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Old 01-07-2008, 05:43 PM   #1
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Default RX-7 LSD setup help please

My diff is out of the car for the 3rd time now, and I really want to get it right this time. Last time I added +.2mm oversize thrust washers from mazdatrix because my cheap little caliper read all the discs and plates as the stock 2mm. That setup got me about 10lb/ft of preload, so I took it out again and this time measured with a dial indicator and the plates were all about 1.95-1.98mm thick. Doesn't sound like much but add it up over 8 plates, and it could be up to .4mm worn. The thrust washers should have made up for that, but I guess they didn't.

I don't think my lube is to blame, it doesn't say anything about friction additives or anything on the bottle. It is Mobil 1 synthetic 75w-90.

So what do I do, short of replacing every plate for about $300 or whatever it costs? I need this thing tight, it's more of a toy than a DD now, and it has to stand up to a turbo and slicks on an autox course.
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Old 01-08-2008, 03:54 AM   #2
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You could try shimming one side at a time, or both. A Coke can works great, but probably depends on how much pre-load you are looking for. Sounds like a trial and error method to me.
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Old 01-08-2008, 10:11 AM   #3
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At this point I'd either get a machinist to measure your plates or get a new caliper. Lots of racers run synthetic gear oil with mazda clutch lsds- so I don't think that's your issue. Remeasure and then decide.

What does the surface look like?
here's mine:
https://www.miataturbo.net/forum/att...1&d=1159484346
https://www.miataturbo.net/forum/att...1&d=1159484321
The all looked like that- like new IMO.

There are several owners citing the "coke can" shim on m.net. I haven't set mine up yet, but you can bet I'm going to remeasure since all six of my disks came out at or above spec. Thanks.
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Old 01-08-2008, 10:47 AM   #4
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This thread reminds me I have to find the list of specific models with the clutch lsd... and start looking for one.
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Old 01-08-2008, 10:51 AM   #5
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Quote:
Originally Posted by Splitime View Post
This thread reminds me I have to find the list of specific models with the clutch lsd... and start looking for one.
http://members.aol.com/solomiata2/RX7LSD.pdf
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Old 01-08-2008, 11:18 AM   #6
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I think the key to getting a good deal is finding the first GSL SE lsd. Most don't realize that the 13b first gen has the right diff - just make sure it's the "large axle".
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Old 01-08-2008, 11:34 AM   #7
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Quote:
Originally Posted by m2cupcar View Post
I think the key to getting a good deal is finding the first GSL SE lsd. Most don't realize that the 13b first gen has the right diff - just make sure it's the "large axle".

yep, when i called 915racing.com (a rx7 wrecker) he had no clue there were interchangeable with the later model. cause when i asked for a 84-85 rear lsd for my miata, he said he only sold the 86-89 ones to miata guys. then i proceeded to learn him one.
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Old 01-08-2008, 01:28 PM   #8
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and now they're more expensive for the rest of us! :gay:
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Old 01-08-2008, 01:54 PM   #9
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well now that shim idea is just what I needed. It looks like I've got some experimenting to do, but at least now I know I'm not going to have to spend any money, always a good thing.

My plates look like yours, but have a few spots of shiny, which is a little worrying. I guess some tiny contaminant was trapped in there.

I'm gonna try aerosol shims first, and I'll be aiming for about 100lb/ft.
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Old 01-08-2008, 05:20 PM   #10
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report back!
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Old 01-08-2008, 06:19 PM   #11
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ok so i made a couple shims and it only brought it up to about 50ft/lbs... but what was bad is that it was notchy, like as you moved the torque wrench it would go from 50 to 20 to 50 to 20... I'm guessing the plates might be worn unevenly or warped?

also, does anyone know if you can just add an old thrust washer to one side? does it have to be symmetrical? it doesn't seem like it would matter. I'll try it again tomorrow with more preload and see if it is still notchy. I may have to buy plates after all...
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Old 01-09-2008, 10:55 AM   #12
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Seems to be some merit in a warped disk causing irregular lockup, given the amount of lockup. But you'd think it would have to visually warped. Do any of the disks have a noticeable uneven wear? that seems to be the more likely candidate for the problem. Run your caliper around on each disk and see what the variations are- that might reveal something.
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Old 01-09-2008, 11:12 AM   #13
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Damn i geuss i was lucky mine had like 100 lbs of lock-up out of the box. I use friction modifier with synthetic on mine though and, from the results you are getting i would think that you might well have a warped or uneven plate or two in there. Hence the notchyness esp. Cause if you where getting even laoding you wouldn't be getting the notchyness.
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Old 01-09-2008, 11:16 AM   #14
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mine has 150-170 ft/lb out of the box might as well be welded.
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Old 01-11-2008, 04:25 PM   #15
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Ok an update, first the notchiness was my own dumbness, I was shimming the thrust washer, because I assumed this was the only non-moving place on the diff. Apparently it does move, and the proper place to shim is between the spring and the outer plate. Once I made new shims and bought a proper micrometer I ended up with this:

christmas cookie tin - .009" x2
Yuengling can - .0035" x2

This gave me 90ft/lb, which should be about perfect. I tried adding one more can shim to one side, but this took it up to about 140ft/lb, so I left it out.

I'm going to put a few miles on it and let you all know if it's still working.
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Old 01-11-2008, 05:32 PM   #16
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Good to see Yuengling is good for more then just getting me drunk enough to sleep with Pennsylvanian women.
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Old 01-11-2008, 06:47 PM   #17
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Awesome. Now i'm not worried about trying this myself later on.
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Old 01-11-2008, 10:08 PM   #18
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Mine needs a shim aswell and after a full rebuild(minus two discs as they shorted me on my order) Looking for 150 lockup
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Old 01-29-2008, 06:34 PM   #19
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Ok after about 150 miles of street abuse and several drag passes, the shims are holding steady at 65ft/lb. They measured at 90ft/lb when I first installed the diff, but after a 5min drive they were at 65. So after an initial settling, they seem to be working well.

65ft/lb is really enough for a street car, it's plenty to make the car handle well while sideways and get the power down a little better and more predictably. I'm sure it won't be enough for autocrossing and slicks though.
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