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Setting Diff Backlash and Preload

Posted by FTodaro , 12-23-2015 · 14,447 views

I bought the tools years ago and never jumped into learning how to Lash a diff and set the preload on the carrier bearings. This is not a full blown rebuild. I did not change the Pinion bearings. I simply swapped out an open diff and put a Torsen unit in it. So I reused the all the bearings and the original ring and pinion.
I will endeavor to do the entire diff but that will be another day. Need some additional tools for that.

The instructions on how to do this have been posted on the internet for a long time. I would read this first before you read my post as i assume you have read it.

These are pretty good instructions, and I bought the tools that are listed in this DIY. I am not going re post the instructions on the link, but rather talk about the questions that I had before I started and the outcome. Someone with some experience is welcome to correct me.

You can also find some helpful instructions if you Google “lash and set preload for RX-7 diff" as it’s the same type of diff.

The first recommended step is to take the measurements of the Lash and the preload measurements before you get started and be sure to record them. I recorded the Lash on 4 positions of the ring gear as recommended. I also recorded the measurements used to measure the preload on the bearing on bearing towers.

The next step was to mark the bearing caps so they do not get mixed up,. I like to use a center punch one dot for the left two for the right. I also marked the bearing races so they went back to the right spot.

To do that you have to get familiar with a dial indicator- Google it and you will find some instructive videos.
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I disassembled the diff cleaned it and separated the Ring Gear from the open diff. Put my Torsen unit on the ring gear, lock tight the bolts and was ready to drop it back into the diff.
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When i set the ring gear into the diff, one of my questions was when do I tq the bearing caps. I put the adjusters in and loosely fit the bearing caps on and turned adjusters a few times to make sure they are in the groves and then i Tq the bearing caps to 20 ft lbs. this would allow me to make easy adjustments.

I then set the lash close by feel. I made sure that the adjuster has some load on them. I then set up the dial indicator and started making my adjustments.
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When I got the Lash close to where i wanted it. I tq bearing caps to spec I went to 35 ft-lbs. I set the lash where i wanted it, (it was much looser than the street spec), and checked it at 4 different locations.
The next step was to set the pre load in the carrier bearings. What I could not understand conceptually was how was moving those adjuster going to expand or contract the bearing caps at the measuring points, when those caps are tq, to the 35 ft lbs. but that is how it works, they flex and that measurement is how Mazda sets the preload.
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I kept making adjustments until i achieved the correct measurements, that are found in the instructions and in our manual.

Once I had the correct lash and preload I checked all again. When all was good I installed the locking plates. This can be a little tricky as sometimes the adjusters are out of position so you have to play with them with out changing your settings. you can also deform the locking tabs a little to get them installed.

A final product.
Ready to race.
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