Learn me: Custom diff backlash/spacing

So I have a 318ti and really need a LSD to make it fun autocrossing without a one tire fire all over the course. It has its own special differential out of the E36's as it uses a slightly different E30 rear subframe. It runs the same 3.45 ratio as all other 318's and the internal LSD unit is the same. I need to keep the 3.45 ratio as I plan on competing in STX and maybe HS, where ratio changes are not allowed.

I have a line on a 318is differential which I plan to swap out the internals and put into my differential. However, it has come up to my attention that there is a chance that preload, backlash, and spacing all need to be adjusted so that I don't destroy my differential when I put one together. I have also been told that it is a drop in and forget job too.

So I ask GRM, what is really required?

Essentially, the pinion and ring gear are the same ratio. I will just be putting the differential and ring gear into another case with another pinion.

Don't change the gears, just change the differential. If the ratio is the same, then changing the gears is just adding a lot of work.

Pinion depth is 90% of setting up gears. If you don't screw with that, and use your old ring gear, then you just make sure the backlash is the same as before you took it apart, double check the mesh pattern, and you're done.

Just to clarify something: Pinion and ring gears are matched sets. So you will be swapping the ring gear from your old diff to the new one.

Absolutely don't mix the gears. As Knurled said, the pinion depth is the toughest part of the deal, and to be honest, most diffs built in the last 30 years are machined carefully enough that they all shim up the same anyway.

I've not had a BMW apart. Basic rules would be to beg, borrow or steal an dial indicator, and measure the backlash before you take it apart. Then use shims or the adjuster nuts (whatever it has) to create the same amount of backlash with reasonable preload on the side bearings. `

The important thing is not backlash, it's the mesh pattern on the gearteeth.
Unless the pattern is correctly centered, your gearset will not last and will not perform properly.

Right. Which is why you measure the backlash before you pull it all apart. If the pinion depth does not change, and the backlash stays the same, the gear mesh will be the same.

Gear mesh on a used set of gears will not look like what the handy-dandy charts show you. The gears are worn in, the pattern will more or less be a wipe across the entire gear face.

I can say from experience that certain OE gearsets are set up very loose of spec. Spec is something like .004-.007 and I would see them in the .008-010 range. I set them up to .004 because tighter is a little better as long as you keep temps in check. This throws the gear mesh out of whack and those suckers are loud for the first five-ten thousand miles until they break in again. I check to make sure that the pattern isn't too far out of whack and have at it.

First thing to do (and this comes from recent hard experience): get some gear pattern paint, like this stuff: http://www.amazon.com/Ratech-500-Marking-Compound/dp/B001P2D27U

BEFORE you remove the ring gears from EITHER diff, use the paint to find the gear backlash pattern. Take a picture of the contact pattern of the gear set you plan to use, and no you should not mix and match gears, i.e. the ring and pinion are a matched set.

Swap the ring gear you plan to use onto the diff you plan to use. If there are shims used to set the diff bearing preload, use the ones that were in the HOUSING you plan to use. Do NOT remove the pinion gear from this housing and be sure to use the ring gear that was originally used with this pinion gear.

Now reassemble everything, then use the gear marking paint again. Compare the gear contact pattern to the picture you took. If the gear contact pattern is the same, you are in good shape. If it isn't, then it may be necessary to move the diff to one side or the other by swapping shims.

The last one I swapped was a Miata diff using an RX7 limited slip, that one uses large 'gland nuts' to set the diff bearing preload. If yours is designed this way then there won't be any shims but you will need to check the shop manual to see how to set the bearing preload. About the bearings: do NOT swap races etc. i.e. make sure that the races for the side bearings go back with the same bearing they came off of.

I basically did the same thing with a 240sx diff and the lsd unit from an infinity. I just took both apart (left the pinion) and kept all the shims and the ring gear from the original diff. Put everything back exactly as it came out.

I then took it to a trans shop to have them check it for good measure. I told them what I did, and they thought I was crazy for thinking it would work. They were totally floored when it was all in spec and good to go. Charged me 20 bucks to check it i think.

rcutclif wrote: I then took it to a trans shop to have them check it for good measure. I told them what I did, and they thought I was crazy for thinking it would work. They were totally floored when it was all in spec and good to go. Charged me 20 bucks to check it i think.

Really? We do this sort of thing all the time. Swap open diff for limited slip, use old shims, if the mesh/backlash is good then ship it.

Heck, tolerances are soo good nowadays, we generally don't have to juggle shims when replacing all of the bearings. Re-use old shims, swab the case out thoroughly, slam together, double-check backlash and pattern, and ship it.

Looks like BMW diffs have bolt-on ring gears so I can swap out the original ring gear of my open diff and put it on the LSD I want to install. This is all sounding very doable for me, as long as I find the right equipment such as a dial indicator and needle-type torque wrench that tells me preload. Would I have to buy extra spacers or parts? The most I expect to do is rebuild the LSD with new clutch packs since the LSD I want to buy has 184k miles.

I am getting this diff for under $200. Ebay has the correct differential for my 318ti for $325 plus shipping. It looks like I am saving a considerable amount of money if everything goes well.

When I did my Miata diff I got a dial indicator from enco, it was fairly inexpensive.

Knurled wrote:

rcutclif wrote: I then took it to a trans shop to have them check it for good measure. I told them what I did, and they thought I was crazy for thinking it would work. They were totally floored when it was all in spec and good to go. Charged me 20 bucks to check it i think.

Really? We do this sort of thing all the time. Swap open diff for limited slip, use old shims, if the mesh/backlash is good then ship it. Heck, tolerances are soo good nowadays, we generally don't have to juggle shims when replacing all of the bearings. Re-use old shims, swab the case out thoroughly, slam together, double-check backlash and pattern, and ship it.

I think the fact that the diff carrier came from a different car was what blew their mind. Dunno if anyone ever told them that infinity really is the same as Nissan, but it was the cross model thing they were concerned about if I remeber correctly.

You would think that, being a transmission shop, they'd know that Infiniti was Nissan. Or that they use practically the same Hitachi diffs that Subaru did (well, I don't think Subaru ever used an R200)