Recommended Posts

Hello,

A forum member has sent us his 'RS' diff sent from the US. He's requested a bit of a thread of what we do, so thought I'd take the opportunity to post up here.

As is fairly common knowledge, the US rear diffs are setup "soft" from the factory. We reset them to be the more aggressive factory-intended RS setting. This thread serves as a "here's what's involved" and could be used as a how to (at your own risk, and as a supplement to the factory manual).

First things first we check a few things out with the diff itself.

IMG_6207_zps5vsmasye.jpg

IMG_6208_zpsm17ujmek.jpg

A bit of wear was evident from the swarf on the magnet. Not overly concerning, as it is a clutch plate LSD unit that does wear down by design.

Backlash felt alright by hand so we went straight to checking preload and mesh pattern.
IMG_6209_zpsm166koay.jpg

IMG_6210_zpsktsw2wvz.jpg

Preload was on the low side (0.1-0.2nm) so I expected a bit of bearing wear.

Mesh pattern was indicating that the crown wheel and pinion had reasonably full contact so I was happy to proceed.

A bit of wear was evident on the small pinion bearing which may explain the low-ish preload reading.

IMG_6213_zps1kq1fbdw.jpg

Once the unit was disassembled, the components were thrown through the spray wash for an initial clean.

IMG_6217_zpsytn2qqot.jpg

We then crack tested the crown wheel and pinion to make sure it will run reliably into the future, and to make sure it's suitable to process further.

IMG_6219_zps3eyavqbr.jpg

After this process the crown wheel and pinion are demagnetised and then ready for further processing (in this case, shot peening and isotropic super-finishing).

It was now time to move on to the LSD unit.

Undoing the screws can be tricky without the right tool - they are sometimes extremely tight and easy to strip. We use an impact screwdriver to easily undo the 4 small m5 counter-sunk screws.

IMG_6223_zps9zvplpko.jpg

From here the unit disassembles into its components easily. Take note of the positions of all the items, specifically which hemisphere goes on which side of the carrier so as not to change the ramp settings. We use texta marks and scribe marks for this.

IMG_6226_zpsedzrpqct.jpg

Each hemisphere has a clutch pack comprising of 2 clutches and 3 plates. The US models order these incorrectly to give a soft breakaway torque action, so as to not induce "unsafe" oversteering characteristics. From the US they are ordered PPCCP, whereas they should be PCPCP. The correct ordering gives almost twice the breakaway torque.

Incorrrect:

IMG_6229_zpsrypjyhwc.jpg

Correct:

IMG_6230_zpswrv1erbt.jpg

While we have everything apart we check for scoring etc. on the plates and clutches. This one is in good condition. Sometimes the tags on the plates bite into the housing and cause all sorts of issues. This needs to be corrected by filing the housing back to a smooth surface, and then replacing the clutch packs.

The next check is that we have an even stack height on both hemispheres, to give an even breakaway value from left to right. This is often overlooked but makes sure it operates correctly and wears evenly.

IMG_6227_zpsdvkwcgz1.jpg

In this case the variation between the two stacks was 0.01mm - about as good as you could possibly expect! The plates and clutches showed zero wear so this was driven by a very conservative driver or for a short time.

Now it's just a matter of reassembling with a good oil, paying attention to your marks.

IMG_6235_zpswoyh1twi.jpg

  • Like 11

Share this post


Link to post
Share on other sites

Good read. Thanks for posting

What are the costs involved here (assuming everything checks out OK so it's a re-shuffle and reassemble)?

Share this post


Link to post
Share on other sites

For a diff use exclusively for race Sam, would it be worth a shuffle of the stock plates? Or is it better to go for a higher plate count replacement? I have one on the bench ready to go back together but can't decide to run with the stock plates or not.

Share this post


Link to post
Share on other sites

Hey George,

I find the housing stretches if you add plates/shims. I have seen up to 0.4mm of runout measured off the back of the crown wheel with a new gearset due to stretching. Also the housings are quite soft and the additional torque strain on the plates means that the "ears" can bite into the housing, causing very unpredictable torque transfer shifts.

Because these diff gear sets are extremely sensitive to backlash change, this kind of runout leads to premature gear failure. My opinion is if you need additional breakaway torque then it's best to upgrade to a Cusco unit. The reality is that very rarely do you need more than the 40nm torque differential that the stock RS centre has in an Evo. I have customers running at the pointy end of both dirt and tarmac rally competitions nationally, with stock rebuilt RS units.

  • Like 2

Share this post


Link to post
Share on other sites

Much appreciated to have a write up like this, i'm keen to give mine a check over soon.

Share this post


Link to post
Share on other sites
Sign in to follow this