I just finished a -SE rear end swap on my 85 GSL and I now have a humming noise coming from the rear end. I had the first differential worked on at a local shop and when I installed it, it still continued to have the same hum noise. I then decided to purchase another complete 3rd member that I believed was set up correctly and I directly installed it without checking anything. It too has the same hum as the previous differential, just not as high pitched. I was wondering if there's any good tutorials on setting the backlash myself as this is one thing I've yet to do as a mechanic. I've replaced both axle bearings, seals, and collars. I also have a brand new driveshaft from Mazdatrix, so I'm about positive its something inside the differential. Any advice, or places I can possibly mail the 3rd member to get serviced would be appreciated. Most of the local shops freak out because it's an RX7 and not a standard Mustang or Camaro

 

http://miata.net/garage/diff_rr.html


Read this, there is one specific to the fb but I couldn't find it, it was in cunjunction with an LSD rebuild iirc.


I did this on my 84 gsl in Jan this year, I've got at least 2,200 miles probably more on it now. Its pretty easy work, except the backlash is phenomenally tight as compared to your ford/dodge/gm "'merican pick-em-up-truck" so a shop will most likely make it too wide. That's what was wrong with mine, the po had a shop replace a leaking pinion seal. And it was howling with three distinctly different noises one under accel, one under decel, and one at light throttle/cruise.

Might as well replace the bearings, I got timken cheaper than anything else by searching and cross referencing, from auto zone and amazon, iirc auto zone doesn't list all the parts in the catalogue, but (again iirc) it shared parts with the GLC.


Good luck, its perfectly doable diy despite what some people will tell you. Just gotta be really really patient and ****. You'll need some tools, like gear marking compound, a runout gage, small "deflection type" torque wrench (if you mess with the pinion, the pinion can cause noise too!), I used a big dial caliper to measure the peload on the carrier.


Mazda made it simple to set backlash with adjusting nuts. Pretty easy compared to the shim method.



Edit: FYI an ex auto mechanic told me while I was doing mine "no shop mechanic ever measures backlash, he sets it by feel and goes, that's the price of piece rate". I'm sure some do, but my opinion of most local shops are shady.

Also, there's a drive line shop in I think forest park ga that specializes in imports, I've never used them but I have researched them, they seemed OK.

 

Study, read these, especially the rx7 one till your eyes bleed. Then Google search "how to set backlash", "how to set pinion preload" and "how to read gear marking compound"


https://www.rx7club.com/1st-gen-arch...ur-lsd-711948/

http://www.pirate4x4.com/tech/billavista/Gear_Setup/

https://www.ringpinion.com/Content/B...h-Patterns.pdf


http://www.4wheelparts.com/aux_incl/...l.pdf&line=G/2


Then study some more.

 

Awesome information, I greatly appreciate it.

Think I'm going to attempt to mess with my spare 3rd member and see if I can't get the feel for it. The teeth on this spare are completely shot so I can't break it. Hopefully I can fix this, I'm afraid to drive over 60

 

One more thing I forgot to mention, I did everything myself except press the old bearing off the pinion and press then new one on, 15$ at a local classic car/race car shop/machine shop. If I did it all over again I'd have them do the carrier bearings also,(actually I'd buy my own press) as I had trouble removing those as well.

 

I don't think i need to replace any of the bearings. This 3rd member is in amazing condition and the bearings are all newer. I know that my first one had issues because I was in a hurry and forgot about backlash when I removed my pinion flange to put on a 12a flange to get back on the road. The second differential I thought already had the preload/backlash properly set and I never thought to check before I installed it. Hopefully I can tinker with it and get somewhere today.

 

I would recommend new bearings if you are fighting noise. Then again, if you wanna pull, reset backlash, and reinstall, the only cost is a little gear oil. I think I spent less than $100 for all bearings and seals including crush sleeve, nut, wheel seal, and wheel bearings.

 

a few summers ago I redid my SE with new axle bearings, diff with new crush washers, and changed the disk stackup to like a GSL not a GSL-SE...When it came time for final backlash adj, I went to a real OLD schooler I was learning from. He refused to use the gear marking compound I provided, and said he used WD-40 (said the thickness of the marking stuff changed readings, and didn't want to clean it up afterwards...) and he also basically refused to go 1000ths closer to spec...I works well, but IS noisy...I've a sound like a HUGE rotating metal mass barely scraping metal upon acceleration/cruising...If you match speed/rpm to no accel/decel, the sound goes away, and it's also a little different upon Decel...I've suspected it's because the backlash is about 1000ths above spec, but haven't done anything with it yet...

 

Thats sounds a lot like my issue. I dont want to damage anything, but if its just tight but safe to drive i guess ill turn the stereo up...