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Shimming my diff - Did I over do it?


Alex3000
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My car is an 84 rx-7 (First Gen)

 

Long story short, I had to replace pinion bearings and decided to rebuild the LSD while I had the thing apart. The rx7 has 4 friction plates on each each axle side so 8 in total. without getting into it too much the stock spec is between 1.90mm and 2.00mm.  All my discs were between 1.97 and 1.98.

 

So for argument sake lets say my entire clutch pack was about .18mm below top spec for stock.

 

I made two shims from .254mm shim stock and stuck one on either end, after the cone washer. So now my diff is (0.254x2 - .18) .328mm over stock spec.

 

To me this seemed like an almost trivial amount, but when I installed the diff this weekend and drove the car around the block when I did tight turns the car lurched and skidded from the rear being locked. It seemed to get a little better after a few laps around the cul de sac but I did the old one tire in the air thing and tried to turn a lug nut with my breaker bar and it didn't turn the wheel (I didn't crank it down too hard as I didn't want to overtighten the lug)

 

So my question is for anyone with experience: Could .33mm over stock be enough to send a diff into locked up territory? 

 

I am driving at Dominion this weekend to shake down my most recent car re-assembly and am a bit nervous about the diff.  My hope is that it just needs a bit of break in after the rebuild and perhaps the breakaway will manifest itself better on a racetrack than going 25 mph around my neighborhood... 

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16 minutes ago, Hi_Im_Will said:

Yes, 0.3mm can lock it.  Put some paint marks on the halfshafts to confirm there is no relative motion, then embrace the locked diff because locked was always the correct answer anyway.

 

halfshafts?  That must be a fancy BMW thing B)

 

I would consider marking my axles but I silicone RTV'd them into the rear so I don't have to worry about my axle seals 

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19 minutes ago, Alex3000 said:

 

halfshafts?  That must be a fancy BMW thing B)

 

I would consider marking my axles but I silicone RTV'd them into the rear so I don't have to worry about my axle seals 

 

Just mark the flanges at 12 o'clock with a sharpie then have a look when you change wheels.

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2 hours ago, mhr650 said:

Lots of first gen racers shim their clutch packs tighter. It is annoying driving around and listening to all the popping and banging coming from the rear end, but it works fine on the track.

 

Thanks MHR.  I was hoping this was the case.  For all the in dept info on the interwebs about people shimming their differentials there doesn't seem to be any good info on how thick of shims people are using or what total thickness yields a particular breakaway.

 

OK, I feel better now.  Thanks forum people!

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12 hours ago, Team Infiniti said:

 

Interesting. A mechanic friend told me to build the diff "wet". So when I assembled the clutch pack I dripped a bit of gear oil on the plate and then smeared it around with my finger.  I just did a search for friction modifier and found instructions to soak the clutch discs in friction modifier... 

 

Maybe I will grab a bottle and take it with to the track, and if the car is behaving poorly from the diff being too tight I'll pour it in. 

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