bearing race up if not careful.

    Now onto shims.... IMPORTANT NOTE. Shims are a vital part of the transmission and should not be taken lightly. They ALWAYS need to be replaced or at least measured when new bearings are installed. Dont simply put the old shims in there, the preload will be off. As noted above improper shimming destroyed the old bearings. All roller taper bearings need to be loaded against themselves with some force to work properly. This is known as preloading a bearing. Axial clearance of the bearing is important in how much force it takes to turn the shafts, (drivetrain losses) how long the bearing will last, noise and play.

    The proper procedure as specified by Ford is to install a measuring shim behind the race and install each component separately and rotate to seat the bearings then set a dial indicator against one edge of component, lift with a prybar and measure the resulting gap or end play. Like so.

    Input shaft end float measurement

    Differential measuring


    You take this end float figure and add a certain amount of extra shimming to get the pre-loading. There is a chart in the FSM I followed as a guide to shim selection. I say guide because I chose to not follow the UNLOAD figure for the input shaft bearing of -.05mm. I used a preload of roughly .04mm for the input bearing. and .33mm for the differential bearings. The shims by the way are individually listed from Ford for the differential by size, and a $100 shim kit F5RZ-7L172-AA for the input shaft can be procured as well as the measuring shim F5RZ-7L172-DA.

    The issue I had again was with the input shaft shims, and the exorbitant price Ford has set for them. F5RZ-7N135-AA is $350. I just cant, after spending all the other money for the bearings and final drive change, justify that kind money on basically ONE shim I need out of the lot of 32.

    So here is the solution I came up with. And brings me to the point I am currently with the transmission. I am going to assemble the bearing into the case with no shim and measure the axial play. After determining the size shim I need I will carefully "mill" the current shim have until it is at the proper size, assemble and done. Hopefully all this will be worth it with a transmission that shifts beautifully and has no noise and lasts a long time at high horsepower. Only time will tell
    Last edited by rbls4ever; 2017-01-10 at 11:43 AM.
    3 point OHHHHH in the works......
    My Baby
    the other

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  3. #2
    3 wheelin Lifetime Platinum MemberSuper Moderator jaged's Avatar
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    Re: changing bearings, final drive and shimming of the MTX-75 a how-to article

    very nice write up. I would go back through it and space some things out for easier reading or compile the part numbers in one place.

    depending on the change in your shim i would use a surface grinder and not a mill. much more accurate. In all the times ive changed out diffs ive never changed shims. everything always measured out to the .001 when reassembled compared to the old unit
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  4. #3
    NECO Member rbls4ever's Avatar
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    Re: changing bearings, final drive and shimming of the MTX-75 a how-to article

    Thanks! It was months of trial and error on this thing so I figured I would save others some grief on this one if they felt ambitious. For the shim milling I was planning to simply glue some 400 grit sand paper to 2 pieces of glass and go to town on the shim. The problem I had was the high level of turning torque required even when I got the transmission. The shimming wasn't right then, and would have resulted in destroyed bearings eventually, low power output to the wheels and low gas mileage if I just put it all back together. I'm sure the Torsen is almost identical to the original because they both required about the same force to turn when installed in the transmission. So if the shims were right from the factory (how likely do you really think that is??) then I suppose a Torsen could be thrown in with no change to shimming. But bearings are all slightly different sizes due to manufacturing variances so I would still want to check the preload, I mean what is a $7 shim and a dial gauge and some time when it comes to the second most vital part of your car?

    In addition to the shimming specs there is a specification for torque required to turn the input shaft when the transmission is all assembled and put into fourth gear. that spec of 1 nm to turn is important if building a higher horsepower car, or care about drivetrain losses. The average person probably wouldn't notice, but I am going for butter smooth and low friction.
    3 point OHHHHH in the works......
    My Baby
    the other

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  6. #4
    NECO Member rbls4ever's Avatar
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    Re: changing bearings, final drive and shimming of the MTX-75 a how-to article

    So I am happy to announce that the transmission is all completed! The shaving of the shim worked perfectly. i used 2 pieces of glass some 100 grit sand paper some spray glue and tape. basically tape one piece of glass to a surface and sand with the other, check every so often for any variances and the thickness and voila, a new shim is born.

    BTW I can spin the input shaft with minimal force and all falls within specs. Here are some pictures

    Had to get creative to get this one in there and mounted properly as you can see.


    And the sanding process.


    I hope all this helps someone in the future... good luck
    3 point OHHHHH in the works......
    My Baby
    the other

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